A series of articles on the technical aspects of FFs
Advantages of different types of FF Single Track vehicles
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
The following applies to all FFs. It is intended to be chiefly useful to designers seeking to define the type which best meets their requirements
Sources.
The Quasar and Voyager projects both involved study of the 'performance envelope' of the FF single track to try to gain a picture of the full potential. Since those projects others have gone on the produce other examples of roofed FF's (BMW, Benili, Ian Peagram) and the Peraves Ecomobile represents an excellent example of full enclosure. All these vehicle appear on this site.
The overall envelope.
Compared with a car the single track vehicle offers a solution to traffic congestion and usually parking problems. In some locations it also allows use of special lanes. Single tracks are normally faster, with more acceleration than cars. There are theoretical, but rarely obtained, efficiency and cost advantages.
Any effective FF, one that offers a genuine functional advantage, must retain these advantages, while avoiding the problems of motorcycles. It must be more comfortable. It must be easier to ride, with less marginal features, and it must be safer.
Or it must offer such a unique level of luxury (Ecomobile) or high performance ( NSU Hammock), that compromising some of these basic advantages is acceptable..
The performance trade off familiar to motorcycles - handling OR comfort - is largely avoidable in an FF. Like a performance car it is not necessary for an FF to be uncomfortable. Comfort, a good fit, is an essential part of a performance vehicle. An FF can also be manageable at higher weights than motorcycles, due to the low CG, although weight should always be considered a Bad Thing.
This implies that, for a road going FF there is no reason why the basics; - Comfort, Handling and Safety - should not be incorporated from the outset in all types of FF. The question of which type hangs on other issues.
Cost.
Any individual constructor will discover that a motor vehicle costs several thousand pounds to complete. The basic 'open cockpit' type, typified by 001 Ducati 450 FF, is the most minimal and hence cheapest type. As an open cockpit single seater it has no seat transport, no wipers, washers, de-misters, closing transparencies or doors. These are substantial savings.
Every extension beyond 001-like, basic, FFs, increases the design and construction costs. FJ is significantly more complex, and better equipped, than 001and cost more in time and money to make. The Quasar, with just a roof, plus a windscreen to wipe, wash and demist, adds further components. All roofs add weight in the last place it's needed, above the riders head. Doors, even with an open cockpit will require careful design and more materials.
Most challenging of all is full enclosure, where opening doors with large transparencies are required along with controlled airflow and heating. Some level of the technical difficulty can be gauged by the number of very expensive supercars that leak through their exotic door arrangements. The Ecomobile demonstrates that all can be done, with some style but there is a substantial cost.
Weight.
Although the 295Kg Voyagers are much easier to handle than a similarly heavy motorcycle, there is no doubt that this figure is close to the top of the acceptable weight for a leg-supported single track vehicle, especially if a passenger is envisaged. I believe the Quasar is above the acceptable envelope for general use.
Above this envelope some form of outrigger system is needed. The Ecomobile demonstrates a functional system and although the dramatic shape and aerospace construction excite more interest the outriggers are actually the key components. Anyone building a vehicle with a design weight above 300Kgs should be considering outrigger systems.
These are not simple, fail-safe is difficult to achieve, ergonomics are tricky and some cost is unavoidable. I have done some design work on outrigger systems but other designers and constructors have real experience of such systems and a better qualified to describe the problems and solutions.
The passenger.
The weight and cost factors are just the consequences of more basic design decisions. Chief of these is the nature of the passenger arrangement. If no passenger capacity is required there is little reason to go beyond 001's level of complexity. If a passenger is to be carried then the open cockpit 'leg-supported' type demands careful packaging to achieve a reasonable weight and drivability. It also requires the two occupants to be closely fitted together. Although the rear bodywork obscures it the Voyagers just use the “King and Queen” stepped seat familiar on 'low-riders' with a movable single seat back..
Two-up, the rider uses the passenger as a seat back. This may be regarded as excessively intimate by some designers, although motorcycle passengers have always been in similar proximity.
If it is desired that the passenger sits further from the rider, using a separate seatback for each, it is extremely difficult to fit these people inside a motorcycle-like wheelbase and urban agility will inevitably suffer. Again, the Ecomobile demonstrates the wheelbase/agility consequences of providing a car-like passenger space.
Specific features.
The open-cockpit type is the simplest, lightest and smallest version. It should also be the most agile and efficient. Quite reasonable examples may be achieved by modifying a suitable motorcycle and Arthur Middleton's simple Kawasaki 500cc twin is a better than average example of a very basic “Cut'n shut” FF. The Comfortmax is also very good.
At the other end of this range FJ and it's relatives demonstrate the upper limit to weight and complexity for this type. Their major advantage might be described as “Utility” The ability to do everything useful a motorcycle can do, whilst approaching the comfort and serviceability of cars.
The next stage, avoiding outriggers but hopefully further improving the comfort, has universally been roofs. Some of this has been done in the hope of avoiding compulsory helmet use. In reality very few open-sided, roofed FFs improve the cockpit environment to the extent that wearing a helmet wouldn't be warmer, drier and quieter. The cheapest and simplest so far, the Benelli Adiva, seems one of the best.
All roofs, whether accompanied by doors or not, impose limitations on entry and exit to the vehicle and prevent the rider standing up and looking around. The visibility needed above the normal limits of windscreens, when turning into a corner has to be considered, as does the need for extremely narrow windscreen pillars to avoid obscuring obstacles.
The Quasar fails in all these aspects. These are important restrictions. Roofs do offer some improvement of the basic aerodynamics, 'hard tops' are generally cleaner than open tops, but a poor roof on an FF may be less use than a well-controlled open cockpit.
It may be worth considering side doors as an alternative route to better efficiency and a warmer cockpit. There needs to be provision for putting the feet down - or outriggers - but the result, resembling a Formula car should be efficient and offer a good cockpit environment. It would be easy to fit a cover over the cockpit opening not filled by the riders helmeted head and the small space occupied by the riders body would be easy to heat. Getting the legs in and out would seem to be the key problem to be solved.
Total enclosure takes the FF to the upper limit of it's range. Cockpit environment is completely controllable and the experience should be similar to a modern car in terms of ride, and noise. Safety should be the best available with seat belts and sensible cockpit design.
In theory, having solved the leg access problem with side doors, a fully enclosed, leg supported lightweight FF could be designed. Sketches of such vehicles, usually looking like a personal spaceship, are popular with potential customers.
The problems of combining so many features in such a small and lightweight package are challenging. If such a vehicle was made by a major manufacturer a team of people would take on the multiple problems, from the ergonomics of door and roof design, through the heating and demisting problems, to the careful integration required to get everything inside that leg-supportable weight limit
In practice it seems probable that the Ecomobile represents the typical total enclosure package, with well separated passengers and outriggers. However all these vehicles are packages of components intended originally for other layouts. The appearance of a powertrain that would fit entirely under the passenger space might allow packaging improvements that would make a smaller and lighter, even leg-supported versions possible.
Summary of types.
Designers will probably have a good idea of the type they intend to produce. Attention should be directed at the most pressing problems of the type chosen.
An open cockpit will need careful attention paid to it's environment, This is a matter of aerodynamic development which may be achieved with a few holes and some ducting.
Roofs need attention to the ergonomics and design, the windscreen requirements are as complex as for full enclosure.
Full enclosure probably requires outriggers, of which there are few successful examples and doors, with transparencies, that seal. These features present the most daunting technical challenges.
Open cockpit with side doors and leg-supported full enclosure have yet to be prototyped.
In reality open cockpit FFs have proved eminently practical all-round vehicles with many prototypes in daily use decades after the projects that generated them ended. A designer or constructor wishing to experience FFs for the first time would have no reason to go beyond this type. If later, a more complex type was desired, a basic vehicle would exist to carry out development on the specific problems of that type.
Clearly there is scope for experimentation and ample technical challenges to meet. Ian Peagrams development vehicle, based on a 650 Burgman scooter is an excellent example of a small roofed FF already showing exceptional fuel economy. Others are addressing the problems of full enclosure and outriggers and it will be interesting hear of their results.
Royce Creasey
Jan. 2005
Copy free for credit.
Aerodynamics of open cockpit, FF single track vehicles
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
This information applies to bodied single track vehicles generally but specifically to the open cockpit type, where several detailed effects are considered. In general the more fully bodied a vehicle is the more critical the aerodynamics will be, regardless of the type
Sources.
The following contains descriptions of specific effects and features. These can all be seen on FFs featured on this site. These are almost all vehicle I have designed myself. This is not because I think they are the best but because I know how they behaved in development.
Some familiarity with aerodynamics is assumed, you'll need to know why an aeroplane flies and a Formula One car doesn't. My own tenuous grasp comes from an apprenticeship on military aircraft, which included aerodynamic theory to supersonic. This was followed by a period in Formula One in the early years of wings, and then the design and development of most the vehicles detailed below.
I continue to learn from this experience that I don't know much about FF aerodynamics. But then, neither does anyone else.
Basic stability requirement.
A car, a horizontally arranged box, wide and low, has great trouble not taking off. The big horizontal surfaces can easily generate enough lift by 120 mph for it to fly. Designers take some care to counter lift in cars. In general, the more 'streamlined' the car, the more likely it is to fly.
An FF, a vertically arranged box, tall and narrow, has some trouble not being disturbed by sidewinds. Designers should take care to counter wind generated side loads. In general the more 'streamlined' the FF the more likely it is to be disturbed by sidewinds.
In practice fortunate dynamic effects, inherent in single track vehicles mean that a simple “Weathercock” rule will provide good directional stability in even storm winds. It is fairly easy to achieve stability in cross winds at least as good as modern cars.
For directional stability all that is necessary is that there is more side area at the rear than at the front. If a shape turns into wind when on a vertical pivot through it's CG it should be stable in side winds
This rule has been applied to all the prototypes associated with the Voyagers and the Quasar shape. It has proved a valuable rule, providing stability ranging from good to excellent. It may be instructive to study the pictures of 001, 002, Production Voyager and Fat Jogger, specifically in respect of the relationship between nose and tail side areas. CG can be assumed to be close to the nose of the seat.
All these vehicles have good directional stability. 001 is exceptionally stable in all respects, The Production Voyager is next best. The Banana had poor stability until it got a new nose and tail, both probably contribute to its good stability now. Because stability was so easy to achieve much of the detail differences between the Voyager shapes has to do with attempts to gain other advantages or solve other problems.
There is some evidence from 001 and FJ, that the lower side panels at the rear, the widest part of the tail, give effective weather cocking at speed but may not be as disturbed at a standstill, or very low speeds, as the flatter-sided Production shape.
Beyond Stability.
The aerodynamic quality usually most desired is Efficiency, low drag, with it's offer of a free ride to better performance and fuel efficiency.
Efficiency.
In reality, at the speeds that road vehicles use, simple frontal area is the main determinant of drag. Almost all FFs that run as well as the vehicle they share an engine with, return fuel efficiency gains of around 20%. Many, especially high powered ones, achieve similar increases in top speed. Even 001, where drag was hardly considered, comfortably out-performed the Ducati motorcycle.
However there are detailed ways to improve efficiency. None of these are mysterious. Smooth, clean surfaces, slow surface direction changes, clean separations, all add up to quite surprising fuel efficiencies - when the vehicle is being run fast enough for these details to matter.
Fat Jogger is optimised for efficiency more than any of the other shapes. In addition to it's clean lines it also has a sharply cut off 'Kamm Tail' and the resultant low pressure bubble is filled by the hot radiator outlet flow. The side and centre stands are part of the shape, with the centre stand, when retracted, forming the 'chin' of the radiator inlets.
It impossible to say whether this detailing is 'worth' the effort. Although it has reached 90 mpg it more recently returned worse figures than Ian's Production Voyager travelling together Vagaries of old engines and states of tune have probably had a greater impact. However the three Voyager shapes are all fairly clean and a computer generated prediction of the Cd for 002 was .3. This level of efficiency is clearly worth having.
Indifference.
