Derailleur gears are a variable-ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another. Although referred to as gears in the bike world, these bicycle gears are technically sprockets since they drive or are driven by a chain, and are not driven by one another.
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.
For more information about the choice of particular gear ratios and sprocket sizes, see Bicycle gearing.
Read more about Derailleur Gears: History, Modern Derailleur Types, Use of Derailleurs, Electronic Gear-shifting System
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“Your work is to keep cranking the flywheel that turns the gears that spin the belt in the engine of belief that keeps you and your desk in midair.”
—Annie Dillard (b. 1945)