Cottered Spindle Failure

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Cottered crank spindle failed along section change. "Firenze" bicycle, spindle not branded.

Cottered and square-taper cranks typically fail at the inboard edge of the crank, where the section is small and loads are high. With good materials and manufacturing, both types of spindles often last 10's of thousands of kilometres. With poor materials and manufacturing, the service life can be short.

This unbranded spindle is from a "Firenze"-branded bicycle ("Buy a television, get a free bicycle!") from 1983. It saw very little use, was clean, and was re-sold in 2012, whereupon the spindle broke.

The dark color on the short stub indicates the failure started on the "back" side (away from the cotter). The light-colored portion is fresh, and the "shark fin" shows the final point of failure.

The spindle shows rust on the broken face, but this is very likely recent.

The failure is almost certainly due to poor materials and manufacturing heat-treatment. The back side of the far race shows a further design defect: a sharp corner between the bearing race and the center of the spindle. Sharp corners tend to break after repeated loads, even when the loads are relatively small.