If you've watched any MotoGP races this year, or read any reports, you'll know that many riders and teams are struggling with chatter. Repsol Honda riders Casey Stoner and Dani Pedrosa are the most vocal, and it's enough of an issue for them that Yamaha rider Jorge Lorenzo leads the points after five rounds so far this year even though Stoner was consistently quicker in testing.
The term chatter has its roots in machining, and refers to the resonant vibration that occurs when a machine tool, such as a milling cutter or drill bit, vibrates against the work piece, leaving a rough or wavy finish. Running your nails over a chalkboard and making that awful screeching noise is an example of high-frequency chatter. Or, if you push your hand across a table at just the right angle and force, it will chatter at a lower frequency. Under certain circumstances, a motorcycle's unsprung masses - the wheels and tires - will vibrate on the pavement at a resonant frequency in the range of 17 and 22 Hz (cycles per second). According to the book Motorcycle Dynamics, by Vittore Cossalter, chatter is often observed in circumstances where the resonant frequency matches that of the wheel rotation frequency. That is between 125 kph and 160 kph depending on tire circumference and the chatter frequency, and European tracks are filled with corners taken at those speeds.
Chatter is not a new and mysterious issue - I remember my brother struggling with it on his Yamaha TZ250 at Shannonville 30 year ago. It does, however, seem to come and go as tire, suspension and frame technologies continually leapfrog each other but occasionally clash. Valentino Rossi and Colin Edwards suffered from severe chatter on the Yamaha M1 in 2006, and World Superbike riders have complained of chatter occasionally. Generally, it cannot be eradicated with the standard setup adjustments available even on a MotoGP bike as the suspension simply cannot absorb vibrations at that frequency. One way around chatter is to use a different tire that can better absorb vibrations at the resonant frequency, or has a different resonant frequency altogether; with a spec tire as used in MotoGP however, that is not an option. Engineers are forced to turn to the chassis, and either change the resonant frequencies of various parts or make them more flexible to absorb the vibrations elsewhere.
Yamaha attempted a temporary fix for the M1's chatter early in 2006 by adding weight to the axles, but ended up reverting to the 2005 chassis until the new bike could be updated with more elaborate modifications. The factory Repsol Honda team is working through a similar process. Following the recent official Catalunya test, Casey Stoner stated in the team press release that, "We didn't work on set up at all, we just tried everything we could think of to reduce the rear chatter." Showing how fickle chatter can be, however, he did say to Motomatters.com that progress was made by changing a small two-dollar part that they did not expect would make a difference. Certainly the various teams will find their individual solutions before too long, and then the current obsession with chatter will likely fade - until the next time the relative performances of tire, suspension and frames and their resonant frequencies collide.