Given your background in vibration analysis, did you ever consider the use of carbon fibre for the Yamaha chassis or swingarm?
Some of the engineers thought about using carbon fibre, but for me no. Carbon fibre is very good for keeping rigidity and it is very lightweight. But for a motorcycle I don't think it is so good.
When you lean over 45 degrees there is almost no suspension, so you need some flexibility in the frame. And with carbon fibre it is really hard to control stiffness. For carbon fibre the stiffer it is, the better, which is why it is perfect for a Formula One chassis.
So how do you find the right balance between enough chassis flex and too much flex?
We do analysis, simulation and also experimental tests to acquire real data, which we feed into the simulation software to help improve the accuracy. But it can be difficult to get right.
This is one of the keys to a good chassis: You must have smooth stiffness changes from the steering head to the rear.
In 2006 we made a mistake with the chassis. We designed a chassis that was much more flexible on the lateral side, but to do that we needed to make some parts with a thin cross-section [so that they would flex more] which can be pretty hard.
So instead of decreasing the stiffness of these parts, we reduced the thickness of the frame in the middle. Point-to-point the overall stiffness was the same, but now the distribution of the stiffness was not 'smooth' and not changing in a consistent way along the bike.
That makes for a pretty bad chatter problem. Chatter is not a linear problem. It is self-excited vibrations. Once it starts, more and more vibrations happen and finally you have a big problem.
One of the solutions is to keep continuous stiffness changes from the head to the rear. Even then you can still get some chatter, but not so much.
So, in the future, what we really need is a damping force for the chassis, but so far that is not so successful. Sometimes we tried a performance damper like on my Miata. Sometimes it works, sometimes not.
This chatter phenomenon is probably the last remaining problem to be cracked by motorcycle engineers.
End of interview