Originally Posted by
AndreyT
This sounds like a description of a typical chain suck. Yes, it happens when downshifting in the front. Most of the time the stuck chain goes a full circle, severely scrapes the chainstay and then slams into the FD cage, deforming it significantly. I.e. it's the FD that gets damaged, not the RD. But, I guess, under some chain length and cage length combinations damage to the RD is also theoretically possible. Including the hanger.
This is what chain suck looked on my bike back in the day
And in my case the FD cage took the damage, It used to happen from time to time with FSA chainrings. Once I switched to SRAM it just disappeared for good.
yet another thing bad about FSA cranksets... i thought the too large screw holes in the chainrings, teamed with the too large BCD cut, causing repeated creaking/loose/worn screws was bad enough.
the wiped out rear ders. i've seen happened at CX races, in the heavy mud/grass, usually after it quits raining.
the mud/grass builds up, a clump breaks free, and the chain wraps up on the cassette... SNAP! instant DNF.