At the beginning of the last section on efficiency I used the term 'free ride' to describe the advantages of a low drag shape. There are no free rides and as the American fuel efficiency competitors have discovered, extreme efficiency can lead to extreme problems.
For FFs these chiefly amount to sensitivity to turbulence, where the vehicle is noticeably buffeted by turbulence, typically on motorways, at speed, in traffic. Ideally an FF should be “Indifferent” to these disturbances.
Poor indifference is caused by the body surfaces of the vehicle reacting to rapidly changing airflow directions. Sideways lift generated across a smooth nose may be balanced by sufficient tail area but the sudden cessation, or reversal of the airflow will instantly cancel the side load and this will be felt in the steering or attitude of the vehicle.
This is avoided, on cars as well as FFs, by enforcing separation of the airflow from the surface at chosen points, rather than allowing the separation line to wander about the surface generating unpredictable lift effects. Any reasonably sharp edge, ridge or groove will do the job.
At the front this is usually done almost immediately, certainly within 500mm of the leading edge. 001 used immediate separation devices on all the leading edges and some of these sharp direction changes, can be seen in the pictures. 002 had less separation on flat surfaces, as nose lift was not seen as a problem but has clearly visible channels running up each side of the nose to enforce separation of any airflow across the nose surface. The ledge in front of the lights is intended to enforce separation of the flow over the lights and features on all the Voyagers in various guises.
002 however had a minor indifference problem. Strong turbulent side winds could be detected as tail buffeting. The probable reason for this became apparent years later..
The production Voyagers are a simple step on from 002. There is more tail area, chiefly due to the use of slides on the head fairing and a complete head fairing. The separation feature on the nose is somewhat more 'styled' and the nose is shorter due to packaging improvements in the basic design. The result was very encouraging with excellent. stability and indifference while efficiency seems as good as any other of these shapes.
Enforced separation of flow across the tail is just as important as across the nose. 001 and FJ both use separation enforcing edges on the upper edges of the tail. Subsequent study revealed that other vehicles have suffered from cross-flow generated tail lift and buffet and this device is a fairly common solution. The sharp upper edge of FJ's tail may be extreme but some device like this is important. It may therefore be significant that 002 has no separators on the upper tail for cross flowing air. It's tail ridge is a gentle curve. Review of wool tufting footage shows the random nature of the airflow around the tail.
Fat Jogger's nose shape paid only lip service to enforced separation. The shut line gap of the nose opening panel is supposed to bleed air into the airflow over the nose to provide separation. It's pressurised by the air entering the headlight opening. There is no evidence that this works and indifference was initially so bad that strip separators were retro-fitted, very crudely, to the nose. These are visible in later pictures of Fat Jogger. Indifference is still only moderate in some conditions and it is planned to revise the nose eventually, although the 'dustbin' style will be retained to maintain flow to the radiators.
In 2006 a new nose was made for FJ and there are several photos and test reports on this site at http://www.bikeweb.com/image/tid/86 (Royce Creasey Creations, FJ, new nose) This uses a bluff 'inverted aerofoil' shape with integrated separation on the upper and side faces. There is also a much bigger duct up the rear of the windscreen. The new shape is entirely successful in achieving indifference and provides better cooling and cockpit environment. Drag has not increased. However the reduction in air washing downwards off the sides of the nose allows road spray to enter the lower cockpit and further work will be done on this problem.
When an entirely new shape is not possible, separation devices are a good and simple solution to aerodynamic problems caused by excessive contact with the airflow. It is possible to buy 'turbulator tape' used on Glider wings to confirm separation, but a length of wire or string under a strip of tape will work fine.
Obviously there are questions about the styling of any of these vehicles. There was only one
((disastrous) session with a 'real' stylist and everything was done for functional reasons.. The challenge for stylists is to achieve a similar nose to tail side area relationship, with some similar separation devices, in a shape that provides their aesthetic requirements.
Once stability, efficiency and indifference have been combined in a beautiful shape there are another important things to consider.
Cockpit environment.
In some respects an open-cockpit FF is similar to an open-topped car. There is a need for a tail, or at least a head restraint panel to eliminate the recirculating bubble of air that both vehicles tend to generate. This is massively inefficient and gives the rider a cold neck. It is possible to stop this effect with a tail that doesn't reach fully to the height of the rider's head. 001 was quite successful in using a cut off tail, otherwise rather similar to FJ.
Because FFs are so narrow recirculating bubbles tend to to run on either side of the cockpit opening as well. In particular the cut outs needed to giver leg clearance tend to allow airflow running alongside the keel to roll up into the cockpit low pressure area. It has proved quite difficult to avoid some airflow through this gap and if anywhere gets cold in FJ's cockpit it's the rider's hips where airflow turning up towards the cockpit runs over the top of the radiator intake fairings.
At lower speed, particularly urban speeds, a smaller recirculating bubble may form in the upper cockpit in front of the rider's face. This is unpleasant and if it continues at any speed head buffeting will cause control problems.
A number of detailed devices have been used to try and eliminate these problems. The most effective have sought to increase the pressure in the cockpit. Arranging for strong ram air directly into the cockpit works very well. The trick is to select an intake that won't carry water, or litter, is directed to the right parts at the right speeds and, above all else, is heated. All the Voyagers have heaters, with fans, and this is a crucial advantage in anything but hot weather.
The headlight opening is an excellent intake for cockpit air, being well out of the spray and exhausts nearer the ground. This realisation came after the creation of the Voyagers which bleed air out of the wheel bay with reasonable success. Both options line up well with a good heater location spilling warm air, ideally, into the riders lap. It will flow over the hands, knees and face without further assistance. The cockpit pressure change caused by turning on the heater fan in FJ results in a detectable reduction in the speed at which rain is kept out of the cockpit.
A duct ramming air up the inside of the windscreen is very effective - and needs no heating. It increases the height of the aerodynamic effect of the screen by several inches at 'main road' speeds. FJ and some of the production Voyagers have these ducts, running from the headlight opening, up to the rear of the screen. These work best at speed and are quite common on motorcycles fairings.
Further ram air can be taken from the front of the keel, on either side of the front wheel. In the Voyager layout the engine heats this air, some of which is exited into the cockpit along the leg clearance cut-outs to counter the airflow mentioned previously tending to take cold air through this gap. These outlet ducts can be seen on the panels running from the top of the radiator inlets to the rear of the footboxes. The radiator outlets on the Production Voyagers were intended to do the same job.
Care has also been taken to seal the rear of the cockpit, to prevent through flow, all the air entering the engine bay is directed into the radiator intake except for a bleed through the gap between the seat base and seat back.
Each of these devices produced a noticeable improvement in FJ's cockpit environment. Some development of features like these should be expected before an optimum environment is achieved in any prototype open cockpit FF.
It is impossible to overstate the benefits of a good cockpit environment. Staying warm, dry and comfortable allows an FF user to fully exploit the dynamic and efficiency advantages of the layout. Even in winter.
Lift.
If a sharp edged tail like FJ or 001 is used it is extremely unlikely that the tail will generate meaningful lift. In any case re-attachment of the airflow onto the tail will not be laminar so even a flat surface is unlikely to generate a lot of lift. The shelf on the rear of the Production Voyagers, caused by the use of seat slides, was given a slight positive angle of attack to eliminate any possibility of lift. However the actual angle was set so that a pint of beer would not slip on it when parked.
Lift at the nose would probably have been achieved by the clay shape offered to the project by a professional stylist, which was why it was rejected. The 'Notched nose' shape used throughout the Voyager project is principally intended to reduce side area at the front but it is easy to provide for positive angles of attack over the entire upper surface.
In practice most prototype shapes generated too much negative lift, or download. Malcolm's SEV Z1300 and the original Banana, both accelerated fast enough for the arrival or this download at around 80 mph to be visible, with the body settling onto it's rubber mounts. This can produce instability at high speed, where the vehicle apparently hunts about behind the huge pressure wave. It can also increase sensitivity to side winds, possibly due to variations in download according the the airflow angle over the nose.
It has proved better to seek to eliminate positive lift, by suitable separators, positive angles of attack, etc., than to attempt to achieve download.
Cooling ducts.
All vehicle body shapes need entries and exits for cooling air. The most efficient form of intake is the 'pitot' type. Basically a hole in the front. Such intakes work well on FFs. The Banana demonstrated that with only 10 degrees of lock each side, a hole in the ('Dustbin') nose just big enough for wheel clearance, would provide ample airflow for cooling the engine. Until the space around the radiator was blanked off it acted as a vacuum cleaner, sucking litter off the road surface and throwing it into the cockpit.
If an air-cooled engine is used the front wheel bay is necessarily the intake and this works very well. 001, with close cowled fins, seriously over-cooled. In winter use water droplets appeared in the oil and no oil temperature was indicated between October and March.
Water cooled engines will have thermostats and 002 placed a radiator in this intake, immediately behind the front wheel. In terms of cooling this is perfectly satisfactory. There are problems with this approach however.
First is that there also has to be an outlet. If this is the engine bay, as in a car, the bay will become extremely dirty, to the point where this may compromise function. It also places the hot exit air in the riders lap and this is unacceptable in hot weather. These problems can be solved, as Yamaha has with the Tmax, by ducting the air into the keel. Yamaha use this technique on their race bikes and it clearly a good low pressure area. However, It can be difficult to manage this ducting when a collection of unmatched components have to be packaged. It was incompatible with a forward mounted Reliant engine.
Most problematical is the increase in wheel base, and the rearward movement of the CG, if a radiator and probably a fan have to be fitted between the engine and the front wheel. 002 had a weight distribution too far to the rear and moving the engine forwards as was done in the production design, by 3”, was a major dynamic improvement. This was done by fitting two radiators either side of the engine and the front wheel, fed from the wheel bay and exiting
into the leg clearance cut-outs on each side. .
This was not very successful, with insufficient airflow through the radiators One Voyager subsequently fitted a third radiator across the top of the wheel, exiting into a hole in the shelf in front of the headlights. This provides sufficient cooling but is costly in components and plumbing. Much later development has indicated that the poor sealing between the wheel bay, the radiator intake, and the outlet areas may be a large part of this problem.
FJ took twin, larger, radiators to the rear. The intakes were placed below the riders seat in attempt to capture the strong airflow turning up into the cockpit mentioned earlier. The exits, as also mentioned earlier were fed into the low pressure area of the Kamm tail. There were initially problems with water flow, due to over-elaborate plumbing but this system works well, with the engine temp falling below 'Normal' (Thermostat fully open) at high speeds. Fans are needed in these long ducts at low speed, but heat balance is reached well within urban speeds.
An attempt was made recently on another Voyager to replicate this system, using a single radiator on one side. Although designed from the outset for a clean duct it has proved very difficult to make it work. It was immediately clear that air was re-circulating through the radiator - out at the top, back in at the bottom, a perfect recirculating bubble.
It seems likely that the problem is very poor airflow into the intakes. This probably relates to the different nose and keel shapes. The production nose is intended to cause a low pressure area behind it, like a dam and the resulting low speed air blanks the intakes. FJ's nose is cut away and shaped to minimise the interference with air flowing either side of the front wheel and keel. Near full airspeed is maintained some distance into the intakes.
Basically the standard pitot intake works well, but needs work on the exit if the engine is to stay clean and the rider cool. Rear radiators solve most of these problems and are a good packaging solution but need careful airflow optimisation from the outset.
Beyond open cockpits
Many FF proponents are interested in greater degrees of enclosure, Roofs and full enclosure are popular features. Every shape produces a unique reaction from the air and it would be extremely rash to extrapolate anything learned with open cockpit FFs to roofed or fully enclosed vehicles.
However the general rules apply. The more bodywork in contact with the airflow the more important it is to get the stability balance right and to ensure that separation is predictable rather than dependant on airflow direction. Efficiency and cockpit environment should be better than an open cockpit unless something is seriously wrong!
There are several other vehicles on this site with roofs or enclosure and the NSU Hammocks are worth careful study. Not least because it was discovered they take off at around 200 mph... The Quasar shape, colossally heavier, also lightens up surprisingly when driven by other engines to speeds above 120mph. It also appears to generate too much download over the nose initially and small spoiler strips across the nose and at the top of the tail were tried, proving to lighten and quicken the steering on 'fast main roads'.
