Vicious Cycles

Vicious cycles

TDC

Kevin Cameron

EVERYTHING IS FLEXIBLE. WHEN A FLY lands on the end of a steel beam, the steel deflects. No structure is perfectly rigid.

As a small boy, riding in my parents' car, I wondered at the rapid “wubba-wubba” vibration that always followed a wheel's hitting a sharp bump. Years later I learned that this was not the wheel continuing to hop up and down-the shock absorber deals with that. It is a phenomenon called “shake,” and it is the flapping of the chassis at the corner where the wheel has hit the bump. Any suspension is three springs in series; first is the tire, deflecting to absorb the smallest bumps; next is the suspension spring; behind that is the flexibility of the chassis itself, being deflected by the forces transmitted through the suspension.

Very early on, stock-car racers learned there is no point in putting stiffer suspension on a chassis too weak to support it. As a bump pushes the wheel up, both the spring and shock absorber resist the motion, passing on the force to that corner of the vehicle, which bends upward, as well. Once the bump has passed, the shock absorber prevents the wheel from snapping back, rebounding off the pavement again, and continuing to oscillate disturbingly. But the chassis has no shock absorber to damp its motion, so it continues to vibrate up and down-wubba-wubba. This continuing motion can be just as disturbing to tire grip as running without a shock absorber.

The correct response was to stiffen the chassis, to force more of the bending to occur in the suspension, less in the chassis.

The same is true of motorcycles-in spades. At a given level of chassis stiffness, there are limits on how firm the suspension springs and dampers can usefully be made. Above that firmness, the chassis simply bends instead of the suspension’s doing so-and uncontrolled motions result.

Yet high-performance vehicles need firm suspension. High cornering speed crushes a motorcycle down on its suspension, forcing use of springs stiff enough to keep it from grounding. Riders cry out for instant turning response, and stiffer suspension can

give it-up to the point that the chassis shows its spring-like nature.

This is why chassis designers, whether working on production bikes or GP bikes, must continually stiffen chassis. Tire grip improves, corner speeds rise, and bikes will ground or bottom unless suspension is stiffened to deal with the increased loads.

Ever drive a big car on soft tires? When you take a quick cut at the steering wheel, there is a longish pause as the front tires deflect, then gradually drag the front of the boat-olac sideways so the rear can follow. Often, the response is so slow in coming that the driver, sensing nothing is happening, increases his turn of the steering. When the effect finally arrives, it is too big, and the driver, now in moderate panic, steers sharply back the other way to compensate. A disagreeable oscillation follows. The only way to keep such a vehicle stable is to apply appropriate control slowly, and patiently await the effect. Robert Mason, in his excellent book Chickenhawk, describes learning to fly the H23 training helicopter in much the same way; the long control delay made it necessary to know in advance how much control to apply, and how fast to apply it. Otherwise a cycle of overcontrol and overresponse would result. This is just the description given by leading privateer roadracer Dale Quarterley of riding current Superbike racers. He said at Daytona this past spring that he can feel the fork flex, the chassis twist and the swingarm wind up, and that bad things will happen to anyone who tries to make the motorcycle do anything suddenly without knowing in detail what the delays are. When the machine is all wound up, that state has to be allowed to decay gradually, for any attempt to force through commands just channels all that energy into chassis oscillation. Skeptical? Racers are very sensitive to control delays. When Dunlop introduced its “anti-chatter” soft-sidewall T26 fronttire construction years ago, riders could feel the increased delay at once. Wayne Rainey says he can feel which frame he is on just rolling down pit lane, from the cues coming through seat, pegs and bars. The human body is a pretty good measuring instrument.

In GP roadracing, where the process of adding stiffness has gone farther, the machines are more resistant to all that winding-up and self-steering and oscillating, but fresh problems have been created. Now, when these bikes are leaned over in corners, their suspensions and chassis are too stiff to respond to bumps. Several teams were perplexed early this season as their newly stiffened bikes skated uselessly on rough surfaces.

As usual, it is the tire companies that are called upon to fix this. A new tire can be made in a week, while it takes the bike-makers more like a month to come up with a new chassis, and usually far longer to create and implement a new concept. Therefore, new tires with far more lateral flexibility are now being tested. Bikes equipped with onboard TV cameras, aimed at tires decorated with painted stripes for easy shape analysis, have been seen at test sites. These tires are said to feel like the earliest test radiais from 1984-85: as though the machine had no swingarm bushings. As the machine is set into its turning attitude, the tire deflects sideways extravagantly, then clunk, fetches up solidly against its own limits, like an MX bike against a berm.

Such laterally soft tires may temporarily supply racebikes’ need for better in-corner suspension, but they will also re-introduce long control delays, making these most responsive of bikes act instead like big cars on soft tires. I think this is what we call a vicious cycle. □