Repair guides › Drivetrain
Check derailleur pulley wear
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 ($$)
- Shop rags + nitrile gloves: shop towels and nitrile gloves ($)
- Bike grease: Park Tool PPL-1 PolyLube 1000 ($)
Compare Good, Better and Best for each tool
Steps
- Look at the teeth on both pulley wheels: they should be square-topped. Pointed, hooked or chipped teeth mean the pulley is worn.
- Wiggle each pulley side to side: a little play is normal; a lot of wobble, or a gritty spin, means a worn bushing or bearing.
- Spin each pulley: it should turn freely and quietly.
- Clean them: scrape packed dirt off with a small tool or a rag.
- To service one, remove its bolt and keep the parts in order (bushing, washers). Clean them, put a little grease on the bushing, and refit each pulley in its own place: the upper and lower ones are often different. Torque the bolt to the derailleur’s spec (often 3–4 Nm).
- Replace worn pulleys in pairs, matching the tooth count (usually 11T) and your speed count.
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)