In general I hope I have made clear that open-cockpit FF aerodynamics are quite complex, but that it is quite easy to arrive in an acceptable window. Providing the basic rules for stability are followed and attention is paid to indifference it is probable that most designers will find the details of the cockpit environment most challenging..
Anyone with no formal or empirical experience of aerodynamics would do well to read widely on the subject. A recently published book “Road Vehicle Aerodynamic Design” ISBN 0954073401, from www.mechaero.co.uk is a good starting point with many references and a non-mathematical approach. Single track vehicle are not mentioned.
Anyone at all who wished to gain a feel for good shapes will do well to look very closely at modern cars. There is virtually no limit to the real complexity of FF aerodynamics. One might well build an FF purely to study this subject.
A final rule of aerodynamics.
Anyone who says they understand vehicle aerodynamics doesn't understand vehicle aerodynamics.
Royce Creasey
Jan. 2005
Copy free for credit.
Safety performance of open cockpit, FF single track vehicles
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
This information applies only to basic 'open cockpit' FFs of the type shown on my site www.hightech.clara.net Open cockpit FFs give the least protection of the various FF types but offer the least weight/size and greater agility. Crash protection is concentrated on initial impact mitigation, mainly cockpit features. In this respect it is at a similar technology level to late twentieth century open topped cars, without seat belts.
Sources.
All this knowledge is derived from the Quasar and Voyager production projects, during which several real accidents occurred. Information from accidents was added to published knowledge of car safety features and these features have worked in subsequent accidents. This process was engineering, rather than academically, based and the accidents were not necessarily representative of any other accidents. However the evidence strongly suggests that FFs are much safer than conventional motorcycles and that simple safety features work.
“Active” safety.
A properly designed FF (see also Basic Dynamics) will be far more agile than a motorcycle of similar size. It will turn faster and brake harder, at the same time if required. The rear wheel will not lift under braking. It is very difficult to 'High Side'. The rider is far more secure in the vehicle and has better control, These features increase the safety performance by making it easier to avoid accidents.
“Passive” Safety.
Foot clearance.
Very early in the Quasar programme it became clear that the footboards were a hazard if the vehicle was dropped as they trapped the riders foot. This caused serious injuries in some cases. It also prevented riders putting their feet down at a standstill if stopped between cars. Subsequent designs allow riders to put their feet down within the widest part of the vehicle. It is important to confirm that the feet will not be trapped at any point if the vehicle falls over.
The resulting cut-outs cause minor aerodynamic problems which are discussed in 'Aerodynamics'
Frontal impact.
This common form of accident kills most motorcyclists, usually due to head contact with the roof edge of a target car. It is primarily caused by the poor design of the motorcycle and in particular the front forks. In such impacts the front of the motorcycle collapses downwards, pitching the rider into the car. On an FF, even a 'cut'n shut' with telescopic forks, this pitching effect is largely absent due to the lower CG and the forks will collapse more rearwards.
This provides an opportunity for the rider to be 'caught' by the cockpit of the FF rather than be pitched out and over it. If 'Hub Centre Steering' (HCS) is used it will transfer far more energy into the target vehicle and may push it out of the path of the FF. It will also collapse downward but lever the front of the FF upwards in so doing. This makes rider capture quite easy and it becomes relevant to provide padding and collapsible structures at the front of the cockpit, especially the hand controls.
In accidents like these, where the vehicle remains horizontal or pitches up at the front, the rider will move forwards until contact is made with the cockpit. This should happen as soon as possible and a steeply sloping seat will immediately capture the rider's pelvis. Further forward movement of the lower body can be prevented if the knees contact the inner face of the footboxes which should be suitably deformable. This immobilises the lower body but provision should be made for progressive structural collapse of the seat nose and the footboxes.
The rider's upper body is more problematical. There is inevitably some distance between the rider's chest and the hand control yoke. This has been addressed on the Voyager design by providing a friction clamped pivot allowing the entire control yoke to hinge upwards under crash loading. The underside of the control yoke is a flat panel which, in the prototype at least, is heavily padded. It is assumed that the rider will be resisting the forwards movement of the upper body and thus the clamp will already be loaded towards the point at which it will slip by the time the riders chest hits it.
The control yoke on the Voyagers pivots in a structure that will itself deform under crash loads and the instrument panel, seen as the 'target' for the riders helmeted head, is too fragile to cause injury.
These features can be seen deployed on the post-crash photos in 'early prototypes' on www.hightech.clara.net There were no injuries to the rider as a result of the impact with the cockpit. In this accident 002 hit the rear wheel of a Fiat 132 which had turned into it's path, with an impact speed of about 50 mph. One might expect a modern production version of this vehicle to feature an airbag on the hand control.
After this impact the rider was ejected from the cockpit, at ground level, and slid for some distance along the road. This produced minor 'road rash' injuries and these are typical of most accidents involving open cockpit FFs where the rider has left the cockpit. However these have all been accidents at fairly high speed (40-60mph) and the 'urban accident at less than 30 mph' that kills most motorcyclists are clearly more survivable in these vehicles than on a motorcycle.
Leg injuries
At the time of the Voyager project (1988-9) leg injuries were the second most common cause of death. Usually the leg would be trapped between the two vehicles and ripped off. Initially this was countered by enclosing the footboxes in the frontal bodyshell (Banana) then steel footplates inside foam-filled bodywork (001, 002), then as a result of the 002 accident the production design was a steel structure that supported the footboxes and the nose bodyshell and was intended to improve the progressive collapse of the front of the vehicle. All these designs were intended to give protection from side impacts and prevent the feet being trapped,
There have been no leg injuries in any of these vehicles and until 2006 none of the production Voyagers has suffered a frontal impact, making it impossible to evaluate the crash performance of the later features up to that point. In the 2006 event a production Voyager slid, on one side, into a concrete block that impacted on the side of the nose bodywork, the right hand footbox and the right hand leg of the front fork. All the steel components were unrepairably damaged. The rider suffered a slight injury to a toe. The footbox collapsed roughly as intended, preventing serious injury to the riders foot or leg. The accident also demonstrated the value of a 'bolt-on' footbox system for repair purposes.
Non-impact crash performance.
As performance, rather than safety, was the primary interest of early English FF development the majority of accidents involved going too fast rather than hitting other traffic. As a result there is plenty of information about the performance of FF's sliding along the road on one side.
In this mode, if the rider is still seated correctly, there have been several occasions where the edges of the wheels have remained on the ground and some control has been retained. Several riders have reported a belief that in the right circumstances a sliding FF could be recovered onto it's wheels.
From the safety point of view there are two considerations that should be borne in mind by designers.
First that any slide should stop as soon as possible. This to avoid painful and damaging contact with road furniture or kerbs. This requires consideration of the detail of the ground contact points that the vehicle will slide on. These should resist, sacrificially if needed, being dragged along the road. The production Voyagers used convenient brackets, shaped to slide, sledge-like, fore and aft, while resisting being dragged sideways. These have worked in 'going too fast' accidents. 001 had alloy caps designed as friction contact pads and these also worked well in a demonstration slide. GRP does not resist slide well and will skate for some distance along a normal road surface.
This ability to slide illustrated the second requirement for a slide on one side. In the Banana crash in the I.o.M. (Creg Na Ba) the vehicle not only slid freely after losing grip but also rotated so that on impact with the straw bales it was going top, or cockpit,-first. This is undesirable. A stable, keel-first, slide can be ensured by fitting the ground contact points well to the front and rear of the vehicle. This means that whether it goes down in under-or over-steer the first point to contact will tend to return it to a stable keel-first slide.
Later, ad hoc, development and some minor low-speed drops have shown that the slide attitude can be quite easily controlled by selection of the friction materials. A slightly 'tail-down' keel-first slide, suitable for damage limitation and potential recovery, and be achieved by fitting some high-friction rubber at the front with just a metal contact point at the rear..
One might see some advantage in racing , in using rotating contact patches (or “Skateboard Wheels”) but in practice, on the road, stopping the slide as soon as possible has to be the priority.
Practical crash padding.
Polyurethane foam, also known as 'Brown Foam', Seat Foam', is the perfect material for deformable crash padding. Enclosed in a Glass/Polyester or Glass/Epoxy envelope it makes stiff, light and fire-retardant bodywork and it is an easy matter to adjust the GRP skin thickness to give appropriate strength for either outer skin or a target for part of the riders body. It's progressive collapse qualities are excellent. It can be poured, as a self-foaming liquid, into existing body sections, or used as a carved shape to build GRP onto.
Pressure on manufacturers to reduce toxicity in their products in the late nineties and early noughties has resulted in a new form of PU foam, used for house insulation. One trade name is 'Seletex' This type of foam is just as suitable for bodywork but does not produce an irritant dust and is much cheaper than 'brown foam' 'Room Temperature curing' Epoxy resin is also now available for GRP, lighter and stronger than Polyester resin. It tends to cause eye irritation as a dust but is not toxic like early epoxies. FJ has a new nose using these materials.
Seat belts and beyond.
The apparently obvious step of fitting seat belts, to prevent the rider being ejected after the initial impact means that the rider may find themselves unable to keep their arms and legs in the cockpit of a cartwheeling or tumbling vehicle. Even sliding into an obstacle strapped to the vehicle may be injurious. This may not be a problem, or seat belts may lead to increased arm, leg or head injuries. Seat belts have been fitted to a Quasar and much more recently to the production “FF-like” BMW C.1 scooter, both open sided at least but I have no information on their crash performance. It may be that the addition of a roof alone will make seat belts safe.
Prioritising safety, inevitably at the expense of cost, weight, complexity, etc., leads to consideration of other rider restraint systems that can be used in conjunction with seat belts. These might be doors, enclosing the side of the vehicle and containing arms and legs or total enclosure up to the level of the Swizz Ecomobile.
All these features, seatbelts, the simplest side doors, and definitely roofs, also compromise basic features of the vehicle. Seatbelts, unless quite sophisticated will prevent body movement needed for easy low speed control. even simple sidedoors are actually quite difficult to do achieve without leg fouling problems. The problems of roofs on FF's are almost legion!
Individual designers must make their own decisions about their design priorities, these basic safety features on open cockpit FF's have however demonstrated a very reasonable standard of protection in real-life accidents and will be of particular interest to designers chiefly interested in the vehicle dynamics.
Copy free for credit
Royce Creasey. Jan. 2005
Dynamics Consequences of FF Single Track vehicle layout.
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
This applies to all FFs in terms of the physics but is specifically relevent to open cockpit FFs.
This is not a description of the basic physical reasons for the superiority of FFs over other types of two-wheeler, it deals with the dynamic consequences of applying the FF layout to two wheelers.
Anyone unsure about FF dynamics generally may find “The Physics” on www.hightech.clara.net informative. There is a more flippant description on Ian Kew's site www.voyager03.co.uk where he has put up the 1978 High Tech series from Bike magazine. There are no formulas involved but some very basic geometry is used.
Sources.
Isaac Newton, his laws of motion. Proved time and time again by experimentation and development with Quasars, Phasars and necessarily the entire Voyager project series of prototypes.
Major effects.
Torsional stiffness.
An FF will be able to roll into and out of lean far faster than a motorcycle. This will be exploited by users who will invariably drive the vehicle to its comfortable control limits. Roll is initiated by counter steering where the forward motion of the vehicle is used as a form of power steering to drive the front contact patch in and out of balance with the CG.
If the vehicle can provide a high roll rate, without wheel lifting or unloading, then the rider will demand a high rate of direction change from the front wheel in order to initiate these roll rates. This is one of the major reason for using a well supported, torsionally stiff, type of front suspension. Suitable types are considered elsewhere. A 'top wishbone' is a common feature.
Front suspension systems that are capable of taking car-type levels of steering input, including violent corrections, are in use on several FFs and are an essential feature of any 'performance' type.
