This Saturday I was riding my 32" on a slow/social group bike ride, and was quite surprised to have my uni transform into a surprise ultimate wheel:
Luckily I wasn’t going very fast or leaning hard on the handle at the time, and was able to hop off without incident.
Consensus amongst the veteran unicyclists I’ve shown is that by not having the bolt tight enough (see below), it was exposed to lateral stress it wasn’t designed for, rather than the teeth of the interface taking on that load. The bit of rust on part of where the bolt sheared also shows it had started to fail some time ago.
Lessons/recommendations for the future:
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tighten to spec (20Nm according to UDC support) with a torque wrench. I had used a little hex wrench initially. After some time I did acquire a torque wrench and re-tighten it, but maybe some damage had already been done at that point.
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grease the bolt to avoid a false torque reading. I don’t remember for sure, but I think when I used anything it was loc-tite to try to keep it from loosening, which it always seemed to do within a few rides.
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if any play/looseness is detected, inspect and re-tighten immediately. It always seemed to develop some play after a couple rides, maybe due to some combination of getting 1) and 2) wrong. After a while I stopped being so diligent about taking it apart to retighten. Clearly this was a mistake
Especially I know now there’s a torque spec