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Recently, on my single-speed bike, the right crank arm just snapped. Was slowly going up a hill, and all of a sudden it broke in two pieces.

I have now exchanged the entire crank & chain set (sadly had to give away my beloved spider crank!)

Afaik this is not a very usual thing to happen. What could have caused it?

Edit: It looked pretty much exactly like this.

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If it is an older model check for a recall, Shimano, Trek, Mongoose recalled thousands of cranks. I believe they were from the late 90's. A friend had his replaced about five years ago at no cost. – mikes Sep 24 '13 at 0:28
If you look at the fractured edge, you can usually see a difference between the area that is freshly fractured and the area that had been fractured for some time, as the fracture "grew" to (slightly) catastrophic size. Likely the crack was already halfway across (for weeks/months) before it suddenly failed. – Daniel R Hicks Oct 8 '13 at 20:11
up vote 4 down vote accepted

Crank arms snap all the time. Odds are pretty good that it was an older crank and the fatigue in the metal built up enough that it was no longer structurally sound. Metals are a lot like crystals on the atomic scale, but when you repeatedly put a load on them tiny cracks begin to form and grow, until the metal is warn out and cracks.

Fatigued metal is also more likely to snap when subjected to above average forces, so it makes sense that this happened when you were hammering up a hill.

Airlines around the world are required to have their aircraft x-rayed periodically for signs of fatigue. When they see microscopic cracks they replace the part.

If it wasn't an old part it's likely that there was a flaw in metal.

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Aluminum parts (cranks, handlebars, stems...) don't last forever, sadly. – WTHarper Sep 24 '13 at 1:10
Aluminum, in particular, is subject to "work hardening", where repeated flex makes the material harder and more crack-prone. This would be exacerbated, especially on a square shaft, by a less than absolutely tight crank, as the slight rocking motion of the loose crank would produce more flex. – Daniel R Hicks Sep 24 '13 at 1:55

Yes, it's true that just like most things crankarms will have a fatigue life, but it should be a very, very long life that extends well beyond the factory warranty period. As pointed out by mikes in his comment it is worth checking for recalls on your model and batch of cranks.

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