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Steven Woo's Ritchey crank failure
This break illustrates how the crank cracked first along the thin ribs of the crank that are on the outside face of the crank (pedal side) that is maximally stressed at or near the bottom of the pedaling downstroke. The material on the two ribs is taking the highest load, so any imperfection in the material can initiate a crack. This crack worked away for a while, shown by the darkened material at the separation. The crank break is light-colored where it failed suddenly. In this case two cracks roughly across from each other, separated by the hollowed portion, progressed until they met suddenly and catastrophically, resulting in complete separation (shiny area) and the rider being dumped onto the road. Photo courtesy of Steven Woo

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