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I was trying to learn how to track stand and I had a strap on my left foot. I failed miserably and I tilled over then since my foot was in the strap my left crank got pulled out. What do I do to fix it.

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    To pull a crank out on a trackstand fall there must be more to it. We need more information on this. A picture from the side among others may be useful. – Carel Sep 14 at 10:18
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Hope you're ok!

Agree with the first comment on the Q. It's hard to discern what's going on from the picture.

Your crank is held on to the bottom bracket (the axle/bearing combination to which the crank arms bolt) by being jammed up on the wedge shaped tapered spindle (axle) of the bottom bracket. This is done by placing the crank arms onto the spindle and tightening down the bolts that hold them in place until the crank is forced up onto the spindle.

For the crank arm to come off, it either has to break badly (altho it's hard to imagine how this could happen) or the arm has to slide off the spindle. For THIS to happen, though, the crank arm bolt that forces the crank arm up onto the spindle and holds it there needs to be missing.

Perhaps your crank arm bolt had fallen out previously and the cran arm was just hanging on due to being wedged on? If this is the case,.consider yourself lucky -- you found out about the missing bolt in an easy manner (assuming your crash didn't hurt too badly) -- if your crank came off in traffic, it could have been catastrophic.

Check to see if there's a missing bolt that holds the crank arm in place. It would match the other side. (There's a few different styles.) If you need a new bolt, you can get them from a shop. Crank arm bolts need quite a bit of force to push crank arms onto the spindle (but not too much!) -- they're often out on with a torque wrench. Your shop could probably do that for you for a few bucks.

Also, and this is important, make sure the metal around the hole didn't crack in the crash (or previously). Cracked cranks do happen and are a cause for crank arm bolts to come loose. Maybe take the loose arm to the shop with you to get a 2nd pair of eyes on it.

Good luck!

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    I'll also point out that a square-taper crank arm that was loose enough to fall off may have become distorted because it was loose. If that's happened, the crank arm will never stay tight in the future. See bicycles.stackexchange.com/questions/14417/… for one example. – Andrew Henle Sep 14 at 14:00

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