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Derailleur

drivetrain

A derailleur moves the chain between sprockets. Bikes can have:

A front derailleur that moves the chain between chainrings (on 2x and 3x setups).
A rear derailleur that moves the chain across the cassette and provides chain tension via a sprung cage.

Derailleurs are precision components — bent hangers, dirty pivots, or worn cable systems all affect shifting quality. Modern derailleurs may also include a clutch that adds tension to the cage to reduce chain slap and chain drops on rough terrain.

Compatibility considerations include speed count (10/11/12), pull ratio (varies between brands), and capacity (the total tooth-difference range the derailleur can handle).