Derailleur
Variable-ratio transmission system commonly used on bicycles
From Wikipedia, the free encyclopedia
A derailleur ( di-RAYL-ər; di-RAYL-yər) is specifically a device that moves a bicycle chain from one sprocket to another, and more generally the entire gearset on a multiple-speed bicycle with such a mechanism. Modern front and rear derailleurs typically consist of a moveable chain-guide that is operated remotely by a Bowden cable attached to a shifter mounted on the down tube, handlebar stem, or handlebar. When a rider operates the lever while pedalling, the change in cable tension moves the chain-guide from side to side, "derailing" the chain onto different sprockets.