RaerDesign

Motorcycle Chassis

Gyroscopic precession is now recognized as the single most important factor regarding motorcycle handling. Even today, most frame builders do not fully understand the dynamics involved.

(The engineers at Honda were a long time before they realized the high-positioned heavy fast rotating crankshaft in their RC45 Superbike was detrimentally affecting cornering potential. Chassis tuners, instead of addressing the problem, worked on the ride height by altering the rear suspension linkages.)

The gyroscopic principles of the revolving wheel (and all other rotating shafts) being turned to the left yet tilting to the right, and vice versa; or, by tilting it to one side or the other and having the wheel turn to that side, are well known. But a far more critical aspect of these principles remains undiscovered. This is, the rear wheel/tyre is used to dampen out oscillations from the front as the rider unintentionally (subconsciously) maintains his balance (by means of gyroscopic precession) at speed.

From the steering head an imaginary line can be drawn down through, or very close to, the centre of the front wheel, on towards the ground in front of the contact patch of the front tyre. This imaginary line is the steering head angle or rake. The distance it protrudes in front of the centre of the contact patch on the ground is the measured trail, where any deflection of the front wheel will have it rotate forward in an arc towards this imaginary line. Nothing very startling here, but as demonstrated in fig. 1. a second imaginary line can be drawn out from the centre of the front wheel at a right angle to the steering head angle backwards towards the rear of the motorcycle. Again, gyroscopic precession operates at ninety degrees hence this definite second line of axis. At the contact patch of the rear tyre, as demonstrated in the drawing, the imaginary line is below the ground. It is imperative that this is so, with the forces rotating around this line forcing the tyre into the ground. Otherwise, with the imaginary line above the rear tyre’s contact patch the gyroscopic forces from the front wheel can become extreme enough as to physically lift the rear wheel off the ground, with disastrous consequences.

In fig. 2., with telescopic forks, when they are fully compressed the trail and steering head angle decrease dramatically but the head angle has to be sufficient so that the imaginary second line from the gyroscopic forces of the front wheel remains theoretically below the ground at the contact patch of the rear tyre. If this is not accomplished it can result in instability, even during braking when weight transfer has made the rear wheel light.

In fig. 3., the final variation in this is to consider flex in the physical structure, therefore a slight amount of tolerance has to be accommodated.

From consideration of these unknown aspects of gyroscopic precession the drawbacks of the telescopic fork can be made more apparent. Telescopic forks operate best when they are splayed out at an angle of 30-35 degrees to absorb the incoming undulations of the road.

The greatest force placed on them is under braking, where because they are held at the top but subjected to pressure at the bottom there is a natural inclination for them to bend backwards towards the engine. This is again alleviated by a splayed out steering head angle. Only, with this new theory in dynamics, the imaginary second line of axis is best placed as close to, but most definitely under, the rear tyre contact patch as possible to provide a light steering yet extremely stable motorcycle. A steering head angle of less than 20 degrees is most ideal on sports motorcycles for this reason.

Without ever knowing why, this was found to be the case with hub-centre steered motorcycles that had rake figures of degrees in the teens. These bikes could run with greater stability than their telescopic forked rivals but not have a steering damper fitted! A long wheelbase is obviously a benefit to stability but also a small diameter front wheel can produce a stable motorcycle, which at face value would appear to be wrong. Both, in fact, allow a steeper head angle with the second line of axis remaining below the contact patch of the rear tyre.

Linkage front suspensions have the possible benefit of increasing the rake as the suspension compresses unlike telescopic forks that do the reverse. Once again this can be used to have a steeper head angle.
Hub-centre steered motorcycles are not without their drawbacks – mainly a lack of feel from the front tyre. Telescopic forks have the most direct connection between the rider’s hands and the front wheel, likewise the greatest amount of feel.

The French oil giant Elf did a lot of experimentation with linkage front suspensions in MotoGP but was unable to capitalize on the said advantages. What their racing motorcycles suffered from was excessive pitch because the front pivot point and the rear swing-arm pivot point were too close together – only separated by the length of the engine. Whereas a telescopic forked bike has a long frame to separate the axis points allowing less pitch with the chassis. The forks can also be considered rigid, theoretically further extending the distance. Such rocking motion of the chassis encourages the tyres to break contact with the road when cornering on an uneven surface.

It is interesting to note, BMW are presently changing their motorcycle image to that of a sports bike manufacturer and are now using a linkage front suspension that has very short double wishbones. All the said advantages could come to fruition without the disadvantage Elf experienced of excessive pitch. BMW’s design has a reasonable distance between the front and rear pivot points by simply reducing the length of the front wishbones. A lack of feel from the front tyre will be more than that of a telescopic fork but less than complicated hub-centre designs.