Bicycle Derailer – The correct spelling is bicycle “derailleur” not bicycle derailer. The “derailleur” is the mechanical device that moves the chain from sprocket to sprocket on a gear set and is used to change the gear ratios to best suit cycling conditions. There are usually two derailleurs on a multi-speed bike.
The front derailleur is attached to the frame and fits around the chain near the front gear set. As the chain moves along one of the sprockets in the gear set, the shift lever actuates the front derailleur to move left or right against the chain. This pushes the chain up or down to the desired sprocket. The rear derailleur is a mechanical armature mounted on the frame near the rear axle. This derailleur is spring loaded to keep constant tension on the chain.
The rear derailleur is also actuated by a shift lever and moves the chain up or down the rear gear set.
A set of gear shift levers, which can be a pair of levers or twist grips mounted on the handlebars, will actuate the derailleurs to shift gears by means of wire cables that run from the shift levers to the derailleurs. The shift levers allow the rider to move from gear to gear smoothly while riding. It is important not to shift when the chain is under direct pressure, as this can damage the sprockets or derailleurs.