Highsides and the FF layout

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.

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.

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.

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
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