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Derailer, Dérailleur
From WikiPedalia
A mechanism for moving the chain from one sprocket to another to change gears on a multi-speed bicycle.
A typical derailer consists of a parallelogram which moves a cage. In the case of a rear derailer, the cage has two chain pulleys, a jockey pulley and a tension pulley. Different derailers have different capacities to handle different gear ranges. Also commonly spelled in the French style "Derailleur."
An extensive article on Derailer Adjustment is available on this site.
Britain's Cyclists' Touring Club Website has a good article on derailer gearing.
