Chassis Flex Is a Crude Technology

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Frames in the time of the RC45 were chunky and massive to reduce chassis flex—stiff was good. (Sport Rider Archives/)

A modest revolution has changed motorcycles in visible ways since 1997. Before that time, stiffness was the goal of chassis design. Stiffness gave quick steering response, resistance to chassis weave, and high-speed stability.

What are the visible differences? Before about 2002, it was common to see bolt bosses on engine cylinder heads coupling the rigid engine directly to the steering head. At the steering head itself, it was common to see the two main chassis beams form a stiff, boxy structure to carry the steering head, as had long been standard on two-stroke 500cc roadracers and on “homologation special” production bikes such as Honda’s RC30 and RC45.

A new direction was taken from 2002 onward, first by Honda, then Yamaha, and later, others. Instead of a solid connection to the steering head at cylinder head level, longer front engine hangers descended to lower attachment points at the level of the cylinder or even the upper crankcase. As explained to me by a Yamaha engineer in 2003 (my good fortune in visiting its Media Center on a trip to Motegi), this use of longer engine hangers would allow the steering head and forward chassis beams more lateral flexibility. Not a lot, just a matter of a few millimeters, enough to take the peak shock out of pavement irregularities with the bike at high lean angle. Just as the few millimeters of crush foam in a rider’s helmet can make the difference between death and headache, so those few millimeters of chassis flex could make the difference between complete loss of front tire grip and a minor slip.

Simple-minded? You bet, for such a “system” has a very high unsprung weight consisting of tire, brakes, the entire fork, steering head, and forward portions of both chassis beams, having to sproing crudely from side to side to reduce the severity of the force spikes the front tire must transmit during cornering. But it worked. More than that, it was decisive.

Ducati proved that by trying the opposite direction. Casey Stoner, on its quite flexible steel tube trellis frame, brilliantly won 10 races and the MotoGP championship in 2007. But when an extremely stiff carbon fiber box beam was put in its place, Stoner won few races and often lost the front. No more championships until he went to Honda.

Soon everyone was hard at work, building and testing one “engineered flex” chassis after another in hopes of finding a sweet spot.

What sweet spot? Not all results of increased chassis lateral flex were positive. First was steering delay, for as the rider steered the tire and side force developed, that steering force had to pass through the steering head, deflecting it sideways. That took time. When Max Biaggi felt it in 1997 on the Honda NSR250 he was riding, it was a huge difference from the contrasting high stiffness of the Aprilia on which he’d been world champion in 1994, ’95, and ’96.

No prob, Bob? Just remember to steer a fraction of a second earlier? Try driving your car fast through a twisty gymkhana course with an eighth of a turn of steering backlash.

Another negative was the effect of increased chassis flex on braking stability. Lots of prose is poured forth on the subject of heroic acceleration out of corners, but the very large forces of braking into the corner must also pass through the steering head. If that steering head can now flex side to side more easily, what might that do for braking stability? Get it wrong and a sudden oscillation develops that can put the rider down. There was a lot of that in the early years of MotoGP, and I’ve seen production bikes that, when pushed hard, go into a ka-boing-ka-boing mode during braking.

Also, because the damping for the higher-speed weave oscillation (rapid side-to-side swinging of the rear “caster”—the chassis and rear tire) decreases with speed, every motorcycle has a weave threshold. One of the tasks of design is to put that threshold at a speed too high for the motorcycle to reach.

So you get the idea. Improving a motorcycle’s front tire hook-up is more complicated than just going nuts with a hacksaw in hope of creating “good” flex.

This Moto2 concept from Taylor Made combined the frame, fuel tank, and bodywork into a single fabrication. The problem here is the cost and effort to make chassis changes. (Cycle World Archives/)

Meanwhile, swingarm beams were evolving from round tubing (1960) to rectangular (1970, stiffer against twist than against side forces), and from constant-section rectangular to braced (1980) and then to beams that were triangular, as seen from the sides, and growing thinner year by year to increase their side-to-side flexibility (new century). Last year some MotoGP bikes actually ran their chains off—something that was common back in the 1920s when bendy bicycle chassis were still the basic fashion.

While the order of the day is compromise—enough flex to be useful, but not enough to cripple performance with steering delay, braking instability, or high-speed weave—surely other engineers are waking up at 4 a.m., staring into the darkness and imagining systems that can turn chassis flex on or off.

One example of a possible direction was the field kit applied to gradually increase the flex of Colin Edwards’ Honda RC51 Superbike in 2002. Back then it was done by selective-fit shims, providing various degrees of lateral motion at bolting points between engine and forward chassis. Changes, of course, could only be made by mechanics in the paddock.

Just as an example of one possible “switchable flex” system, imagine that the cylinder head or some part of it is made to clear the inside of the forward chassis by a few millimeters on each side. In the head at those points are a pair of small pistons, about the size of bucket valve tappets, supplied with engine oil pressure through check valves. Each also carries a solenoid-operated drain valve. With the drain valves closed, the pistons extend to prevent lateral motion of the chassis with respect to the engine. Braking stability is good and high-speed weave is prevented. With the drain valves open after corner entry, full side-to-side chassis flex is possible.

Other, and likely better, ideas will come to readers.

The danger in such tech would be if it actually worked, for then it would have to be banned as “destabilizing” the series. Remember: The primary goal of riders and teams is not to win, but to be entertaining.

In that vein, I remember sideways stylist Garry McCoy at Valencia getting roars from the massive crowd for his long smoky slides, “autographing” the track with cursive black lines. His bike, although very powerful, did not hook up well, so he was last or next to last at the time. The crowd didn’t care.

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