Originally Posted by
WhyFi
The derailleur and/or hanger could be bent inwards - that's a part of what caused my thrown chain, I think. Pedaling forward, that alignment might not be problematic much of the time, but pedaling backwards the chain is too far out of alignment for the teeth at the bottom of the chainring to engage.
If it was bent inwards would it only do this on one cog? I believe the hanger is steel and part of the frame. I havnt crashed it so I can’t imagine ifs bent out of the blue. Could it be an indexing issue?