Having initiated a rapid roll, into or out of some desired lean angle, using a suitably stiff suspension system, it is necessary that this input is transferred to the mass of the vehicle without loss of precision. Torsional stiffness is a crucial element in multi-track dynamics, where it defines the ability to balance roll stiffness, front to rear, . It is similarly important in an agile FF where displacing the front contact patch in a flexible chassis will merely result in prolonged understeer, possibly followed by an unpleasantly sudden turn in.
Torsional stiffness defines control precision in an FF. It has been neglected in motorcycles where a degree of flex helps a rider stay perched on top.
Although the consequences arrive a little later it is also important that the rear contact patch reacts as though it is connected to the front contact patch by torsionally unyielding structures. To achieve this is is useful if the rear suspension is as stiff as the front and this requires a top wishbone, controlling an upright, as is common at the front in English HCS systems. Graham Robb's Production Voyager as been fitted with such a device, A top wishbone controls a simple inverted 'U' shaped upright, running off the rear swing arm and he reports that it controls even the fearsome Guzzi rear suspension.
Although this innovation came after all the Voyager, project, it is so central to FF performance that I would not build another single track vehicle that did not use a top wishbone at front and rear.
From the design point of view a designer should seek the highest torsional stiffness available in the package being assembled.
Weight Transfer.
Minimisation of weight transfer is one of the great advantages of FFs. This has implications for suspension set-up. A vehicle that does not stand on one or other of it's wheels simply as a consequence of control inputs does not need suspension capable of dealing with such huge variations in loadings. This means that less travel is needed, even if rising rate systems are used, with three to four inches of travel proving adequate for general road use. In particular travel at the front can be similar or identical to travel at the rear.
The position of nominal ride height, usually requiring some guess at the total weight during the design stage is also different to motorcycles and this has safety implications. Motorcycles, with their huge weight transfers, tend to run ride heights around 30-40% of 'Full Droop', or fully extended. This allows the system to deal with the positive weight transfer and assumes wheel lifting under any major negative transfer.
FFs do not suffer this weight transfer but can roll much faster. A well supported rider will tend to initiate roll rates up to the point where they become insecure in their seat. If this exceeds the ability of the suspension to cope with a wheel unloading it will lift and control may be lost. These factors mean that ride height should be set further towards 'Full Bump' or fully compressed.
For highly agile FFs like 001 ride height at 65% to full bump was appropriate. The heavier Voyagers, after development, ended up at 50% of travel, largely due to the amount of travel needed to cope with the high weight.
To avoid wheel lifting in development it is always best to start with springs that are too soft and damping that is barely adequate, and then to increase these, to just avoid the bump stops and prevent excessive movement. Using motorcycle settings initially may be painful.
Brake performance.
A stiff and strong front suspension, allied to a low CG and a secure seat means that the rider may brake up to the limit of front tyre adhesion. This may exceed 1G and certainly this is one situation where a seat belt would aid the rider. Any structure strong enough to take normal loading will be able to cope with this although designers should consider these loads, at least in details like fuel surge and rider security and particularly the hand control support. 'Underseat steering' as used in some HPVs is ruled out by the potential braking performance of FFs.
This performance relies on substantially more braking effort from the rear wheel than is usual with a motorcycle. In any case, total weight on the wheels does not alter but less weight transfer to the front wheels means more weight remaining on the rear - and hence the ability to generate more rear braking effort. Almost all FFs converted from a single motorcycle suffer from an inadequate rear brake. The Voyagers were fortunate in that a Moto Guzzi 'Cruiser' (The 'Californian') had a larger than usual rear disc, although it has to be fitted with front brake pads to fully exploit it's potential.
Any motorcycle-based FF should use the largest rear brake available for that model.
Ground clearance
The lack of weight transfer and suspension set closer to full bump than full droop means that conventional motorcycle ground clearances may be ignored. Lowering the ground clearance is be the cheapest and most immediate way of lowering the centre of gravity. Development of this specific feature has shown that 95mm is about the minimum for general road use at 1525mm wheelbase. This allows a vehicle to be ridden up kerbs without contact but FJ, with this setting, just touches on a road hump found in a local industrial estate. The 1600mm production Voyagers are set a little higher at around 100mm and seem to clear everything. These figures are 40-50mm below common motorcycle settings and represent a valuable gain.
Wheelbase.
FFs resemble other single track vehicles in the way the wheelbase setting affects agility. It seems clear that, in a balanced turn, wheelbase has no effect on the angle of lean but in the transition to that state a long wheelbase vehicle will need to move the front wheel faster and more acutely for a given rate of transition, this will need to be balanced initially with greater lean. This can be seen as apparently exaggerated counter steering from a following vehicle and is a feature of Quasar performance.
This is an important consideration in design terms. It is pointless to build a long street racer, unnecessary to build a short cruiser, just as with motorcycles. The intended use envelope should influence designed wheelbase.
In England regulations permit single track vehicles to drive between cars, changing lanes, as and when deemed fit. This rather democratic approach to road use seems to produce good safety figures but is definitely illegal in several parts of the world. The agility required for routine 'elk tests' in moving traffic call for wheelbases similar to motorcycles. It is easy to tell the difference in this environment between FJ and the 75mm longer Production Voyagers.
It is probable that the agility conferred by the low CG allows the wheelbase figure to be relaxed slightly compared to common motorcycle figures which are usually 20-50mm shorter.
001 ran a minimum wheelbase of 1475mm and turned slightly faster than FJ but both these vehicles have a maximum roll rate faster thasn required in normal use. 001 was handicapped by the inherent geometry of the Difazio hub, forcing excessive trail even at 10 degrees of rake, FJ, as usual, by it's weight
A racer, or even hot street design might reasonably call for wheelbase at 001 levels or shorter, in order to increase roll rate. However other factors such as CG height and suspension quality also limit roll rate, by defining wheel unloading. It is not worth compromising other design features to achieve sub-1470 wheelbases for maximum roll rates which may be difficult or unnecessary to achieve.
Long wheelbases also require more lock to turn at very low speed and this is a real handicap in European cities. The Banana, at 1728mm, with a Difazio (20 degrees total lock) HCS is awkward in town, especially back streets. 001 with a similar HCS was sufficiently agile for a policemen to explain that it was frightening the traffic.
I believe that for general purposes a leg supported, open cockpit FF should have a wheelbase in the 1500-1575mm window. This requires attention to packaging but fully competitive specifications with passenger capacity have been achieved. Outriggers turn a single track vehicle in a car when deployed and completely alter the dynamic case. I have no information about this type.
Other problems
Clearly the design of any motor vehicle requires consideration of many factors, several of which usually conflict with each other. However, compared with the design of a multi-track vehicle, and the subjective criteria involved in different motorcycle designs by different people, FF design is rather straightforward.
Physics rule. Different riders will tend to agree on an FF's qualities, rather than disagreeing as is common in motorcycle development. It is possible to take wrong turns in FF design but these are usually based on unthinking application of some motorcycle-based theory or other. Once any thought of these devices have been banished from the mind, designers may proceed through the normal vehicle design process with some confidence that, if the above specifics are taken into account, their FF will meet dynamic expectations.
Royce Creasey
Jan. 2005
Copy free for credit.
Ergonomics of open cockpit, FF Single Track vehicles
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
Single track vehicles.
Sources.
There have been articles published on the subject of the interface between the human and the machine, to my knowledge, since the middle of the last century when German aircraft engineers demonstrated the advantages of the reclining position over the prone position.
The subject became widely known as “Ergonomics” during the 1960's when car designers began to take it seriously. Any designer, engineer or user seeking optimal performance should be aware of it's importance and it is the principle failing of the motorcycle that it's designers have not done so.
In addition to my own interested reading the Quasar, Phasar and Voyager development programmes have allowed exploration of the subject.
Basic requirement
The principle advantages of FFs over motorcycles have been described as “Comfort, Handling and Safety” (Voyager rider summary results)
To qualify the term “Comfort” it is convenient to consider the two other conditions an open cockpit FF most closely resembles. These are the open air, standing in the street - and the open topped car.
If the level of comfort achieved by a pedestrian, assumed to be wearing clothing appropriate to the weather, can be maintained in the FF, and if that cockpit environment can be maintained at a similar level to an open car, then the rider can be said to be comfortable.
This implies that they will be seated normally, fully and securely supported by the seat, so that the steering is not used for support. It means that the cockpit area should be heated to a level at least as good as ambient, with no unpleasant draughts or buffet. Equally important, all the controls, including parking, refuelling and servicing access, should be easy and natural to use.
Relevance to specific design.
Jack Difazio (Front suspension pioneer);-
“Once you take the wind blast of your chest you'll worry about the cold draught down your neck”
Like most design areas there is no end to the improvements in Comfort that can be sought. The open cockpit type has the simple requirements noted above, full enclosure would need to achieve car-like levels of soundproofing and tranquillity to justify the loss of urban agility and convenience. A record breaker, or short circuit racer, would leave out heating and settle for a simple, secure seat.
The degree of comfort chosen will define the quality and possibly even the type of the vehicle. It might be regarded as the start point for a design process of any road vehicle.
Basic features.
The Seat.
The FF differs from other single track vehicles in it's seat height and type. The height is a response to the need to dramatically lower the CG. The type of seat, with a full seat back, is the feature that seems to cause motorcyclists most disturbance. Such seats, universal in all vehicles except motorcycles, are essential to comfort and handling.
By supporting the upper body they remove almost all the effort required to stay on a single track vehicle. Properly designed, they make good ride quality much easier to achieve. Finally, the extra support provided allows much greater control over the steering. The seat back, although run close by the front suspension, is the most important single component on an FF.
Even FF designers often have trouble taking this seriously, a seat back may not be as exciting to do as a four-valve, twin-cam cylinder head but may contribute more to performance satisfaction in use. Good comfort has been noted as a feature of several of my designs but in reality the common factor is the same Volvo seatback - from the 'legacy' 340 model.
This unit has everything a seatback needs in an extremely simple and lightweight package. During the Voyager project units were supplied for less than £30.00 and may be acquired from scrapyards for around £10.00. It has a 'tip-forwards' feature, allowing easy access to the passenger and rear compartments. There is a simple lumbar support adjustment, allowing correct support in different clothes and most important, rake adjustment.
Volvo spent more design time and money on this component than most individuals will on a complete prototype. Even if a different unit is used it would be wise to study this one as an example of excellent design. Other 'quality' cars also provide fine seatbacks and a trawl through scrapyards for an individual perfect fit is entirely sensible.
If a seat back is to be designed from scratch it should at the very least be free to flex from it's base. If rigidly mounted to the top there is a strong possibly that it will hit the rider between the shoulder blades over bumps, causing neck pain. Some trouble was taken to separate the top of the Banana seat from the structure during it's decade-long development.
Rake adjustment is the next important feature. It is very difficult to set this angle correctly at the design stage and in any case the angle that fits best in urban dodging may be steeper than that ideal for leisurely motorway cruising. Lumbar support adjustment is often regarded as a luxury but needs to be correctly set to support the upper body over bumps.
If rake angle and lumbar support must be fixed it still makes sense to use a simple car seatback, with the angle set at the manufacturing stage so that the riders helmeted head just falls forward at a standstill.
The seatback of an FF is used differently from the car unit in one respect. The FF riders upper body will slide across the upper seatback in low speed manoeuvring and if the full range of FF rider body movement is considered it will be seen that there is a need to look around, and upwards, similar to a glider, or light aircraft pilot. In the absence of seatbelts this will be done by rotating and flexing the entire upper body and excessive lateral support, as provided by most 'sportscar' seats will be intrusive. Kart racing type GRP seats, with support right up to the armpits, will seriously intrude on the normal riding experience.
If seat belts are planned this movement requirement should be considered in their design.
The seat base is very simple by comparison. Fully sprung seat bases were used on 001, 002 and the production Voyagers but these are not noticeably more comfortable than the 'chopped foam' seats used on the Banana and FJ. These resemble motorcycle seats in structure, a rubber mounted steel base, with about 50mm chopped foam, covered with PVC 'leathercloth'.
