What Are The Causes Of Motorcycle Failures?

Cycle World Technical Editor Kevin Cameron answers your engineering and mechanical questions

You might wonder why I dwell on failed parts so much. Soichiro Honda said famously—and no doubt, many times—there is more to be learned from failure than success. Here, for example, is an ignition-side flywheel. You’ll see a taper and a keyway so that the ignition rotor, which is just a magnet, fits on to the taper.

As it rotates during the operation of the machine, possibly at 10,000 rpm, this whole affair can move around a bit. There can even be some slight back-and-forth movement, which is why you see reddish bands of discoloration on the taper. That is iron oxide, resulting from minute oscillatory frictional movement, which is called frettage. When you see that red powder in a joint, it means some slight motion is taking place.

This rotor is heavy, all this action is going on, and that results in stress. Stress can initiate cracking by being amplified by the presence of a defect in the metal or a sharp edge, which can act as a crack nucleation site. Here is a failed ignition rotor, and you’ll notice that the key slot is part of the failure. This kind of thing has happened to all manufacturers at various times.

When Kawasaki took its H1RW (the "W" standing for "water-cooled") Grand Prix racing in 1975, some ignition rotors broke off in just this fashion. The rotor was substantial; the taper was not. Therefore, a necessary countermeasure part: a bigger taper. This kind of thing goes on in engineering constantly.

Naturally, for reasons of common sense, we want to keep successful parts in production for a long time. That’s why each new model has as few new parts in it as can be managed. But when there’s a problem, there has to be a countermeasure. In this case, it’s simple: Make the parts stronger. It goes on all the time.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin's unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”