I haven’t been active on the forum, but thought I’d share this and see if others have had any similar experience.
Commuting to work yesterday, I was standing to accelerate away from traffic lights when something happened. I cruised to a stop and found my pedals were misaligned (running clipless). Couldn’t do anything roadside, but when I got back home I removed the drive side and found this.
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Personally I haven't had this happen but I've seen about three in my time from my customers. All of them were on bikes used by men, large and heavy and heavy on equipment. This type of failure is bound to happen in a very, very small percentage of cranks after prolonged use or damage to the spindle. The spindle is a thin CroMo steel tube that transmits torque from the left pedal to the chainring during forward pedalling and, resists rotation between the two pedals when the rider stands on both pedals such as during a bunny hop or trackstand or just riding downhiill without pedaling.
A tube that fails in torsion doesn't make a clean circumferential break as you'd expect, but a spiral break as can be seen in the photo. This mechanism is nicely illustrrated lusing a pool noodle or pipe lagging (with the zip closure removed). there used to be a nice video on YouTube demonstrating how this works but I can't find it now. Do your own experiment. Find a flexible tube - the cardboard inner from a roll of kitchen paper or, a pool noodle will do. First draw a line from one end of the tube to the other Now cut along that line so that the tube has a single straight split along the length of it. Now draw little lines perpendicular to the cut (or draw these lines first and then cut). Make several of these about one inch apart. Now twist the tube and you'll see how the material moves. The obvious part is that the cut now twists in a spiral, as expected, but surprisingly, the lines now separate and significantly so. This means the tube undergoes both twisting and stretching at the same time, showing why the failure above is that shape.
These cranks are an excellent design and the tube is just the right diameter and wall thickness to prevent 99.9999% of failures under normal use. The number that do fail is insignificant and expected. A small flaw or damage to the tube can induce a stress riser. Damage is usually from just underneath the bearing where dirt scores the spindle. Repetitive brute force is any material's enemy too.
So there, perfectly normal and the OP falls within the .00001% percentile of the cycling population.
Edit: Because the spiral crack is directional, you can figure out from the broken spidle whether it was broken during normal pedalling or bunny hopping, but only if you know whether the rider hops goofy or regular*.
* Surfers will know.
