TO FLEX OR NOT TO FLEX
Motorcycle dynamics can be a complex conundrum. Balancing the design elements that make up a bike’s system for optimum performance is a never ending challenge. That system includes an element that plays a major role in the overall performance of the machine, the rider. Bikes are more of a holistic package compared to cars, where all the elements must work well together for optimum performance.
The perfect example of this conundrum has been demonstrated by the mega experienced brains trust of Preziosi, Burgess and Rossi currently struggling to cure the Doctors front end woes. One area on which the Ducati boffins are working hard is lateral flex. But just what is this lateral flex, what does it mean?
Well lateral means sideways and flex is bending, so we could assume that the boys are working on getting the GP12 chassis bending sideways? It seems pretty crazy that you would want your bike to bend sideways? From the beginning of time, or motorcycle time anyway, chassis manufacturers have been trying to reduce chassis flex and increase stiffness (stiffness being the chassis resistance to deform under load) in an effort to keep the wheels in line and improve handling.
Uncontrolled frame flex whether it be lateral, torsional (twisting) or vertical can be thought of as the bike behaving in an elastic state. It has a clear influence on the bikes handling, feeling slow and heavy due to the dynamic geometry changes taking place with trail, wheel alignment, etc. In the good (or bad?) old days, Norton’s 1950’s chassis was so poor it was nicknamed the ‘Garden gate’. Belfast Engineer, Rex McCandless then revolutionised motorcycle chassis with his ‘Featherbed’ design!
However fast forward to 2012 again, where Engineers are finding that increased stiffness can amplify vibration and chatter generated from acceleration, braking and cornering forces. At today’s 60 degree lean angles, suspension effectiveness is much reduced when the bike is on its side and this can be a big problem for MotoGP racers. Did anyone see Ben Spies chatter induced seat vibrations at Qatar!
Another problem with too much un-damped chassis stiffness is lack of feel. The Doctor complains that his Ducati front end feels vague and isn’t providing sufficient feedback. This leaves the rider element with a lack of confidence.
Also worth mentioning is the Engineers rule of thumb, in any design, and that is to change stiffness gradually. You will often find a component fails at the point where there is a sudden change of stiffness.
To combat these problems, the MotoGP boffins have been engineering tuneable flexibility into elements of the chassis while retaining the stiffness in the planes where it is needed. Check out the GP12 frame rails around the swingarm pivot and fork clamp below, allowing an element of lateral flex to absorb the bumps whilst retaining stiffness in torsion (twisting) and the vertical plane to control braking and acceleration forces.
The latest GP12 aluminium chassis elements are much longer than the previous GP11 carbon fibre chassis, with the frame rails running from the headstock to the swingarm pivot. These new longer chassis elements or levers, allow flex to be designed in more gradually and effectively. The previous GP11 carbon fibre chassis was basically a sub-frame connecting the very stiff 250hp motor to the headstock, while the swing arm hung off the rear of the engine. So you effectively had two short laterally flexible elements, the subframe and swingarm, attached to either end of a very stiff element, the engine.