Derailleurs are a variable-ratio transmission system which consists of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another. Modern front and rear derailleurs typically consist of a moveable chain-guide that is operated remotely by a Bowden cable attached to a shift lever mounted on the down tube, handlebar stem, or handlebar. When a rider operates the lever while pedaling, the change in cable tension moves the chain-guide from side to side, "derailing" the chain onto different sprockets. The main differences between derailleurs are the gear capacities. These range from 5 speed to 11 speed and the higher quality the derailleur the lighter it will be, the more accurately it will shift and the better it will shift under intense pedaling pressure.
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