Jerez Questions: Chatter, Chatter, Chatter

I understand the feelings of the fans of Jorge Lorenzo and the Yamaha YZR-M1 he rides, but the fact is that Casey Stoner and Honda cleared off at the start of this season’s first round at the Losail International Circuit in Qatar. Just as Stoner showed last year, his combination is strong.

“When I got to the front,” said Stoner, “I did one lap really hard and noticed I dropped everyone immediately and figured they weren’t able to run that pace.”

Although Honda MotoGP boss Shuhei Nakamoto said earlier that the Yamaha, with its remarkable stability and direction-changing ability, is the best bike, we can all see that the Honda has a performance edge, enabling Stoner’s title in 2011.

Then, Stoner was hit with arm pump—a self-intensifying condition in which muscular effort restricts blood circulation, leading to cramp and further compression of blood vessels. The affected arm, the right one, turned into a useless claw.

“Eight or nine laps in,” he said, “I could barely hold on.”

Stoner’s crew had worked hard to abate the chatter they’d had through preseason testing in Sepang, Malaysia, and in Qatar practice.

“In the first half of the race,” said Stoner, “we didn’t have chatter.”

Stoner could hardly keep the throttle open and scared himself badly by being unable to grip the bar. “I was pathetic on the brakes,” he said.

The Repsol rider mentioned that he had used new gloves in the race, which he called “a bit tight.” This made it harder to make a fist. “I had to put more effort in to grip the bike, and that puts more pressure on your arm .”

Despite all his improvisations, Stoner’s pace slowed five laps from the end, allowing Lorenzo and his Honda teammate, Dani Pedrosa, past to finish one-two. Stoner specifically denied that the arm pump and chatter were cause and effect. He has had chatter since pre-season testing began but not arm pump, until now.

If Honda has not overcome this problem by Jerez, results could repeat those at Losail.

Although the Yamahas of Lorenzo and American Ben Spies have been much less affected by chatter, Spies’ bike chattered so severely at Qatar that he finished 11th, only 1.1 seconds ahead of Colin Edwards’ CRT bike. Spies felt the sudden onset of “the disease” might have been caused by damage to his bike in his two crashes in practice.

Edwards, this year riding a BMW-powered Suter, said, “Now, I have the worst chatter I have ever had in my life! We tried more variations than we ever did at Yamaha but nothing made a difference.”

Three decades ago, American great Freddie Spencer encountered chatter for the first time on Erv Kanemoto’s Yamaha TZ250. The harder he tried to “ride through it,” the more violent became the rapid bouncing of the front tire, and the faster the bike went off line to the outside of the racetrack.

Chatter seems to be an interaction between rapid grip/release cycling of the tire’s footprint and the vibrational frequency of some flexible part or parts of the motorcycle. For example, if a bike is placed on a two-post shaker (front and rear tires supported by pistons that can oscillate up and down at various frequencies) and a vertical frequency sweep is made, the front wheel, vibrating forward and backward on the springy fork tubes, becomes a blur of motion at around 22 cycles per second. The wheel, too, is flexible, as is the steering head of the chassis.

Long-serving rider Mick Grant (now retired) once said, “Chatter occurs primarily under conditions of good grip and heavy load.” What this probably means is that large forces help to flex chassis parts in step with the tire’s grip/release cycle rate.

Kenny Roberts regarded chatter as a major obstacle to good chassis performance and developed riding-style alterations in an effort not to excite chatter. Rather than trail-brake deep into a corner, placing heavy weight-transfer load on the front tire at the same time as it was required to generate turning force, he would do most of his braking with the bike upright, then flick the machine into the corner and turn it without weight-transfer load. You can be sure that Stoner is also adept at such style changes.

How do teams deal with chatter? There is a “chatter-relief cookbook” that has accumulated over the years. It includes things like varying front-tire stiffness by altering tire pressure, changing the vibrating mass of the front wheel with a second brake disc or a lead-filled axle, and making sure that gas compression inside the fork tubes is not making front-end action overly stiff. Edwards is on his third variant chassis from Suter, which probably means that lateral stiffness or steering-head twisting stiffness has been changed to move their vibration to a frequency not in step with tire grip/release. Yamaha even tried Formula One-style tuned-mass dampers some years ago, but the chatter just moved to a different frequency.

And why chatter now, when Stoner seemingly waltzed away from competitors in 2011? Edwards points to this year’s altered tires, which have been given softer construction and rubber that comes more quickly to operating temperature.

“In my experience,” he said, “we get chatter every time we go to a rear tire with a bigger contact patch.”

In 2006, in response to Honda’s request for increased traction, Michelin developed a larger, lower-pressure rear tire. In testing, Honda initially had heavy chatter but was able to overcome it. Untroubled through preseason testing, Yamaha suddenly had bad chatter—enough to stop Valentino Rossi’s drive for points cold.

Edwards blamed the current chatter on the new, larger-and-softer rear Bridgestone. He also said, “This new rear tire is a lot better, especially for traction and safety in the first few laps.”

Like a soft qualifying tire, but to a lesser degree, the new rear comes to operating temperature more quickly after the start and then loses grip later in the race. This is something new when you remember that in the past three years, riders have often set fast lap in the closing stages of races.

Edwards continued, “But we have changed the rear , and we haven’t changed the front.”

What might this mean? In cornering, teams seek to balance front and rear grip. But when rear grip increases, the rider must increase steering effort at the front or the bike will run wide. So, a bigger rear puts a heavier cornering burden on the front tire.

Some think the Hondas are chattering because their chassis were optimized for the 2011 tires, but Honda is not a babe in the woods. If necessary, it will prepare new chassis or take other measures.

Some riders have sensibly pointed out that a spec tire requires all bikes to be adapted to it, while in a series with tire competition, each tire maker looks after its contracted riders—tires are adapted to bikes. Tires, which are now made on automated machines, can be rapidly altered, as Michelin did in the days when Sunday’s tires were made Saturday night and transported to the circuits. Chassis take weeks to produce, weeks during which positions can be lost in the points race.

Bridgestone will provide a new front tire with altered construction and compound at Jerez, but each rider will receive only two. It will become the standard front tire from Silverstone onward.

Racing is a series of problems to be tackled in the limited time available. “Solutions” are most often gambles. Riders come to the line with the best setups they and their crew can devise then must make the best of what they have. Chatter can come to any rider or make of machine, and there is no sure formula for eliminating it. As one crew chief recently put it, “You need Ghostbusters equipment to deal with chatter.”