Auntie Helen said:
after 12.5 miles my rear gear mech hanger bent out of shape (it's bizarre, we can't work out how!)
My experience (based upon our unique viewpoint of the bike in front) is that an unexplained bend in or around the rear derailleur is typically triggered by the chain locking up as it tries to pass through the jockey wheels. The resulting tug pulls the mech in an unexpected direction onto itself, with more than enough force to bend it.
As often as not, post bend, the chain then frees, and (as it all happens on the spring loaded return line) there's not even a break in cadence. The exception is if it holds on (in which case you suddenly run out of chain slack) or you hear the 'scccchhhhlock!' sound of it biting and backpedal (in which case one finds the derailleur stretching out to meet the pedals.
Or, Wowbagger is deliberately sabotaging you because he has:
- a beard and anger management issues.
- committed to rohloff and can't bear the idea of anyone using a cassette.
- deep seated psychiatric problems.