Derailleur hanger

The derailleur hanger is the small piece on the frame that the rear derailleur bolts to. It is made to bend easily, so in a fall or knock the hanger takes the damage instead of the costlier derailleur and frame (Park Tool hanger guide).

What it's used for

A bent hanger is a common cause of "mysterious" shifting problems. Park Tool says this kind of misalignment usually can't be fixed by adjusting the indexing or limit screws (Park Tool). Park Tool names crashes, falls and rough handling as the usual causes (Park Tool).

What to look for

• The hanger made for your frame. Hangers vary between frames. Get the one listed for your bike from its maker or a shop.
• Integrated or replaceable. Some frames have a hanger that is part of the frame and can't be replaced, so Park Tool urges extreme care when aligning them (Park Tool).

Using it safely

• Alignment is done with a gauge that compares the hanger to the wheel. Park Tool bends in small steps, correcting about half the gap each time, and rechecks after each bend (Park Tool).
• A hanger that has been bent repeatedly may fail during alignment and need replacing (Park Tool).
• Recheck all rear derailleur adjustments after aligning or replacing a hanger (Park Tool).
• If you don't have a gauge, a shop can check alignment.

Guides that use it

• Adjust a rear derailleur
• Check and replace a chain

Sources

• Rear derailleur hanger alignment, Park Tool