RaerDesign

Suspension Technology. (Motorcycle.)

In recent years tire technology has advanced to give incredible levels of grip, which translated into lean angles that were not thought possible a few years ago, with MotoGP racers reporting lean angles of as much as 60 degrees from vertical. It is this advance which is bringing the telescopic fork to the end of its development life. Electronic active suspension, under development today, will only increase telescopic fork lifecycle for a few more years – at great expense to the rider – because it cannot address the fundamental problems. This is because active suspension operating within telescopic fork tubes can only change the compression and rebound dampening ‘on-the-move’ but not the spring rate which needs to be reduced during cornering to provide lighter suspension action thereby reduce the risk of cornering ‘chatter’.
To explain…

In ordinary travelling, telescopic forks need to be angled backwards to function properly, particularly so during braking. Likewise, they simply cannot cope with the extreme lean angles of today’s motorcycles during cornering where they have to slide at angles unsuited to their nature. Lateral flex has been designed in to the forks (more accurately into the yokes which hold the fork legs) to give a degree of side flex to assist the forks in their action and to provide an amount of suspension motion of its own. This side flex, or lateral flex, further exacerbates the problem as telescopic forks act like a set of giant tuning forks (as do modern twin spar frames and swinging arms) where sideways displacement will have them rebound back further than where they were first displaced from, this being the longitudinal centre-line of the motorcycle. A method of dampening out this oscillation is required before it builds into a harmonic sequence – this is what has became known as cornering chatter. The simple answer, not currently adopted by motorcycle manufacturers, is to have shorter suspension members i.e. shorten the overall length of the telescopic forks. For such, suspension travel has to be restricted (difficult to achieve given the fact telescopic forks dive so much during braking) plus the bottom yoke has to extend down the fork legs – a practice that has been adopted but only for increased longitudinal stiffness and not for what it should be utilized for. This raises the natural frequency of the members at which they will resonate. Again such practices will only extend telescopic fork life for a few more years before they become obsolete.

Linkage front suspension can have the benefit of shorter structural members and of additional members interacting with these short suspension members where such can be at a different frequency thereby dampen out oscillations present. With which longitudinal stiffness can be retained in the motorcycle to the benefit of braking performance and a higher natural frequency can be attained with a high degree of lateral flex to the benefit of cornering suspension action.

Directly connected suspension has the benefit of altering the spring rate automatically where displacement of the outer ends of the shock absorbers can orientate the shocks to lighter geometric rates. This is the case during cornering as centrifugal forces compress the suspension systems, such can be utilized to provide lighter rates by simply displacing the shocks to more acute angles. This also has the beneficial effect of providing lighter rebound and compression dampening. To date, directly connected suspension is the only way of lightening spring rates (and dampening settings) on a motorcycle during cornering, where it does it automatically without any mechanical or rider input. Other considerations have been factors preventing active suspension achieving this such as weight complexity and vulnerability, making fully electronic active suspension impossible on motorcycles at present.