Repair guides › Drivetrain

Check and straighten derailleur hanger

Moves the chain across the cassette and keeps it tight. The cable pulls the parallelogram inward to bigger cogs; a spring pulls it back out. Two limit screws (H and L) stop it from throwing the chain off either end. The B-tension screw sets the gap between the guide pulley and the cogs. The cage length sets how much chain slack it can take up (total capacity). For example, a Shimano Altus RD-M310 handles up to a 34T cog and 43T capacity.

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Tools

• Repair stand: Park Tool PCS-9.3 Home Mechanic Repair Stand ($$$$)
• Derailleur hanger alignment gauge: Basic derailleur hanger alignment gauge ($$)

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Steps

1. Look from behind: the derailleur cage should hang straight down, parallel to the cogs.
2. Remove the derailleur from the hanger (one bolt).
3. Thread the alignment gauge into the hanger. Swing the arm to the rim at 3, 6, 9 and 12 o'clock.
4. Gently bend the hanger with the gauge until the gap to the rim is the same at all four points (within 4 mm).
5. Aluminum hangers crack if bent too much. Buy a spare hanger made for your frame.
6. Refit the derailleur and re-index.

Diagram

Signs it needs doing

• Chain hesitates or clicks between gears (cable tension)
• Chain falls off the smallest cog (H screw too loose)
• Chain goes into the spokes (L screw too loose: dangerous)
• Will not reach the biggest cog (L too tight or cable too slack)
• Bad shifting in every gear after a fall (bent hanger)

Problems this fixes

• Chain skips or jumps when pedaling hard
• Shifts slowly or won't go into some gears
• Chain falls off
• Shifts by itself

Related jobs

• Adjust rear derailleur (limits + indexing)
• Check derailleur pulley wear
• Replace rear derailleur