Originally Posted by
Chuckk
Since everybody is guessing, I'd propose that the root main cause of failure was an overshift.
The chain went over the big gear, dragging the derailleur into the wheel, where it hung up on the spoke long enough to stop the rear wheel and peel the tread before it popped the spoke out of the wheel and bent the hanger.
I've had similar things happen twice to me in the last humpteen years. Once on a bike I was test riding (seller had not set the inner stop screw on the derailleur he installed) and one that I built and just messed up.
That's a thoughtful post-mortem Chuckk.
The OP will want to observe while tightening the derailer bolt (after straightening the dropout) that the derailer doesn't rock as the bolt turns, would indicate a bent bolt. I've got a New 600EX rd here with a badly-bent mtg bolt.
Before I got a proper hanger alignment tool, I used to use a known-straight, long-cage rd to use as a hanger straightness indicator. Got the Microshift long cage mech from Nashbar for a few dollars and the cage was very long. I could swing the cage through 90-degrees rotation and observe the hanger alignment versus the larger cogs, checking for bend and for twist of the hanger. When the cage looked parallel to the cogs even an indexed setup always worked perfectly if everything else was in good order.
I might use a large adjustable wrench on this one for initial gross straightening, but only bend it
with the rear wheel tightened into the frame backwards to support the dropout and to provide wrench clearance away from the cogs.
Did this a hundred times this way myself, always made sure the wrench jaw completely supported the hanger all the way around the hole so as not to distort the threaded hole(!).
This can be repetitive/tedious since the rear wheel needs to be reversed each time one bends the hanger and each time one checks the straigthness against the big cogs, so best to allow enough time to patiently do it right the first that one goes at it.