“eraser Stick” Management

“Eraser Stick” Management

TDC

KEVIN CAMERON

AS I HAVE PREVIOUSLY RELATED IN THESE pages, I once watched the motions of a motorcycle on a “two-post shaker” at MTS Corp. near Minneapolis. MTS makes hydraulic-powered testing machines that can simulate various road conditions in a completely repeatable way—something that has been of great value to motorvehicle industries. Testing programs that could be named “Forty Miles of Bad Road” can reduce new cars, trucks or motorcycles to junk in just a few hours, providing valuable durability information to manufacturers.

As the up-and-down frequency of the hydraulic post under that motorcycle’s front wheel was increased, the vehicle at first just comically rocked back and forth in pitch; then, at a higher frequency, the motion was absorbed mainly within the front suspension; but finally, above 20 cycles per second, the suspension’s movement was replaced by fore-and-aft flexing of the fork legs. At a particular frequency, the whole front wheel became a blur. This was resonance, the cyclic rate at which the frequency of the exciting force came into step with the natural frequency of the front wheel/fork system.

If we were to apply such cyclic driving force to other parts of the machine, we would find that the front wheel can also be made to vibrate from side-to-side on the springiness of the fork tubes and the twisting and lateral bending stiffnesses of the chassis’ steering head.

We could apply our driver to the side of the front tire, just above the footprint, and find that we could make the front wheel tip rapidly in and out of plane as the front axle acted as a spring. This is a critical flexibility, as riders who have just survived a front-end wobble find that this out-of-plane motion of the front wheel has knocked the brake pads back, allowing the lever to come to the bar on the first pull.

We can also apply the driver to the rear wheel in the same way. Now, we can see why racing swingarm beams are made extremely deep vertically yet quite thin (32-35mm) laterally. Side-beam depth keeps the rear wheel from tipping outof-plane, but side-beam lateral thinness serves as a “lateral spring,” acting at high lean angles as a “sideways suspension” to keep the tire on the road and following the pavement’s irregularities. With the bike supported by a cable, our hydraulic driver can make the rear wheel shake side-to-side until it, too, becomes a blur at the resonant frequency of rear-wheel assembly mass, moving back-and-forth on the “spring” of the thin swingarm beams.

Through countless experiments, manufacturers and their racing teams have studied these motions and their value as sideways suspension in improving corner grip on rough surfaces. On MotoGP tires, bikes regularly corner at angles of 63 or more degrees from the vertical. This kind of lean angle makes the machine’s conventional suspension nearly useless at absorbing bumps. Sideways chassis flex can help.

At the same time, they have learned how such flexibility can also hurt by leading to grip-destroying tire chatter. A simple demonstration of chatter: Hold a pencil by its pointed end and then put the eraser end against a table, with the pencil almost vertical but slightly tilted toward you (10-15 degrees). As you try to slide the eraser away from you across the table (this may take several tries to get right), it will begin a rapid cycle of gripping, hopping and gripping again. The chatter can be varied by holding the pencil more stiffly or less stiffly, or by changing its angle to the table.

This is what so many MotoGP riders have fought with this season. Their tires are the “eraser” (usually, it’s the front that chatters, but we’ve also seen the video of Ben Spies’ rear tire oscillating violently midcorner), and their fork tubes and chassis are the pencil and the hand that holds it.

Clearly, not all combinations are beneficial. And the harder the researchers and trackside people work to understand and defeat chatter, the more versatile and mysterious have become the interactions of “cyclic tire grip” with chassis flexibility.

Back in 1989, the great hope was that by switching to “inverted” forks (those with the stiffer outer tubes at the top and the smaller inner tubes at the bottom), the fork would be stiffened, raising its natural vibratory frequency above that of cyclic tire grip.

Sorry. Didn’t happen. Cyclic tire grip just adapted upward in frequency, to remain as damaging an effect as before.

Chatter pretty much arrived with the higher tread stiffness of slick tires, which were first used in 1974. In the late 1970s, there was no mystery about how to stop chatter: You just put on a non-slick front tire and, bingo!, it was gone. So was a lot of grip. This is why, in many photos of racebikes from that period, you will see a slick tire on the back and a non-slick or “patterned” tire on the front. Every club racer knew that if you put your modified Yamaha RD350 on slicks, it would chatter like crazy, and the harder you pushed, the faster it would head for the outside. So, you just put a slick on the back and adapted as best you could.

All of the above is why engineers have given up talking about the stiffness of swingarms, forks or steering heads and are instead interested in the vibratory frequencies of a motorcycle’s flexible structure.

Why not make everything really rigid? Sorry, been tried. Didn’t work. Lacking the “lateral suspension effect” of flexibility, the super-stiff bikes of 1993 just could not be made to hook up in corners.

Okay, then, let’s make them really floppy. Sorry, been tried, too. Didn’t work. Below a critical stiffness level, bikes oscillate in a mode called weave. Some bikes like this were tried in 1997, but they wouldn’t go straight. Bad idea.

Okay, is this the moment when some kind of alternative front end has to be considered, one that doesn’t put the “eraser” out at the end of long, flexible pencils (the fork tubes)? Think about it. A top team spends $10-15 million a year. Could you lightheartedly toss the highly developed telescopic fork in the dumpster and bet that some new arrangement of rods, links and bearings would a) solve all your old problems without b) opening a Pandora’s box of new horrors? One day, it may come to that. □