In the picture above the chain is too short and the rear derailleur is stretched out too far with no more available movement left. In this scenario the rear derailleur can be damaged … bent from not having any more movement available. This is one of two ways that come to mind when it comes to damaging the rear derailleur.
The other way is to try to pedal without first down shifting into a low enough gear. Applying brute force on a pedal trying to move when in too high of a gear can result in making a pretzel out of your rear derailleur. One needs to be very careful not to “force” the rear derailleur as they just are not designed to take such abuse. Sometimes a rear deraileur can be bent back straightening it out to where it will work. However, sometimes one can work and work on it and never get it to shift properly again. Sometimes ya just got to bite the bullet and toss it in the scrap pile and install a new one. I had that happen to me once with a very expensive rear deraileur which was almost brand new. I had picked up a small tree limb about 5/8 of an inch in diameter. It went right into my rear derailleur and spokes doing a lot of damage. I straightened it out the best I could but it never worked right after that and I was forced to replace it. The main problem was the hangar had gotten bent and stretched out. I staraighted it but because it had stretched it would not go back to the right position and alignment. It required replacement. Needless to say, it didn’t make my day.
KEEP ON TRIKIN’
You are invited to come join the Tadpole Rider Facebook Group. Please note that in order to join you must first answer the two membership vetting questions and agree to comply with the group rules.