Parts of a Rear Derailleur – Labeled Bike Gear Diagram

All 11 parts on the diagram

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From the side 9

Cage It is spring-loaded and swings to take up slack in the chain. It does so when the chain is on a smaller sprocket.
Guide pulley (jockey pulley) It leads the chain onto the sprocket the rider has picked. It sits just under the sprockets. It is also called the jockey pulley.
Tension pulley The chain wraps under it. From there it goes back to the front of the bike. With the guide pulley it bends the chain into an S shape.
Sprockets (cogs) The derailleur moves the chain from one to another. A bigger sprocket is an easier gear.
Chain It comes down off a sprocket and over the guide pulley. Then it goes under the tension pulley and runs forward again.
Derailleur hanger It is part of the frame, by the dropout. On many bikes it is a separate piece that can be replaced if it bends.
B-tension screw It sets how far the guide pulley sits from the sprockets. It is also called the B screw or the body-angle screw.
Barrel adjuster Turning it makes small changes to the cable. That lines the guide pulley up under each sprocket.
Shift cable The shifter on the handlebar pulls it or lets it out. That moves the derailleur under the sprockets.

From behind 2

Parallelogram They swing like a gate. They carry the cage sideways under the sprockets while keeping it lined up with the chain.
Limit screws They set how far the derailleur can move in and out. The chain then cannot go into the frame or the spokes.

How this list was compiled: Part names checked on 2026-09-30, a name kept only when two of these use it for the part. (1) Wikipedia, text only: Derailleur (cage, jockey or guide pulley, tension pulley, parallelogram, sprockets, chain, derailleur hanger, shifter and cable). (2) Park Tool repair help, Rear Derailleur Adjustment, https://www.parktool.com/en-us/blog/repair-help/rear-derailleur-adjustment (limit screws, B screw or body-angle screw, barrel adjuster, guide pulley, cage, cogs, derailleur hanger). (3) Sheldon Brown and John Allen, Derailer Adjustment and Maintenance, https://www.sheldonbrown.com/derailer-adjustment.html (limit-stop screws on the back of the parallelogram, B-tension adjustment, adjusting barrel, jockey and tension pulleys, hanger, parallelogram linkage, sprockets). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry, a composite of no one make; nothing traced. Licence: own work

How it works

A rear derailleur moves the chain from one sprocket to another on the back wheel and takes up the slack in the chain. These two drawings name 11 parts, from the hanger it bolts to down to the tension pulley.

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Two views

The first view is from the side of the bike, front to the right. The derailleur hangs from the hanger under the rear axle, and the chain comes down off a sprocket, goes over the guide pulley, under the tension pulley, and away to the front. The second view is from behind the bike, where the parallelogram and the limit screws show.

How it changes gear

The shifter on the handlebar pulls the shift cable or lets it out. The cable moves the parallelogram, which swings the cage sideways under the sprockets, and the guide pulley leads the chain onto the next sprocket.

The barrel adjuster makes small changes to the cable, to line the guide pulley up under each sprocket.
If the hanger is bent, the other adjustments will not work, so a mechanic looks at it first.

Questions and answers

What are the parts of a rear derailleur called?

A cage that holds two pulleys, the guide pulley on top and the tension pulley below; a parallelogram that swings the cage sideways; limit screws, a B-tension screw and a barrel adjuster; and the hanger it bolts to.

What does a rear derailleur do?

Two jobs. It moves the chain from one rear sprocket to another, and its spring-loaded cage takes up the slack in the chain when the chain sits on a smaller sprocket.

What are the two small wheels on a derailleur called?

Pulleys. The top one is the guide pulley, also called the jockey pulley, and the bottom one is the tension pulley. The chain runs around them in an S shape.

What do the limit screws on a derailleur do?

They stop the derailleur from moving too far in or too far out, which keeps the chain out of the spokes and the frame. They do not move the derailleur; they only set where it stops.