Back to the Future?

Is this yet another example of a Japanese manufacturer copying Harley Davidson, or is this the cutting edge of chassis technology?Yamaha recently unveiled its latest prototype version of its MotoGP M1, whose most immediately prominent feature is something that has not been seen on world-class race bikes for several decades – twin rear shocks. What are they thinking?

For many years, the mantra of high-performance chassis design, whether on 2 wheels, 3 wheels, or 4, has been much the same as in the bedroom – “stiffer is better.”For those of us old enough to remember the flexible frames of sportbikes as recently as the ‘80s, this concept is about as self-evident as gravity.However, as tire grip and chassis design advance to present levels, this concept is approaching, if not exceeding, the limits of usefulness. If you picture, for example, Eric Bostrom leaned over so far that his elbow regularly grazes the pavement, and visualize the suspension movement as he encounters a ripple in the asphalt, it is clear that the suspension is relatively useless in coping with that bump. At this time, suspension travel is more nearly horizontal than vertical.Honda first publicly addressed with this problem with the 900RR which was specifically designed to have some controlled flex in the area of the steering head. How successful this was is open to some debate, as some found this bike to be subject to instability under certain circumstances. However, many attribute this to the choice of a 16” front wheel and/or insufficient trail, rather than any fundamental problem with the controlled flex concept.It is generally believed that the concept of controlled chassis flex is a key design element in the Honda RCV, and in the updated versions of the Yamaha M1 introduced mid-season last year. This is also believed to be one of the key changes in the Aprilia Cube from last year.One tricky part of this approach is to get flex in the areas and direction you want, while maintaining rigidity in all other directions. This can be seen in the construction of swingarms of the newest racebikes -- eg Aprilia Cube The structure of the arms is visibly very deep vertically and relatively thin horizontally, giving great torsional rigidity while allowing some horizontal deflection.If you again picture EBoz dragging his elbow while the rear tire hits a ripple, you can envision the swingarm deflecting, while still maintaining proper alignment.This however does nothing for the front wheel. You do not want the fork itself to be flexible, because it is very hard to have it flex laterally but not fore and aft or torsionally. Thus, the flex must come from the frame itself.A tricky issue with using frame flex as a suspension element is that the frame is essentially an undamped spring. This is precisely the characteristic of the frame on my old late-‘70s XT500 roadracer that conspired with the race compound tires to throw me off at every opportunity. The thought of combining 220 hp, modern slicks and rubbery frame elements is too frightening to contemplate!The new Yamaha M1 prototype appears to be an experimental attempt to deal with this. Note that there are what appear to be damping rods extending from near the shock mounts to somewhere near the steering head. Click Here and Here .According to Yamaha, the twin shock layout is a concept being developed jointly with their subsidiary Öhlins and is intended to better permit controlled chassis flex. For a better explanation of how (or if) this works, we will have to wait for an evaluation by someone with more expertise than I have (eg Kevin Cameron).Meanwhile, here is a link to the Yamaha Racing site as well as a couple of others that discuss it Motorcycle-USA, Crash.net and Dork Edge
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George Obradovich
George Obradovich

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