Derailleur

A derailleur is a specialized mechanical device used to move a bicycle chain between different sprockets. This mechanism allows a cyclist to change gears, making it easier to climb hills or go faster on flat ground.

The term comes from the French word "dérailleur," which means "derailer." This name originally described a tool used to knock a train off its tracks. On a bicycle, however, the device is essential for managing the drivetrain.

Modern derailleurs typically function through a moveable chain-guide. This guide is operated remotely using a Bowden cable. The cable connects to a shifter located on the handlebars or the down tube. When a rider operates the shifter, the tension in the cable changes. This change in tension moves the chain-guide from side to side. This movement "derails" the chain, forcing it onto a different sprocket.

The rear derailleur performs two distinct roles. First, it moves the chain between the rear sprockets. Second, it acts as a tensioner to manage chain slack. When a rider shifts to a smaller sprocket, the chain becomes loose. The rear derailleur uses a spring-loaded cage to take up this extra length. This cage contains two small wheels called pulleys. The top pulley is the jockey pulley, and the bottom is the tension pulley.

Cycling history shows a long evolution of these mechanisms. In the late 19th century, early versions like the Protean two-speed were built. In 1905, Paul de Vivie invented a two-speed rear derailleur for his trips through the Alps. Early systems often used metal rods to move the chain. By 1937, the Tour de France introduced derailleurs to the race. This was a major change because riders no longer had to dismount to change wheels.

Technological breakthroughs have shaped modern riding. In 1964, Suntour invented the slant-parallelogram rear derailleur. This design allows the jockey pulley to maintain a constant distance from the sprockets. Most modern rear derailleurs still use this pattern. In 1985, Shimano introduced indexed shifting. Unlike friction shifting, which requires manual adjustment to center the chain, indexed shifting uses a ratchet mechanism. This mechanism stops the lever at specific distances for precise gear changes.

Today, the industry is dominated by three main manufacturers: Shimano from Japan, SRAM from the USA, and Campagnolo from Italy.

These companies use different standards for their shift ratios. Shimano systems often use a 2:1 ratio, where a unit of cable movement causes twice as much derailleur travel. SRAM often uses a 1:1 ratio, meaning cable movement and derailleur movement are nearly equal.

Some high-end mountain bikes even use electronic shifting. In these systems, a battery and a small electric motor move the derailleur instead of a manual cable.

Gear capacity and cage length are important technical factors. The cage length is the distance between the two pulleys. This length determines how much chain slack the derailleur can manage. A longer cage is often needed for mountain bikes with many gears. A shorter cage is common for road bikes to reduce weight and improve shifting precision. Some mountain bike derailleurs also include a clutch. This clutch keeps the chain tight to prevent "chain slap," which is when the chain hits the bike frame.