Rear Mech Misbehaving

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Rear Mech Misbehaving
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Posted: Aug 19, 2020 at 9:55 Quote
Riding some rough trails in the Swiss Alps. Bike behaved itself for 5 or 6 days then massive clatter from rear end of bike, mech had gone forward, pivoting around the bolt at the bottom of the derailleur hanger, which had loosened. I pulled the mech back, tightened the bolt very tight, and no problem for rest of the day. Then the same thing happened today. I attach photos of rear mech in good postion, bad position, and the allen key on the loosening bolt. Thanks in advance.
Correct position
Correct position

Bad position
Bad position

Key in problem bolt
Key in problem bolt

Posted: Aug 19, 2020 at 10:06 Quote
It's a Vitus Sommet 2018 Carbon Enduro bike, if anyone needs to know...!

Posted: Aug 19, 2020 at 11:28 Quote
Your derailleur hanger is broken. There's a little tab on it that's broken off. It's to stop the derailleur from moving pivoting down.

Posted: Aug 19, 2020 at 16:45 Quote
I don't think the derailleur hanger is broken, sounds like the bolt is backing out past the point where the screw contacts the tab that keeps it up. Threerock I would put some Loctite on the bolt, or just get used to tightening it every couple rides. Check that your derailleur pulleys don't have excessive play, as that vibration can accelerate the bolt backing out

Posted: Aug 19, 2020 at 23:56 Quote
Thanks for the advice, folks. I have a spare hanger so I can look to see if the tab is broken or not. Otherwise I'll go down the loctite route.

Posted: Aug 20, 2020 at 3:02 Quote
Looks like the tab on the derailleur hanger is fine, but the corresponding bump on the mounting part of the rear mech is too short to engage with it, so maybe that ha broken off....

Posted: Aug 20, 2020 at 4:52 Quote
the tab that the B adjuster sits on........

Posted: Aug 21, 2020 at 14:19 Quote
the-one 1 gets the prize. A sliver of the bump on the hanger has been sheared off, enough to stop it working. Seems to be made of an alloy containing at least 30% cheese.
Broken mech hanger

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