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How does a derailleur change gears on a bicycle?
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Family bicycling on a country road.
A derailleur is a mechanism that changes gears on a bicycle by physically moving the chain from one set of exposed gears to another.123
The system functions through two primary components:
- Rear Derailleur: This device moves the chain between different sprockets mounted on the rear wheel's hub. It features a spring-loaded pulley designed to maintain proper chain tension by taking up any slack.2
- Front Derailleur: This component shifts the chain between the chainrings (the gears connected to the pedals and crank).12
By shifting the chain to different gear sizes, the rider can adjust the ratio between their pedaling cadence and the rotation of the rear wheel to better match the terrain. While early systems relied on simple levers, modern bicycles typically use trigger or twist-grip mechanisms connected by wire to precisely position the derailleurs. The term itself is derived from the French word meaning to derail, reflecting the process of moving the chain off one gear and onto another.213