In the current issue of Inside Motorcycles, I wrote about the 2012 MotoGP bikes and touched on chatter and chassis flex. I addressed chatter in my previous blog, and will go into more detail about chassis flex here. Chassis designers have long known that some flex is a good thing as it acts as the motorcycle's suspension at extreme lean angles. With tires getting grippier and wider over time, motorcycles are getting increasingly more lean angles and the importance of chassis flex increases accordingly.
Incorporating flex into a chassis is not easy, as strength and stiffness are still desired in some directions while flex is advantageous only in certain directions. Typically, manufacturers express chassis rigidity in three ways: vertical, lateral and torsional. The chassis is generally desired to be stiff vertically, to absorb braking and acceleration forces, while some flex is desirable in the lateral direction to act as suspension. Torsional stiffness keeps the wheels in line, but some twisting can also translate to lateral movement at the contact patch. Yamaha published a graph showing the M1's changing chassis rigidity over the past few years, with vertical stiffness increasing and lateral stiffness decreasing - just what you would expect as tires get stickier and the bikes lean further.
Changing flex can be accomplished in subtle ways. For example, the top triple clamp of the GP12 Ducati Desmosedici shown here has slots that allow the forks to move from side to side. The Yamaha M1's top clamp is similar, and the Suter BMW takes this to the extreme with just a small amount of material connecting each fork tube to the steering stem. Teams constantly experiment with fork tube inner and outer diameter to find just the right combination of stiffness under braking and flex while cornering.
In the frame itself, there are almost infinitely more variables. The height and width of the spars or individual tubes, and the material and its thickness, help to determine how much the frame flexes in each direction. The engine mounting design determines how much the engine itself contributes to stiffness in each direction. And even how the individual components are welded together can make a difference. Most teams machine each part of the frame - even the spars - from solid pieces of aluminum; four or five machined pieces welded together requires less welding and gives more consistency than a collection of extrusions or stampings.
At the back of the bike, the swingarm is usually constructed in a similar manner - individual components machined from billet aluminum and welded together - and it's here that some development has been concentrated over the past couple of years. Bracing has moved from the top of the swingarm to the bottom on most factory MotoGP bikes and World Superbike machinery, which reduces lateral movement at the tire's contact patch when the swingarm twists, improving feel and feedback. This is just one example of how lateral and torsional stiffness are related.
In the end, it all comes down to what the rider desires for that feel and feedback. Repsol Honda riders Casey Stoner and Dani Pedrosa each have frames custom-built to their requirements. The Ducati team has used carbon fibre swingarms on the Desmosedici since 2009, but recently tested an aluminum version with mixed results - riders Valentino Rossi and Nicky Hayden are divided on their preference. The finished chassis is as much a function of the rider's input as the engineer's computer, especially when it comes to flex and rigidity.