It is necessary to shape the riders seat base to allow comfortable 'legs down' use. The well-known 'Tractor seat' is a good shape for the base. This supports the rider at the rear and the sides, with a raised horn between the legs, usually flowing into the engine cover between the knees, and the parts under the thighs are cutaway to allow the 'legs down' position.
The riders seat base, as is usual in cars etc., should slope upwards to the front quite steeply, so that the riders hips naturally slide into the back of the seat. It is absolutely worthwhile considering and defining these details by comparing and measuring available car seat units.
In the above assumes an interested individual considering the design of an FF. Thus the details of the ergonomics will be set to fit that individual. The designers of any production vehicle will need to go further to suit the majority of the population. It is normal to use a '90 Percentile Human Ergo' or standard human model to achieve this and although real 3D 'Ergos' as used by the auto industry are extremely expensive, 'Virtual Ergos' can be readily downloaded. The Voyager project, somewhat preceding commonplace computing, made a 2D Ergo many years ago, featured in some Comfortmax pictures.
Seat position.
The position of the seat in the vehicle impacts on the riding experience and possibly the handling. It is easy when converting a motorcycle to use the space often found just ahead of the rear wheel for the seat. The Banana and Arthur Middleton's Kawasaki conversion are classic examples of this and basic packaging will usually define the position of the rider.
This rearward positioning can lead to a rearwards weight bias, which in turn can cause snatch-locking of the front brake and excessive understeer although this can be minimised by packaging other components.
It also intrudes on the riding experience, especially at low speed. Although it is difficult to pinpoint an exact relationship, it seems that the rider is more comfortable if it feels as though the head is turning with the front wheel. If the front wheel is between the riders feet, as in the Voyagers and the Comfortmax, it is easy to achieve this feel. Steering on the Banana, by comparison, feels more remote.
This is a minor effect, unless urban use is to be common, however FJ is optimised to move the rider as far forwards as the Reliant engine will permit and I believe there is a detectable improvement in the feel of low speed handling. In an ideal situation, where packaging is not an issue, one might start by placing the riders head as near to the centre of the vehicle as possible.
Passenger provision.
If a passenger is to be carried there must either be a separate seat back for each occupant, or the seat back used by the rider must be transportable to support the passenger, whose body will also support the rider. This works well in practice and FJ has carried passengers right at the top of the 90% Euro range. This is clearly the most efficient way of packaging two humans.
Although the close proximity this system provides is commonplace on motorcycles and other single track vehicles, some designers regard it as unacceptable. The Ecomobile uses two separate seats and some system like this will be required if the occupants are to be physically separated. There is a penalty to be paid in weight, complexity and packaging for this feature.
Seat back transport.
A seat back capable of being adjusted to allow use by either a single rider or an additional passenger is a feature of the Quasar and several Phasars, designed by the late Malcolm Newell. The Quasar uses a hammock-style arrangement with a wide, upholstered leather 'strap' which can be adjusted to provide support for the rider or passenger. It major failing is that it provides no lateral support and I found that a skateboard, conveniently jammed behind the hammock, provided much better support in sporting use.
Two-seat Phasars, with various powertrains, used a seatback mounted on telescoping tubes from the rear bulkhead. This provides better support as the seat back itself can be better shaped and a developed version might allow full adjustment of rake, lumbar support and position. The telescopic supports however intrude into the luggage space.
The concept was further developed in the Voyager series with an entire seatback transportable into either rider or passenger positions. The seat base resembles a 'King and Queen' seat, originally found on some American low-riders but also now common on large touring motorcycles. To allow the two occupants to fit together comfortably the passenger seat needs to be around 125mm higher than the riders seat.
002 and the Production Voyagers used the seat slides that come with the Volvo 340 seat unit, including the adjustment latches. These slides are inclined upwards to the rear so that the seat back is at the correct height at either end of it's travel. This system allows limited movement of the seat back to accommodate different solo riders. Similar slides were used in the Production design to transport the head fairing which covered the top of the passenger opening in the bodywork.
Some other prototype FFs also use slides to transport the seatback and it is clearly a solution worth considering in a two-seater design..
The major problem seems to be that. although convenient and easy to arrange, slides intrude badly on the packaging of components under the seat. The need for slides, and the carriage sliding on them, to be parallel, seriously constrains the shape of components which might otherwise be packaged more efficiently. The 'shelf' and side extensions on the top of the Production Voyager tail are principally there to accommodate the head fairing slides.
FJ accordingly used parallelogram arms to swing the seat back from it's single seat position up and rearwards to the rear seat position. Anchorages in either position are engaged by latches that transport with the seat back. The head fairing uses a single yoke to achieve the same end, although if a full parallelogram had been used it could have avoided the body damage done to the upper edge of the tail by the engagement latch in the head fairing during careless transfer to the rearward position.
These linkages have proved to be a better solution in terms of packaging and shaping the rear bodywork - this may be clear from photographs. It also allows the actual distance between the two seatback positions to be set entirely by ergonomic considerations; it is difficult to find car-type slides with sufficient range of movement. The common 'Drawer-type' slides do not provide sufficient location support at full extension. It is more difficult to provide adjustment of the seatback to accommodate different riders, making adjustable footboxes and hand controls more important in a production design.
More recently Honda have introduced a seatback, on a 250cc 'Urban utility' vehicle, which hinges back down flat so that a passenger may use it as a seatbase. It is good that Honda have finally fitted this essential device to one of their vehicles although it seems unfair that only the rider can experience the advantages, and then only when alone. Honda should be capable of designing a fully transportable seatback and it is to be hoped that they can afford the time to do so.
Footboxes.
In the absence of an Ergo the location of the footboxes in an FF design can again be found by sitting in cars. A friend with a tape measure will be needed. In a production design it will probably be found easier to make the footboxes and controls adjustable, rather than the seat. Even if slides are used to transport the seatback it is difficult to provide a full adjustment range. CVT FFs like the Comfortmax do not need foot controls, making a one-size-fits-all footbox quite easy.
The FF seating arrangement needs slightly better security than the car application. There may not be seat belts and the vehicle is subject to vertical accelerations during normal use. The normal agility of a low CG, HCS equipped, vehicle may provide enough vertical acceleration to unload the rider and in any case severe bumps may also cause problems.
The nearest vehicle to this is the racing kart, where heels cups are routinely used to allow the rider to hold themselves in the seat. The floor of an FF footbox needs to be set at an angle steep enough to provide a similar but more comfortable support. The Banana, in it's current form, most closely approaches this angle in the vehicles I have designed. It is considerably steeper than the Production Voyagers. FJ is somewhat inbetween, not steep enough.
This angle is not hugely critical, FJ is acceptable in moccasins, where the ankle can flex freely, but noticeably shallow in winter boots. The Production Voyagers had heel rests, like sportscars, intending the that the feet would rest on their heels, but resting the whole foot on a correctly angled plate is more comfortable and secure.
Width
The width between the footboxes is usually set by the space needed for suspension or wheel lock. If there is no machinery in the way the feet can be set at a natural sitting separation - typically 300-350mm.
Normally however packaging forces the feet further apart. FJ is already 415mm wide by the time it has front suspension and a wheel that steers. Adding 250mm to this, for the two minimum width 125mm footboxes, is what defines it's overall width. Some other FFs have even wider foot positions. This is not a problem ergonomically and any FF approaching standard door width will not trouble the rider with excessively wide foot positions - Unless the engine bay also forces the riders knees apart.
Height
If the knees are significantly further apart than the feet, because of such a wide engine bay, there is a possibility that they will tend to fall apart in a relaxed sitting position. this is uncomfortable and places them outside the cockpit. The problem is reduced if the feet are below the seat base. As the wheel centre (and front suspension arms) are around 320mm from the ground and a typical seat height is around 450mm, there is space to arrange this. Sweeping back angled footboxes also assists keeping the knees tucked in.
Detail.
It is normal to include a high friction surface to the footbox floor, and also a small raised ridge along the outer, but not the lower, edge, usually not more than 10mm high. These increase foot security in the event of hitting big bumps or during accidents where rider location is important.
The 'leg clearance' cut outs, mentioned in the submission on safety performance, and aerodynamics, should start immediately behind the footbox, with the rear edge designed to avoid sharp corners or edges.
The outer edges of the footboxes are the widest and strongest part of the front of the vehicle. They protect the legs from impact and trapping if the vehicle is on it's side. These structural demands, including high friction surfaces as noted in the safety notes, should be integrated into the footbox design.
Hand Controls.
Type
A wide variety of hand controls have been used on different FFs without seriously intruding on their riders conciousness. With one exception the standard motorcycle control set is adequate for FF use.
The exception is the twist grip throttle. Although suitable for a motorcycle it demands that the handlebar be gripped hard enough to keep the twist grip open against the throttle return spring. It was noted that, riding the Banana at speed on motorways, this gripping effort was the highest muscular effort being used. To the extent that, in long runs, it was the limit on comfortable use.
An HCS equipped FF should require virtually zero steering effort at speed. Exactly as in a car, the hands will merely hang on the controls. A similarly low effort throttle control is required. The Voyager solution has been a right hand trigger throttle lever. This was easy on these Reliant-engined FFs, where the car-type carburettor has a single throttle disc. 001, with a slide type motorcycle carburettor was also acceptable. This solution allows easy simultaneous use of the throttle and front brake, as in changing down into a corner while braking and also permits very fast throttle-to-brake transitions. It may not be the only low-effort throttle solution but it sure beats a twist grip!
The trigger throttle also frees the design of the actual hand grips and this was use to provide 'pistol grips' which support the whole hand in a natural, relaxed position. Other controls are similar to the motorcycle set, although combined light/horn/indicator and ignition/start/engine kill switches from cars were selected for their superior ergonomics and protected in a waterproof hand control yoke.
These details may seem excessive compared with the universal motorcycle set. However they pass the definitive ergonomic test - they're unnoticeable - and the motorcycle set does not.
Position.
Height
The requirement for rider security and the braking performance mean that the hand controls need to be higher than they are in a car, where seat belts help with the braking loads. Again, racing Karts provide a good insight with hand positions nearer shoulder height than the centre of the chest. The Production Voyager riding position was largely copied from the Ford Sierra and even with the hand controls at their highest adjustment point it is clear that they should be higher. FJ and the Banana are better in this respect.
Individual designers can get a good idea of the correct position by kneeling on the ground and supporting their upper body on slightly bent arms, there is a position of natural balance where the head and chest are balanced on the hands and this relationship will be correct for handling braking loads.
Width.
Total vehicle width will closely confine the width of the hand control and the 660mm-720mm width range of most of my designs is narrower than normal motorcycle handlebar width. Fortunately HCS allows far narrower handlebars, in terms of control effort, and the Voyager hand controls have generally been set to provide comfortable clearance to the inside of the cockpit, itself close to the full width of the vehicle. Other designers have used hand controls where the minimal width is set by packaging the motorcycle control set.
The Comfortmax is interesting in this respect. It's standard, rather wide handlebars are clearly necessary to control the original scooter but the increased steering authority given to the rider by the seat back gives an initial impression of excessive leverage and over-control, until the limits of the telescopic forks are reached and it becomes quite useful again.
Adjustment.
It has proved important to be able to correctly adjust the hand controls. Pistol grips should be adjustable for angle, to allow the wrist to form a natural angle. It is also very useful to be able to adjust the hand control for height and distance to shoulders. During development of a new vehicle other problems, damping, springs, engine set-up etc., tend to obscure ergonomic details and it may be some months before it's realised that the hand controls are uncomfortable, or fail to provide good security. In a production design adjustability of the hand controls seems essential.
Adjustability generally is seen as something as a luxury on single track vehicles but to return to Jack Difazio's quote, it is the key to eliminating irritating discomfort, cramps, aches and pains, the ergonomic equivalent of the draught down the neck. A vehicle which fits well, causing no discomfort, will be used. An uncomfortable one will either be modified or abandoned.
Heating.
This essential feature, normally entirely absent on single track vehicles, will cause some technical difficulties due to that fact. There are no heater assemblies in production designed for such vehicles. However, if water cooling is used for the engine there will almost inevitably be a hot water pipe which can be used to supply a custom made heat exchanger. Even a manifold heater line can be used, after the manifold, for a heater matrix, but the main coolant circuit can be tapped into if needed.
