Motorcycle Chassis Design | The Elephant in the Pit Box | Drawing the Line

That motorcycle chassis design is often more art than science has never been more apparent than during the last couple MotoGP seasons. Last year, the only European factory in MotoGP struggled and failed to give Valentino Rossi the “front-end feel” he needed from the Ducati in order to regain his winning record. In 2012, while Ducati continues to struggle, it is Honda’s turn to grab handling headlines with chatter problems; first front-end chatter, then rear-end, and sometimes both.

Ducati has thrown new frame concepts, new swingarms, different materials and revised engine tuning at its problems without unlocking the secret to their lack of competitiveness. Though Honda has been more covert about its solutions, there have at least been new frames and swingarms built, even if the look of the bike doesn’t change much, and both Casey Stoner and Dani Pedrosa have spoken openly about chatter and the challenges they face.

While Ducati and Honda try to tune in chassis feel and tune out chatter, MotoGP analysts and observers spend plenty of time giving opinions and guesses about how the factories should proceed. But in the last two seasons I haven’t heard it said even one time that the telescopic fork might be a factor in the handling problems facing these two motorcycle powerhouses.

With problems as clearly related to front-wheel control as “front-end feel” and “front-end chatter,” the fork doesn’t get discussed. What’s going on here?

Chassis designers end their structural work at the steering head, or perhaps at the triple-clamps. From those triple clamps to the front axle, chassis structure is designed and provided by the fork manufacturer. Change the fork and you change both the structure and the action of the structure. But that’s not all, because the action here is not only suspension action, but steering action as well.

The fact that the factories don’t build the front suspension leaves the fork under-represented as a potential cause of the chassis problems. The fork is the elephant in the pit box—maybe the biggest part of the handling problems in MotoGP today, but not discussed in polite company.

In a way, you can understand the reluctance. The telescopic fork has two advantages. It provides workable steering geometry, and it is simple in concept. But the fork has three inherent characteristics that chassis designers, if they had a choice, would reject.

First, the fork is a lever. Front-wheel loads are multiplied by the fork’s lever effect, so the chassis has to resist multiples of the front wheel forces. The designer wants to reduce loads, and certainly not multiply them by a factor of two or three.

Second, the fork is a pendulum. Clearly, the front wheel must turn in steering, but with the fork, the entire front suspension must also pivot back and forth around the steering axis. This is a lot of mass to control, and in headshake that pendulum effect gets our attention. The designer would like to reduce steered mass and the resultant inertia as much as possible.

Third, the fork is a spring. Being essentially a couple of long tubes, it bends under load, storing energy. When the load is removed, it releases that energy, springing back to its original shape. The amount of bending can be well over an inch of deflection. And this is a spring without a damper. The designer doesn’t want bending motion, much less undamped and oscillating bending motion.

Even the most sophisticated MotoGP fork has these three characteristics, and even when relatively well controlled, the extreme forces seen in MotoGP can still overwhelm the fork and the chassis as a whole.

The fork is not the cause of all handling problems confronting the MotoGP teams. But until motorcycle chassis designers also become front suspension designers, the telescopic fork will continue to be the elephant in the pit box with a problem that isn't discussed.