Repair guides › Drivetrain
Replace rear derailleur
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
- Ball-end hex key set (1.5–10 mm): Park Tool HXS-1.2 Professional L-Shaped Hex Set ($$)
- JIS #2 screwdriver: Vessel Megadora JIS screwdriver #2 ($)
- 1/4" click torque wrench (about 2–17 Nm) + bits: Pro Bike Tool 1/4 inch drive click torque wrench set (2.2–16.9 Nm) ($$)
- Chain tool: Oumers bike chain repair tool set ($)
- Quick-link pliers: Basic master link pliers ($)
- Quick links (match chain speed): Wippermann Connex Link 808 ($)
- Cable & housing cutter: Park Tool CN-10 ($$)
- Cable puller ("fourth hand"): Pedro's Cable Puller ($$)
Compare Good, Better and Best for each tool
Steps
- Shift to smallest cog, release the cable anchor and pull the cable out.
- Break the chain at the quick link, or remove the pulley bolt to slide the chain out.
- Unbolt the old derailleur from the hanger.
- Check the new derailleur's max cog and capacity fit your cassette (the Counter tab does this).
- Thread the new one on by hand first, then torque.
- Route the chain through both pulleys and the cage tab, reconnect it.
- Attach the cable and do the full limit + index adjustment.
Torque
| Fastener | Torque |
|---|---|
| Derailleur mounting bolt | 8–10 Nm |
| Cable anchor | 5–7 Nm |
Diagram
Side view, chain side toward youRear derailleur
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)