My rear derailleur hanger went yesterday on the commute home. Luckily it was as I started off again after a turn. Still managed to take out 2 spokes itself and possibly the derailleur itself. Luckily, I had spares back to my knackered old Shimano for now.
I’ve seen this happen before, I guess it’s when the derailleur gets too close to the spokes and they make contact.
A good reason for us to all check our rear derailleurs and limit screws.
Friendly reminder from a jaded bicycle mechanic - it’s more often than not a bent hanger (or bent jockey wheel cage), not a limit screw issue.
The limits don’t tend to work their way out of ‘set’ (unless you ride really hard or are very unlucky).
Good shifting, regular maintenance and checking your drive train goes a long way to avoiding mishaps like this! Thankfully our lord and saviour Calvin at Park Tools can help teach how to trouble shoot little niggles to avoid them becoming problems like OPs! It also means you’ll get to feel when shifting is a bit suspect (ghost shifts, chain skipping etc.) and you can pick up on them earlier.
Thankfully it’s not rocket surgery!
I’ve seen this happen many times because of riding downhill.
It’s not necessarily a misaligned derailleur. It will eventually happen to any bike because the derailleur hanger is made of sintered metal which will break easily. This saves your derailleur from more severe harm. Usually for road bikes this happens from age and natural vibrations. For us downhill bikers it’s usually from smacks against trees and rocks.