Why do rear mech derailleur hangers bend or break?

If you’ve just broken your mech hanger, it’s very disheartening to realise you can’t use your bike again until you get a replacement – BUT – you should take some joy from knowing that your mech hanger has done its job by saving your expensive carbon or aluminium frame from damage by acting as a sacrificial, easily replaceable part! If your hanger was built into your frame and it broke, you’d need to replace the entire frame.

There are a few different causes of mech hangers breaking or bending:

  • A problem which causes your chain to snag while pedalling. Another half turn of the cranks will not move the chain, but pull the whole derailleur assembly around and finally break the hanger.
  • Chain snagging could be caused by: the chain itself breaking and becoming caught in the cassette or front chainrings; damage to the cassette or front chainrings themselves; some debris caught in the drivetrain.
  • If your derailleur end stop is not adjusted correctly, the derailleur or chain can crash into the spokes and become entangled, and the hanger will break.
  • Accident damage when you crash and the weight of the bike (and you!) is taken by the derailleur.
  • In transit when the wheels have been removed and the bike is not packed sufficiently and the weight of the bike or other items are resting on the derailleur.

Things to look out for to stop it happening again:

  • Always check your derailleur, chain, cassette, chainrings and spokes after a broken mech hanger. Often just the mech hanger itself breaks but damage to these other components could either have caused or resulted from a broken mech hanger.