The first of two pre-season MotoGP tests at the Sepang Circuit inMalaysia ended with “Mr. Big Hair,” Marco Simoncelli, on top. He is on an RC212V leased to Team San Carlo Honda Gresini. When the test began, it appeared at first that Casey Stoner (newly on a factory Repsol Honda) would be top man. Then it was Stoner’s teammate, Dani Pedrosa, seemingly out of reach at more than half a second faster than the field. And, at the end, Simoncelli on a “satellite” machine.
That makes one thing clear: Honda has achieved something solid. At the beginning of last season, the Hondas had speed but were rough—clearly not easy to ride. By the middle of last season, Pedrosa looked to have enough machine stability that he could give up some of his defensive style of sudden maneuver in favor of a degree of grace. Now, at Sepang, the Hondas were solidly on top.
Bear in mind that times at pre-season tests reveal little about future race results. It is normal to first build a “one-lap motorcycle,” whose speed may fade in five laps or 10.Valentino Rossi, his shoulder still weak from recent surgery to fix damage resulting from a dirtbike fall last year, worked only on those track areas that did not require full strength, and his best time was far down the list.
Jorge Lorenzo and Ben Spies, who are now the Yamaha factory team, were near the top, but Lorenzo had problems with “vibration” at the front (called “chatter” in the lingo of the previous era. The latest understanding of this is that the major cause is “constructional inhomogeneity” (try saying that one really fast five times), which means that, like our bodies, not all parts of the tire are equally stiff. As the tire rotates, this “variable softness” generates a cyclical force. Transmitted to the front through the springy fork tubes, this can, when the tire slides a bit, generate a rapid vibration or grip-and-release cycle.
Lorenzo commented that this vibration arose at Sepang particularly in the fast corners and with the hard Bridgestone rear. Years ago, chatter usually had a frequency of 20-22 cycles. Then, for a time, it moved up to more like 25 cycles per second and now has come down under 20 cycles.
What does this mean? I can only speculate that the 20-22-cycle era represented the natural frequency of the system composed of the fork as a (bending) spring and the front tire as the oscillating mass. This frequency rose when the coming of the “upside-down” fork so greatly increased stiffness. Now, the popularity of making the whole front of the frame somewhat flexible laterally changes things again. Instead of just the front wheel as the moving mass, now we have the wheel plus the fork moving laterally together, with the front of the frame taking the role of spring.
Rossi, too, encountered some chatter. As he is newly on a Ducati, he inherits the front-end-feel problem that slowed Stoner last year. As is characteristic of a low-feel bike, Stoner could make fast laps but would then crash, having done nothing different. Lack of feel means the front end gives no clear, progressive warning that it is approaching the limit of traction. It is the rider’s job to go fast. So, he does. But without adequate warning, a crash can occur any time.
The lack of feel has been blamed on the initially very high stiffness of Ducati’s new carbon-fiber chassis, which employs the large cross-section of the engine airbox as a structural member. Carbon-fiber-reinforced plastic (CFRP) structures have much higher stiffness-to-weight ratios than metals but have been little used in GP bike chassis because it is so much easier to make rapid changes to the geometry of metal chassis by cutting and welding, while changes to CFRP have generally required manufacture of a new mold.
On the other hand, CFRP construction can potentially allow changes to be made to stiffness in desired directions. This has been a major attraction of this material for aircraft construction.
With regard to the chatter Rossi encountered, he is quoted as saying that although they had available a fork with more-flexible 42mm sliders, they had not used it. A usual remedy for chatter is to change the stiffness or mass of a major part in order to “detune” it from coming into step with whatever cyclic force is driving it.
The 42mm fork was most likely present in the Ducati armamentarium as an emergency means of creating a degree of feel. Last year, when Colin Edwards was asked, “What is feel, physically?” he replied that it was often an isolated front tire slip or two—the beginnings of chatter—that indicate that the front end is close to going away. Parts that flex, like suspension, may keep the tire in more consistent contact with the road. The trick, then, is to permit the flexure that the rider needs for feel without allowing cyclic motions to build up into chatter.
If it occurs to you that maybe these flexing systems now need some damping, I’m with you! I keep thinking of the early experience of the makers of metal skis, which chattered terribly. By building some wood into the skis, the chatter was brought under control, just as hollow steel aircraft propeller blades had to be filled with a wood-like composition to damp vibrations that would fatigue and break them.
What’s next? Plywood chassis? Probably not. Damping materials can be incorporated into CFRP. And, for all we know, that’s happening right now.
