Originally Posted by carpediemracing

When pedaling, the top of the chain determines whether or not the chain stays on the chainring. THat's why the front derailleur moves the top of the chain, not the bottom.

Likewise, when pedaling, where the chain is at the bottom of the cassette (laterally) determines where it'll go, hence the rear derailleur hanging under the cassette.

In a sprint, if the chain came off the big ring, that means the chain was not in a straight line with the rear cog (the 13 in this case). If the chainline is excessive, it can cause the chain to derail (and probably to the outside if you're in the 13).

If you have a flexy frame or crank or chainrings (or they're not tightened properly), then that is one potential cause - you torqued the stays/cranks/rings sideways, screwed up the chainline, and caused the chain to pop off.

You may have a bent tooth on the ring, but I think you'd have noticed that. Finally, your chain or rings may be worn excessively.

The hanger is independent of derailing the chain off the chainring, but it could have been bent as a result of your near fall.

hope this helps,
cdr

Originally Posted by JayC

I had the same issue, it was a bent tooth on the ring.