Derailleur

A derailleur is a bicycle gear-shifting mechanism that moves a chain between sprockets on the chainring or cassette, enabling changes in gear ratio. The term typically refers to the rear derailleur, but most setups also include a front derailleur to switch between chainrings. Modern systems may be mechanical (cable-actuated) or electronic and, in some cases, wireless.

Rear derailleur: Mounted to a derailleur hanger at the rear dropout, the rear derailleur uses a parallelogram linkage to move a chain-carrying cage side-to-side. The cage holds two pulleys, the guide and the tension pulley, which keep the chain engaged and tensioned as gears change. Cable actuation from the shifter pulls the derailleur, while indexing aligns the movement with each sprocket. High and low limit screws restrict outward travel to prevent overshift.

Front derailleur: Clamped to the seat tube, the front derailleur moves the chain between inner and outer chainrings. It uses a cage to guide the chain and is controlled by the shifter via a cable or electronic signal. It also employs limit screws to prevent derailment and rub on the frame or chainring.

Electronic shifting and compatibility: Modern systems include electronic groups such as Shimano Di2, SRAM eTap, and Campagnolo EPS, which move the derailleur by a small motor and are paired with compatible shifters and power sources. Compatibility depends on the system family and drivetrain speed (for example, the number of gears in the cassette).

Maintenance and notes: Proper setup requires correct derailleur hanger alignment, cable tension, and limit-screw adjustment. Regular maintenance includes cleaning pulleys, lubricating pivot points, replacing worn components, and ensuring chain, cassette, and derailleur speeds are matched.