The heater matrix will probably need to be custom made simply because of packaging problems. This is not particularly expensive, radiator repair companies are fairly common and a wooden pattern, with hose connections defined, will usually suffice. The heater outlet should be into the area under the hand controls. If the cockpit is reasonably well protected from buffet this will distribute hot air right up the riders front and out to the hands. The matrix should basically be as big as there is room to fit it, with good ram air supply from the front of the vehicle. The Voyagers use the front wheel bay for the heater intake but the headlight opening might be just as good.
Although not fitted originally several owners have added a small fan to the heater matrix for use at a standstill and it has been noted in in the submission on Aerodynamics that the cockpit pressure increase these fans generate keeps rain out of the cockpit at a lower speed.
None of the heaters fitted so far provide enough heat to trouble riders in an English summer. Users tend to turn the fan on in October and turn it off in April, but a really effective heater, basically bigger than those tried so far, might need to be controlled to supply cold air in summer. designing the control system and air supplies is quite difficult due to the need for fairly large air passages and very constricted space.
Like the seat back, detailed heater design seems to be thought insufficiently exciting by designers to be worth real consideration but for the user heating will make the vehicle genuinely useful in all conditions. Even with the matrix fitted to the Voyagers it's possible to park, step out of the cockpit and realise that it's cold out there....
Rider Access.
Further irritations that justify attention at the design stage concern access to the cockpit and the servicing areas. The Quasar is quite difficult to enter and leave but even simple open cockpit vehicles can present obstacles. It is worthwhile considering entry and exit while finalising bodywork and seat arrangements. If the vehicle is to be entered while balanced there needs to be a way of holding it up while entering. If it is intended to be on a stand it must be possible to operate these stands from the cockpit.
The Voyagers all use lever operated centre-stands to make this easy. The Banana demonstrates that the operating lever should be on the left hand side, opposite the front brake. If there is a side stand it needs to be well forward, like the riders feet, so it can be easily reached.
Servicing access
Access to basic service areas, oil, water, ignition, carburettor should be as simple as a car. It is appropriate to use car type latches for this and under any bonnet in a scrapyard a wide variety of simple, rugged, cable or rod operated latches are available. Dzus, for one, also offer similar latches as catalogue items. Although this is also an area ignored by motorcycle designers the easy access provided by such latch systems mean that servicing and development adjustment will be carried out. As a result the vehicle will last longer and work better.
Less routine areas, charging, cooling, any wearing parts will eventually need attention. If these are easily accessible this will be done in the routine course of vehicle maintenance. This will hugely increase the chances of the vehicle surviving for long enough to justify the cost and time spent in building it. There are number of companies supplying latches and fasteners, the Voyagers use very effective and lightweight “Swell latches” from the American company “Southco” but study in scrapyards will provide many alternatives.
As a general rule servicing access panels should be removable without tools.
Conclusion.
Ergonomics, the science of comfort, is almost completely ignored on motorcycles or scooters and is therefore an unfamiliar area to many single track vehicle designers. However all designers have a body to experiment with and most have access to a variety of cars providing a rich seam of design solutions. There was never any excuse for the appalling ergonomics of motorcycles but given the similarity of the FF cockpit to that of a car it would be ludicrous to ignore the subject in FF design.
Royce Creasey
Feb. 1005
Copy free for credit
Front Suspension for open cockpit, FF Single Track vehicles
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
This applies to all FFs but is specifically relevant to 'agile' or 'performance' FFs.
Sources.
The history of vehicle suspension was enthusiastically plundered. “The Motor Vehicle” (Newton, Steeds and Garrett, ISBN 0-408-01082-7) is a good source for multi-track derived systems. Contact with early seventies Formula One cars was also instructive. Of the various systems bolted to motorcycles only Normans Hossack's system, recently appropriated by BMW and the system pioneered by Tony Foale, subsequently used by Yamaha on the GTS, bear consideration for road use..
Many of the vehicles featured on this site have contributed development knowledge.
Introduction.
At first glance it might seem that the front suspension system universal in motorcycles and 'performance' scooters, Telescopic Forks, would be entirely adequate for a simple FF. This is entirely true, but chiefly to demonstrate beyond doubt that this 'legacy' system is not adequate for even an FF directly converted from a motorcycle.
This doesn't mean that such systems become more unsafe when applied to the FF layout, just that their performance limits that of the vehicle.
There are three reasons for this, all inherent in the FF layout, and this subject is also considered in the paper on 'basic dynamics'.
Braking performance.
The secure riding position and low CG of the FF layout means that braking performance is limited only by available adhesion, rather than the rider losing control or rear wheel lifting. This means more than 1G deceleration in reasonable conditions and telescopic forks are quite incapable of dealing with these loads - while also providing suspension. Only the dive under braking and lack of rider security, inherent in high CG motorcycles, make telescopic forks tolerable under braking.
A system is needed that, at least, can withstand full braking forces without deflection. Any vehicle engineer will assume that the suspension and steering will also continue to work.
'Violent Correction'.
The low CG and secure riding position also allow very fast direction changes, including those needed in correcting some loss of adhesion. Performance use naturally exploits this ability and reasonably lightweight FFs like 001approach kart racing levels of agility. Even FJ can be slalomed violently enough at low speed to lift the front wheel.
Only systems in which the mass of the suspension is not steered can be used this violently without feedback wobbles. Applying the 'opposite lock flick' common in a momentarily oversteering vehicle to a motorcycle normally produces a 'tank slapper' as the considerable mass of the steered suspension resonates with the geometry.
Torsional stiffness.
Any single track vehicle is steered by moving the front contact patch from side to side, relative to the CG, in order to control the lean angle of the vehicle. Much of the 'steady state' is dealt with by the geometry and inherent features like tyre coning but rider inputs in an FF can be rapid and violent.
If this is to result in a precise steering reaction the movement of the contact patch must result in a prompt reaction from the vehicle. Control precision in an FF is defined by the torsional stiffness of the whole vehicle. A flexible single track vehicle will behave like a flexible multi-track; prolonged understeer, followed by an unpleasantly sudden transition to oversteer as everything is finally wound up and the message gets through to the rear wheel.
First of all the steering system used to direct the contact patch must be torsionally stiff in order to transmit steering inputs and detect feedback. Secondly, the structure of the front suspension must precisely transfer the torsional load imposed by a suddenly displaced contact patch into the structure of the vehicle.
Summary.
These three requirements can only be ideally met by systems that are stiff enough to cope with the braking and torsional loads and do not steer the suspension.
However the Comfortmax points to a possible lightweight FF 'window' where the loads involved may be low enough to permit adequate performance with Telescopics. A small, lightweight, wheel may avoid steering wobbles in violent inputs, especially if combined with BMW's telever system, which does not steer the suspension. This should also improve the torsional stiffness of the slider assembly. The actual forks can be better supported with the bottom yoke much closer to the ground than possible on a motorcycle.
The existing Comfortmax example does not meet these criteria but makes noticeable movement towards them. It is still limited by it's front forks,.particularly under braking, but less so than motorcycle derived cut'n shuts.
Telescopic forks are basically an unfortunate accident of history. If the people who developed 'girder forks' especially those used by Vincent on their post-WW11 models, had possessed the understanding and innovative ability of Norman Hossack much pain could have been avoided.
Suitable systems.
Hossack.
It is appropriate to start with Norman Hossack's system. This is, geometrically, “double wishbone” but with the 'upright' extended downwards to form a pair of forks with a wheel axle across their lower ends. It is virtually identical to the “Girder Forks” almost universal on motorcycles until BMW introduced telescopics before WW11. The crucial difference is that the pivoting links and springs of the girder system are not steered on the Hossack. The steering head is moved out to the apex of what are now two wishbones, firmly based on the chassis and the suspension picks up on the lower wishbone in normal automotive practice.
As the fork unit is a single component it can be made light and stiff. The lower wishbone can just clear the tyre, minimising the fork extension and bending loads and even torsional stiffness can be reasonably good. Resistance to lozenging in the wheel axle/fork leg/bottom yoke assembly can be obtained by adding thickness to the fork extensions and the FF layout allows this.
This system has performed reliably on motorcycles and providing the higher loads imposed by FF use are accounted for, it should be worth consideration on any cut'n shut. Comfortmax-style scooter conversions would seem to be an appropriate application for this system where no lightweight full-HCS system is available. Fabrication is easy, no special engineering is required and there is full control of geometry.
“Full HCS”
Single-arm
The very simplest system, Single-Arm, is used on the Reliant three wheeler, which has sacrificed so many engines to the FF cause. Although good in terms of steered weight and braking performance it lacks a top wishbone and any claim to torsional stiffness. The geometry also varies with suspension movement and although five or six degrees can be tolerated over full suspension travel, it seems unlikely that Single-Arm would be a stable arrangement in a full-size FF. The Italjet scooter uses this system and there are some reports of reasonable handling, although this particular example appears to suffer rapid 'king-pin' wear
One might consider it for an ultra-lightweight, a fuel efficiency challenger for instance.
Double-arm
The obvious development, this system has a long history in cars, with the pivots in line with the direction of travel instead of across the front as in a single-track chassis. Both the Citroen DS series and the pre WW11 Auto-union racers used this system. Turned through 90 degrees to become a single track system, the French-based ELF racers of the mid-eighties are probably the best known examples. As is usual in automotive practice suspension loads are taken on the lower arm and the upper arm is used to control the geometry, brake torque and provide torsional stiffness.
This system, in common with other single-sided systems, needs a wheel with considerable offset. The steering axis must be on the wheel centre and the brake rotor necessarily outboard of that, with the wheel disc furthest outboard. Making such a wheel is the major constructional task.
It is also impossible to reconcile the need for the top arm to be reasonable stiff with anything like adequate steering lock. This arm has to tuck in under the wheel rim and inevitably restricts lock to that side unless it sweeps through a wide curve that is clearly incompatible with stiffness.
The Elf racers, like other race bikes, were untroubled by the lock restriction and it seems that racing is the only real application for this simple system. However it is also worth noting the single brake disc, another disadvantage when considered for a heavyweight road going FF.
Arm-and-Wishbone
This is a logical development of double arm, where the upright is continued up, around the tyre and over the top to the wheel centreline. Here a simple top wishbone can be fitted giving excellent torsional stiffness, full braking and geometrical control and no restriction of steering lock.
Normally attributed to the English motorcycle engineer Tony Foale, this system is probably more familiar globally due its use on the Yamaha GTS cruiser.
Significantly this design uses a fairly exotic ventilated disc to overcome the single disc problem and this does cost more than two conventional discs. Also the thickness of the single lower arm needed to give stiffness under braking loads intrudes on the lock to the right hand side, opposite the arm. It would also seriously reduce footbox space, or increase overall width in an FF design with a 'forward seated' rider.
For the fabricator the design of the single lower arm presents a real challenge if all the stresses are to be satisfactorily contained. Again a deeply offset wheel disc is required. However, once these problems are solved this system offers everything that is needed in an FF front suspension system. Rider reports of both the GTS and other privately made systems of this type report no problems with asymmetrical deflection under braking, the most apparent weakness.
Double wishbone.
This is evolved from the oldest and most successful FF yet produced, the Sheffield Simplex Neracar. This used what may be called a double-sided single arm, where the arm, instead of ending at a king-pin pivot to provide steering at the wheel centre, continued through the wheel centre and back to the fork pivot across the chassis.
A component I call the “Barrel” in my designs, mounts on the king pin, to provide the steering movement, and the wheel rotates on bearings mounted on the outer diameter of the barrel.
This would have balanced the braking loads if front brakes had been fitted but confers no other advantages over single arm except for a symmetrical wheel. It appears to have worked reasonably well on the Neracar.
Many years later Jack Difazio allowed the king-pin at the wheel centre to rotate on the axle, and then controlled the geometry and brake torques by carrying an upright, bolted to the outer faces of the barrel, over the wheel and steering it from the upper cross piece using two track rods connecting to a transverse steering arm on the chassis.
