Drivetrain
What is a Rear Derailleur?
The spring-loaded arm that pushes your chain between sprockets.
Simplified schematic — not to scale.
In plain English
The rear derailleur hangs below the cassette and physically pushes the chain sideways onto a bigger or smaller sprocket whenever you shift.
A simple analogy
“It's a little robotic arm, nudging the chain exactly where a cable tells it to go.”
How it works
A cable (or, on electronic groupsets, a motor) moves the derailleur cage sideways. Two small jockey wheels inside the cage keep tension on the chain throughout.
Signs of trouble
- ⚠Chain won't shift into the largest or smallest sprocket
- ⚠Noisy, hesitant or delayed gear changes
- ⚠Bent or twisted-looking derailleur hanging at an odd angle
Rough UK cost
£30 – £400+ depending on mechanical or electronic groupset
Always get a written quote. Prices vary by bike, region and parts brand.
Heads up: Bikeautonomy is for general guidance only. If your bike is behaving oddly or you're not confident, get it looked at by a qualified bike mechanic.