Dynamic Consequences of FF layout

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.