What Do You Think This Thing Is?

Many parts, like the shift shaft on a motorcycle, have stories behind them

This engine is an evolution of an earlier engine, and the earlier engine was an evolution from an engine before that. Each time a given design is judged to be obsolete—that is, it would cost more to improve it than it would cost to make a new model that fixes all the problems of the old model—engineers get an opportunity to fix things that have been bugging them.

This is the shift shaft. It is connected to a mechanism inside the gearbox that rotates the shift drum. Cam-like slots milled into the drum move the shift forks that engage and disengage the gears. On an earlier style of bikes, the shift pedal was attached to the shift shaft, and a pinch bolt held it in place.

Fifty years ago, motorcycles vibrated a great deal more than they do today; people won’t put up with it any more. But in those days, overcoming that vibration was regarded as “manning up.” When the engine vibrated at a certain speed, the shift pedal also vibrated, so fast you couldn’t see it. This is known as sympathetic vibration.

It wasn’t long before a crack would develop, and the end of the shaft would drop off with the shift pedal. It didn’t matter if you had safety wired the bolt and the nut. Here, engineers have made the shift shaft thicker. I wonder why. Some engineer said, “This is the last time that is going to happen.”

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