This eliminated geometry change and permitted twin brakes. The double-sided lower arms are ideal for braking loads and apart from the upright, only the wheel assembly is steered.
Torsional stiffness still relied on the king-pin at the centre of the wheel and, as this rotated on the axle necessitating free play, stiffness on the original systems was poor. The use of one of these systems on the Banana highlighted this weakness and a top wishbone capable of taking torsional loads was fitted to the top of the upright, exactly as is done in Arm-and-Wishbone.
This system allows the Banana to be driven with all the enthusiasm of later Double Wishbone systems
The Difazio systems were fairly lightweight, with limited lock, each one turned from solid billet and nearly all a slight development on the last. Later versions of the 152 systems made had ball bearing king-pins rather than plain bearings and some had rubber bushes on the axle pivot instead of greased bushes.
Both Malcolm Newell and myself needed a simpler, more rugged system with adequate lock and lower costs for our respective FF projects. Bob Tait, Malcolm's designer and I both used spherical joints instead of a king-pin at the wheel centre, relying entirely on the top wishbone to keep the wheel vertical.
The Mk11 Voyager system shown on my site and used on FJ, is the most developed of these systems with ample lock, minimal component count and reasonable weight. It uses a common automotive ball joint at the wheel centre, from a similar application in a small car.
This system copes fine with heavyweight open-cockpits like FJ and might be considered the simplest in design terms, with symmetrical loads and ample stiffness. In common with the other systems major parts have to be designed and made and in this case the most difficult is the design of the axle, ball joint and barrel set at the centre of the wheel.
Application.
These appear to be the only systems to meet the requirements for an FF. For a cut'n shut, where the priority is simplicity, ease of conversion, quite possibly to study some aspect of FF performance apart from outright dynamic performance - aerodynamics, ergonomics, etc. - the Hossack system appears ideal. One might even find one of Norman's conversion on a suitable donor motorcycle. In any case fabricating such a system to use an existing frame and wheel is not particularly difficult.
Only Arm-and-Wishbone and Double-Wishbone offer practical solutions to full scale FFs and the decision as to which to use might be based chiefly on the braking requirements. Ultimately the double-sided system can carry more brakes.
Footbox clearance is also a factor. If the FF is intended to fit through a European doorway (a useful feature) and the rider is to sit forwards, as in FJ and the Comfortmax while enjoying normal steering lock, the room for the lower fork is severely restricted. FJ, with an overall width of 660mm, 38 degrees of steering lock each side and a 16” front wheel, has 35mm left on each side for the lower fork, 5mm of which is clearance to the footboxes. It is difficult to see how this could be achieved with the much greater thickness of a single lower arm.
This problem can be avoided by moving the foot position and GTS FF conversions are in progress.
Steering.
It is inherent in all these systems that the steering is entirely separate from the suspension.
On a motorcycle a lever is simply attached to the top of the suspension and steering passes through it. With either Hossack or HCS some part of the unsuspended assembly must be firmly attached to a steering system. This system, as noted above, must be as torsionally stiff as the suspension while maintaining a low weight and good packaging. Several options are available.
Single link.
The simplest way to transfer a steering load from the unsuspended wheel assembly to the suspended chassis, is a single link (“Drag link”) parallel to the vehicle centre line and exactly aligned with with the pivot points of the top wishbone of any of the suitable systems. At the outer end this is attached to the upright or Hossack fork unit and at the inner end to a lever pivoting on the vehicle centre line at the same angle as the steering axis itself.
This provides bump-steer free steering. This system was used on Malcolm Newell's Phasars.
Anyone familiar with the geometry of these parallelogram suspension linkages will be aware that the steering link can be designed to fit anywhere on the the upright, providing that the geometric requirements in terms of angles, lengths and pivot points, are met. This may be useful to improve packaging but a link below the top of the wheel needs to be wide based to clear the wheel on lock.
Double link.
The Voyagers use this geometric method but 'couple' the steering links so there is one on either side of the top wishbone. This may be seen as extravagant but is an effective way of resolving all the resultant loads in the steering, providing exceptional torsional stiffness with minimal weight increase. Steering input torques are high enough to make flex in single link pivots detectable from the cockpit. In theory a coupled system can use much lighter individual components, while maintaining a degree of fail-safe. In practice component availability tends to define actual size and weight.
Connecting to the rider.
Once a chassis-mounted pivot has been established, the actual steering control can be connected directly to it or a further linkage can be used.
A direct connection is obviously ideal and the Comfortmax shows that, if the rider sits well forward it is possible to connect directly to the fork tops without linkages of any sort. This certainly allows direct use of the chassis-mounted steering pivot of a Hosaack or HCS system.
The most obvious further linkages is to repeat the single, or coupled, drag link system further to the rear where a second pivot may be established to support the actual hand control. Such a link was used on the Quasar to connect to the rider to the motorcycle-style 'Earles Forks' and they were normally used on the Phasar series. The Banana, with coupled forward links, used coupled rear links in a similar way, with Jack Difazio exploiting the limited lock to use very light aircraft-style rod ends to minimise size and weight.
This approach has two major disadvantages, one is the further multiplicity of joints on the steering, tending to increase stiction and free play, the other the packaging problems imposed by the clearance needed for link rods and levers in an area already filled with components and constrained in size.
001 introduced a universal joint on the top of the chassis mounted steering pivot with a steering column taking steering inputs upwards and rearwards to the hand control. This feature continued through all the Voyagers up to FJ. This is very efficient in packaging terms and allows a steering control at an ergonomically, rather than geometrically, set angle.
However the universal joint must be converted from needle rollers to close tolerance bushes if irritating free-play in the steering is to be eliminated.
In any case development has shown that a forward riding position is desirable and that this allows a direct connection to the chassis mounted pivot in most cases. The hand control can be some distance behind the steering axis to bridge moderate gaps to the riders hands, and it seems that even FJ might achieve direct connection with more sweep back on the hand control. Excessive tiller effect can cause ergonomic and packaging problems but the exact limits of this compromise have not been identified. These details can be studied by looking at the Voyager engineering drawing, side view, on my site www.hightech.clara.net
From a design point of view it might be better to move the rider further forwards, rather than replicate the UJ/steering column. The Reliant engine prevents this in the Voyager series.
Hand control support.
The final point worth looking at in the steering system is the hand control support. This structural feature will have to withstand the braking loads imposed by the riders upper body at around 1.2G, plus safety factor. It will also be used to lift the vehicle back to upright when it falls over and this usage seems to have broken more support structures than anything else.
Finally, under the most extreme circumstances, it should collapse non-injuriously if the riders upper body crashes into it. This is considered in the paper on crash performance.
Summary
Now that BMW have adopted a front suspension system that meets some useful requirements apart from traditional appearance, it is possible that similar systems may appear from other manufacturers. The Mk11 Voyager system is rather cheaper to rebuild than the GTS arm-and-wishbone but there is no doubt that it could be further improved by a full production design process. At present it represents what can be done relatively cheaply, fairly simply.
Yamaha could certainly improve it - but probably not as much as Ford!
Whatever system a designer chooses for an FF design, the front suspension is one of the two key components. The other is the seat back, a part that can be lifted straight out of a car and this is considered in the paper on ergonomics. The front suspension cannot be lifted from any existing vehicle except the GTS and unless this, or a Hossack system is used, must be created from scratch or bought new or second hand from very limited prototype supplies. None of these routes are particularly cheap and anyone seeking to become an FF builder must first define and acquire a suitable front suspension.
Royce Creasey
Jan. 2005
Copy free for credit.
Highsides and the FF layoutS
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
This information applies 'open cockpit' FFs of the type shown on my site www.hightech.clara.net and Quasars. FF's with outriggers may be a special case and this is considered speculatively in the text.
Sources.
This knowledge is based on the Quasar and Voyager production projects and personal motorcycle experience since 1961. Study of video records of other peoples highsides has also informed this study.
Highside/lowside.
First it will help to define the above terms for the purpose of this study. For STVs Highsides and lowsides are the opposite edges of the control envelope.
A lowside is a loss of control resulting from overcoming the grip afforded by the tyres even though the vehicle has correctly balanced the lateral and vertical loads generated by the cornering force and gravity. It is the normal 'failure' mode of cars, where the driver attempts a direction change at too high a speed for the conditions.
A highside is a loss of control where the balance between cornering force and gravity is not correct and the STV rolls rapidly out of the turn. This is the same effect as a car rolling over in a turn. It is important to note the vertical component in the movement of the CG of the STV in particular.
Riding a motorcycle at the limits of adhesion requires a fine balance between these two failure modes. If the performance of STV's is to be increased it is neccessary to extend the control envelope into the highside and lowside modes.
In the case of the lowside mode it is easy to show how the FF layout improves control. The chief requirement is always that the rider will not lose balance on the vehicle and the secure seat of an FF means that a lowside can be literally driven into the ground without the rider leaving the seat - or contacting the ground. This allows the same sort of control correction that would be used in a sliding (lowsiding)car, reducing tyre loading by steering and power adjustments. The low CG of an FF reduces the potential energy of the falling mass, reducing the amplitude of control corrections required. Additionally HCS front suspension allows the neccessary rapid steering corrections to be made without feedback (Tank slapping) In practice this means that even heavy FF's like FJ can be driven into lowside failure mode and then recovered. This makes them significantly easier to drive in marginal conditions (rain etc.) or at the limits of lowside adhesion when full grip is available.
The highside is more complex due to the rapidity with which this failure occurs. There have been suggestions that only high-powered vehicles can be highsided but in my experience even a bicycle can be highsided and it is a basic STV failure mode. Under normal circumstances a rapid roll out of a lean would just be part of a fast direction change, the steering would turn into the roll in the normal course of STV geometry until a new balance point was obtained. Highsides however normally start as a result of over-correcting a lowside, usually by closing the throttle.
In this case the vehicle moves very quickly from the low side mode, where the (usually) rear tyre is beginning to slide as the cornering force overwhelms it, to the point where full grip has been restored and the cornering force is suddenly fully resisted. This tends to initiate a roll out of the turn and as this raises the CG it increases tyre grip, leading to a very rapid, runaway roll out of the turn. A car driver, finding the inside wheels lifting would simply turn into the roll, reducing cornering forces. This might require a very rapid half-lock steering movmement, easy in a race car but guaranteeing a tank slapper on any 'steered suspension' system like telescopics.
An FF with HCS can achieve such rapid steering corrections but this ability is not the main advantage of Low-CG FF's in dealing with highsides.
It is important to consider what actually happens when a motorcycle highsides, and study of video records will be helpful. There are plentiful examples on the internet. It is immediately apparant that the major problem faced by riders is retaining control of the vehicle. Riders are normally thrown off their seats and footrests by the vertical acceleration generated by the high seat. Usually they are left hanging on to the steering. As a result, even where the wheels stay on the ground it is common for the rider to lose control and crash the motorcycle. The low CG of the FF layout, achieved chiefly by lowering that seat, directly reduces the vertical acceleration of both vehicle and rider. It is clear that a sufficiently low CG will reduce the vertical acceleration in a rapid roll to the point where it can be accomodated by (damped)suspension movement and that in this case the wheels will definitely stay on the ground. In addition the more secure seat makes it easier for an FF rider to retain control of the event. Again, in practice, this has meant that circumstances that would normally lead to a highside, a rear wheel slide, corrected by a closed throttle, can be 'caught' without much trouble.
This is not to say that FF's cannot be highsided - or lowsided. Most do not have a CG low enough to eliminate wheel lifting in very fast directions changes. Add high-grip race tyres and it might be possible to generate enough vertical acceleration in a highside roll to lift wheels, although the rider would still be well placed to regain control when the vehicle lands.
In early development, with the Quasar and Banana, it became clear that even extreme 'tail-out' conditions could not be converted into a highside by any normal means. On one occasion a Quasar was ridden into a tail-out slide where smoke from the sliding rear tyre was visible in peripheral vision, but recovery (getting off the rear brake)produced a smooth return to normal. This might relate to the very long wheelbases of these vehicles. This slows all vehicle effects and it may be that even a highside roll becomes slow enough to be absorbed by suspension damping. Later work, during Voyager development, however indicated that road tyres, with their gentle transition from slide to grip, are also a factor in this easy recovery from rear wheel slides. Closing the throttle on a rear wheel slide merely results in the slide reducing. There is no discernable vertical acceleration.
