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.