How Does Stress Concentration Affect A Stepped Crankpin In A Motorcycle?

I’m going to harp once more on the problem of stress concentration in mechanical design. In the early days of pressed-together crankshafts, it was very popular to use a stepped crankpin, wherein the smaller-diameter end pieces are pressed into the flywheel, and the bearing and the connecting rod run on the larger-diameter center part.

Where did the failure in this particular crankpin occur? It occurred in the radius where the small part joined the large part. And believe me, the people who designed these crankpins made very sure this radius was as big and gentle as they could make it because they wanted to avoid the stress concentration that they knew would be there.

But Mother Nature doesn’t put up with bad design; she punishes us with occurrences like this one. This is another case of a sudden change in cross section, bringing about a stress concentration at the point of transition. From this radiated many little cracks. Finally, the crankpin broke in half, the engine stopped, and the rider walked back to the paddock.

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 In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions.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.”