An additional helpful feature of FF's is the basic suspension set-up. It has been noted elsewhere that the absence of dive and squat, a feature of low CG, allows the suspension set up to be closer to full bump at ride height, rather than full droop as used on motorcycles where dive and squat must be accomodated. This means that an FF will have more suspension droop available to control vertical acceleration.
Reports from Peraves, running the Ecomobiles at Brno racetrack, and in other circumstances, appear to contradict the above positive hypothesis. Several highsides events have been reported. I have no information about CG heights in these vehicles but no reason to expect that it is at motorcycle heights. The very long wheelbase should also mitigate against highsides according to the Quasar experiences noted above. It may be significant that the outrigger wheels on the Eco's only allow 45 degrees of lean. This is rather less than the maximum lean normally achievable before low-side mode occurs (55-60 Degrees) This implies that the vehicle may be turned into a corner at a speed requiring more lean than available and well above low side angles. In this case a highside might be inevitable, in the same way as a car will overturn in a corner if the CG is too high and the grip sufficient. This is merely a hypothesis, but there is clearly an anomally between Quasar/Voyager/etc. and Ecomobile results. No doubt there wil be plenty of alternative theories to explain this...
In Summary, the following features have been shown to eliminate, or reduce the effects of, the highside failure mode.
Low CG, Especially low rider CG
Secure seating
Steering capable of very rapid corrections, i.e. with very low rotational inertia, e.g. HCS generally.
Suspension set closer to full bump rather than full droop.
Long wheelbase
Road tyres
Royce Creasey
2007
Copy free for credit
Please find attached a written article covering some of the less favourable aspects of low CG design. The article is in part written as a reply to the theories put forward by Royce Creasey in 'Feet First - The Physics'.
Acceleration
The average sports-tourer has nowhere near the amount of power to make wheelies a serious disadvantage. It is only over a very narrow rev range, between 500 - 1000rpm, at peak power and in first gear that such bikes are able to lift the front wheel from the ground (not the same as continuing a wheelie).
This = just a split second. Provided that a modicum of skill and throttle restraint is applied in first gear, the front wheel can easily be kept on the floor without serious loss of acceleration, and who needs to accelerate THAT hard anyway? It is wildly unlikely that a ‘looping’ situation would occur without the direct intention of a wheelie.
In the article, little account is made of weight transfer rates and their effect on grip. The statement about 85% loading and 15% slip giving maximum grip is clearly an error – as already pointed out. Also, only a single situation is considered, where the available grip is very high – the maximum possible – giving 1.5G.
On the road, particularly in the UK, a traction coefficient in the range of 1.5μ is not always available – a wet road is more likely to provide just 0.5 μ, same for gravel, tar strips, drain covers etc. The stuff road riders deal with everyday.
If the transfer rates are worked out for two bikes with wheelbases of 1470, and CG heights of 400 (as proposed by Royce) and a more typical 650, with the CGs being central between the wheelbase, we will see the limitations of low CG.
If the 400 CG bike has a weight transfer rate (k) of 400/735 = 0.54, the downward load on the rear tyre (L) is as follows:
L = mg - (mg – mak)/2
For 1G acceleration (a), the load would be 0.77mg
This means the minimum traction coefficient must be 1/0.77 = 1.3μ
If the available traction is anything less than 1.3μ, the result will be wheelspin.
The 650 CG bike transfer rate is 0.88. For the same 1G acceleration, the rear tyre load will be 0.94mg, and so any surface above 1.06μ will give grip.
The high CG bike will start to wheelie when acceleration = 1.14G, requiring traction of 1.14μ (100% transfer)
This means that the high CG bike will hold the advantage on all surfaces below 1.14μ – which, in my opinion is the more prevalent, and critical level for road riding.
Cornering
Where a low CG will be a real disadvantage, even in high grip circumstances, is in cornering.
When a bike is leant over in a bend, the centrifugal forces are using up a portion of the tyre’s available grip. There is less grip available for acceleration, and this means that a lack of weight transfer will mean that significantly less acceleration can be applied in cornering before rear spin / slide will occur.
A high-side accident is highly complex, but the start of a high-side always happens as a result of a severe rear slide / yaw. The fact that the front has less transfer unloading with a low CG, and therefore is less likely to drift in combination with the rear, means that the easily sliding rear will create exaggerated yaw (for the same wheelbase).
The fact that rear slides are more easily provoked and yaw is more pronounced means a high-side may be more likely to happen with a low CG. Arnold Wagner seems to agree with this.
The final disadvantage of a low CG for cornering is the fact that the bike will have to lean further into each bend. This is due to the way a bike rolls on its tyres. On typical tyres, a bike with a 400mm CG will have to lean 3 – 4 degrees more than a 650mm CG bike. Since ground clearance is the limiting factor for cornering speed on road bikes (at least all the road bikes I’ve ever ridden) it’s not unfair to assume that ground clearance on a low CG bike will also be a critical issue.
Deceleration
The case of rear wheel lifting under braking is also not necessarily the greatest problem for traditional CG bikes. To lift the rear wheel of the average sports touring motorcycle under high speed controlled braking is actually very hard to do. My figures suggest that -1.23G is achievable for our high CG bike allowing for some suspension compression. It is my direct experience that, even on a good, dry surface, a front lock-up is the likely ultimate result of heavy controlled braking at speed, on a sports-touring type machine. If you have ever lifted a rear wheel under braking, you will also know that there is no immediate loss of control, and it is usually when the wheel bumps back down that the lifting is noticed.
The braking performance of two tyres more evenly loaded, rather than 100% weight shift onto one is theoretically better due to a larger total contact patch. Exactly how much better has not been stated by promoters of this approach.
Also, in wet conditions, the higher contact load/mm2 (smaller contact patch) may be preferable. I’ve had difficulty finding precise data on these factors (and I’ve not seen it anywhere in defence of low CG) and suggest that it will be specific tyre sensitive. I hope to do some practical experiments soon.
The biggest problem with two-tyre braking is the fact that it will be very difficult, well-nigh impossible for the rider to precisely control both tyres to a level that he could control one. Holding the front tyre on the very point of locking requires extreme skill; doing the same thing, at the same time with the rear also is just not viable.
Don’t agree
This means that a linked system is often used. The balancing of this system is critical and requires different settings for different surface grip. Although riders are able to set the balance, the settings cannot be changed as fast as road surfaces change.
If the increased contact area gives a 5% advantage, but the balance is just 5% out, you have no advantage.
A dry surface providing 1.2μ for our low CG bike would require a balance of 82% / 18%, where a wet surface of 0.5μ would require 63% / 37%.
On a wet road with the dry setting you could only apply 76% of your potential braking power to the front, leaving only 13.7% at the rear (23% down on total braking power) – otherwise the front will lock.
The ultimate solution is ABS systems to control front and back. These are still not as efficient as a skilled rider, but getting pretty close. Don’t forget that it is more weight and cost though.
On the whole, balanced loading theory is good, but in practice, concentrating effort on the front tyre is hard to beat, all conditions considered. If you have a low CG without ABS, or without the balance being precisely set for the precise grip available, chances are you’d be better with a higher CG.
Practical use.
Toppling over at low speed or standstill is always a risk in the operation of motorcycles. This is an inescapable fact.
On a high CG bike the rider has the best possible stance to hold the bike secure. The seat is high, so his legs are straight, his arms are able to directly pull at the bars, and his bodyweight is separated from the bike.
On a low CG bike, typically the rider has to be seated. This is a very poor stance in holding the bike secure. The legs are bent at around 90 degrees, meaning that the thigh muscles take all the strain. Arms are unable to pull upward on the bars, and the bodyweight remains a part of the mass to be supported.
An easy experiment demonstrates this, please try it. Sit on a chair and put a bathroom scale under one foot. Press as hard on it as you can - 45kg is about the maximum you will achieve.
Sit on a motorcycle; place the scale under one foot. Lean the bike over as far as you can (without lifting your backside off the seat) – 100kg is easily achieved. With standing – 150kg or more is achieved (my scales wouldn’t go any further).
A rider on a high seat bike is easily able to provide 2 – 3 times the stabilising force that a low CG leg supported rider can. Even though the toppling force may be lower due to the reduced CG, it will have to be 2 – 3 times lower without any additional weight to equal a high seat bike. This is a considerable safety and practicality problem for low CG bikes. Solutions to this problem (stabilisers) are heavy and expensive.
Conclusions.
The initial impression of a low CG as put forward by Royce Creasey are very good, but further analysis shows that the advantages are not always real or achievable in practice.
Once realistic, varying and wet surfaces are considered, many of the advantages of low CG dry up.
If substantial added weight is a result of trying to get a lower CG (as it can be, and often is), it is likely that any of the proposed gains will be cancelled out and that the extra weight will have a very negative effect in other areas. A lower CG could be used to offset excess weight, like in the case of the Voyager, which has impressively low roll inertia for its large weight, but was trying to get a low CG largely responsible for the excess weight in the first place?
The above theories are limited to static analysis, and may be simplified to a point. They are intended to provide balance to the CG height debate, rather than a definitive piece of work.
CG height is just one aspect of PTW design, and I believe it is an area where on its own, little or no improvement will be made for practical, road going vehicles. Increased safety …….. Increased comfort …… Improved economy….. Improved practicality…… This is where the user focussed advancement will be made, and CG height is not central to any of these. Improved safety is certainly no.1 on the Government’s agenda. Maybe we should find the compromises WE want to make before the Government decide for us……. but that’s another debate, and these are just my opinions.
Who am I?
I’m a motorcyclist, rarely driving a car – because two wheel travel is more fun. I’ve ridden all sorts of bikes in all sorts of places over the last 22 years. I’m a person who wants to see the open minded development of two wheel road transportation. There are obvious problems (much more real and solvable than CG height) – let’s see what we can fix.
I am an apprentice trained mechanical engineer with ten years’ experience and have recently achieved a first class honours degree studying Industrial Design (BSc). My final year project was a ‘feet first’ motorcycle which I continue to develop.
I welcome any informed comment to the theories shown here.
Performance Advantages of FF Single Track vehicles
Definition; “FF”
A single track vehicle with a seat base less than 20” above ground level at ride height, fitted with a seat back capable of supporting the rider. The front suspension should not be steered.
Application.
The following applies to all FFs and is a comparison with motorcycles.
Sources.
This was developed to explain the performance of the Quasar in 1978, and elaborated after testing and development of the Banana in 1981. It closely resembles the “Technical Advantages” chapter in the FF Information Pack that I published in 1986. The major advantages can be confirmed using basic geometry (A compass, a ruler) and all are supported by experience with a number of subsequent FF designs.
Familiarity with Newtons three laws of motion is necessary.
The major advantages are as follows;-
1. The FF layout substantially lowers the Centre of Gravity (CG) of the vehicle and it's rider/s.
This eliminates excessive or intrusive weight transfer during braking and acceleration. Weight transfer is proportional to the height of the CG.
It allows direction changes to be made faster and with less effort - The CG moves through an arc as the bike banks from side to side. The higher the CG the longer the arc and the greater the effort required to move the CG through the arc in a given time.
It reduces the unloading of wheels during fast direction changes - the arc has a vertical component, rising and falling as the bike banks. The higher the CG the greater the vertical movement. If the CG is high, enough upward momentum may be given to the CG by a fast direction change to lift the rider out of the seat and the wheels off the ground.
It permits better suspension and less ground clearance, due to the lack of excessive weight transfer.
2. The FF layout provides the rider/s with a secure and comfortable seat.
This prevents entirely accidents where the rider falls off a motorcycle.
It provides a secure platform for rapid and precise control corrections - The riders hands do not support the rider who can thus operate the controls without fear of losing balance.
Comfort is sufficient to eliminate fatigue and cockpit heating is easy to arrange.