'CHATTER CAN MAKE THE TYRES JUMP UP AND DOWN SIX MILLIMETRES'

Chatter is MotoGP's C word and is playing a big part in this year's MotoGP championship, helping Ducati dominate and causing KTM big headache. So, what is it and how do engineers fix it?

- Mat Oxley

CHATTER COMES AND goes in motorcycle racing, like headlice in the school playground. It is a technical disease, an engineering infestation that plays havoc with lap-times, can give riders blurred vision, and send engineers a bit crazy.

Some riders prefer to call it vibration, but, either way, it is a tiny thing: a few millimetres of high-frequency resonance between the tyres and the racetrack that can have huge consequences.

Chatter helped Valentino Rossi win the 2004 MotoGP title, his first with Yamaha, because Michelin's super-grippy 2004 rear slick caused Honda's RC211V to chatter like crazy.

"The new Michelin doesn't work so well with our bike; we're getting a lot of chatter, which we call "the jackhammer" in the US," said Honda's Colin Edwards at the time. Having chatter is like having syphilis; it's all you think about all day!'

And chatter played a part in Rossi losing the 2006 MotoGP crown, because that year's YZR-M1 chassis was too stiff.

'We were getting desperate, so we drilled holes in the frame and filled it full of expanding foam, the same stuff you use under the kitchen sink to stop rats getting in,' recalls Rossi's mechanic, Alex Briggs.

Nothing worked, so at Le Mans [one-third distance in the championship] we went back to the 2005 chassis. It was all done in great secrecy people were sent around Europe to collect 2005 bikes which were being used as show bikes.'

Chatter is all about harmonic resonances, frequencies, and oscillations-and it is a huge headache for riders and engineers because it can be so difficult to find the cause and so difficult to fix.