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.
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
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....
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.