chain replacement after rear derailleur failure?

[oskar]

Hi

The rear derailleur on my MTB bent and broke off from the derailleur hanger today, so I need to replace the whole mech. Strangely enough, the hanger didn't break. Is there a recommendation whether the chain needs to be replaced as well? I have done a quick visual inspection and can't see anything wrong with it, but I don't want another failure due to a bad chain (or whatever).

On another note, this summer I was doing a 3300km/2000 mile bikepacking tour on another bike (cyclocross build). One day I rode into deep gravel (road construction), the bike got stuck and the derailleur hanger broke. I managed to get it replaced only to have the new one break again - this time on a regular paved road without any issues (as far as I remember). I haven't figured out why it broke, but what is the most likely explaination? One theory I have is that is that the rear derailleur was running to close to the wheel and simply caught a spoke. (But why after 600km/350 flawless miles?) Another theory is that the first gravel "crash" caused some misalignment or whatever, resulting in the second failure... I should mention that I built this bike myself, including wheels, so no guarantees everything was right from the beginning...

Have a good weekend!

Oskar

[jarrett.morgan]

I have broken my rear derailleur and rear derailleur hanger multiple times on my mountain bike and none of them ever resulted in me needing a new chain.  As long as your chain isn't stretched or any visible defects, it will most likely be fine.  I hope that is decent answer for you.

[jaymtb]

Hi,
It is worthwhile  from time to time to use a derailleur hanger alignment tool, which ensures that the hanger screw holding the derailleur is in perfect alignment  (parallel) with the axle  when looked at from the top and back.    The inner/outer limits should be checked/set  as well- especially the  inner.  It is always good to check chain for wear/stretch.  A 114 link chain can gain half a link or more in length  from stretch. 
 
Jay

[JRA]

I use chain measurement tool to monitor chain stretch.  As long as the chain isn't overly stretched, the pins seems to move freely and there's no obvious damage to the chain, I would keep using it even after a derailleur failure.