Derailleur

A bicycle mechanism that moves the chain between sprockets to change gears. Front derailleurs shift among chainrings, while rear derailleurs guide the chain across a cassette or freewheel.

Rear derailleur: A derailleur that shifts the chain among the sprockets on a bicycle’s rear cassette or freewheel. Its pulley cage moves sideways to align the chain with each rear sprocket.
Bicycle gear: A combination of chainring and rear sprocket that sets a bicycle’s mechanical advantage. Derailleurs change gears by moving the chain among available sprockets.
Bicycle hub gear: A bicycle transmission that changes ratios through mechanisms enclosed inside the rear hub. It shifts internally rather than moving the chain between rear sprockets.
Mountain bike: A bicycle designed for riding on rough, unpaved terrain. Mountain bikes commonly use rear derailleurs with wide gear ranges and chain-control features.
Front derailleur: A derailleur that shifts the chain among a bicycle’s front chainrings. It pushes the chain laterally across the chainrings, complementing rear shifts.
Chainring: A toothed ring attached to a bicycle crank that drives the chain. The front derailleur shifts between chainrings of different sizes.
Continuously variable transmission: A transmission that varies its ratio continuously across a range rather than selecting fixed gears. Its ratio changes without the derailleur’s discrete sprocket choices.
Road bicycle: A bicycle designed primarily for riding on paved roads. Road bicycles often use derailleurs to provide closely spaced ratios for varied speeds.
Bicycle chain: A linked metal assembly that transmits pedaling force between bicycle sprockets. The derailleur shifts this flexible loop without disconnecting it.
Freehub: A rear-wheel hub mechanism that allows coasting and carries a cassette. It supports the sprockets that a rear derailleur selects.

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