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    Mountain bike (MTB) rear derailleurs

    A rear mech, or derailleur, is the device at the back of a bike that moves the chain from sprocket to sprocket on the cassette. It consists of a sprung parallelogram body and lower cage with jockey wheels that take up the chain slack differences in different gear ratios. On mountain bikes rear mechs are made by two companies: Shimano and Sram. Although they look virtually the same Shimano and Sram mechs are not interchangeable. Sram mechs require Sram shifters. Shimano mechs require Sram shifters. Two recent innovations in mountain biking are the low profile (Shadow) hanger and the clutch. Low profile or Shadow mechs don't stick out as far as previous mechs. This makes this less susceptible to trail damage. Clutch mechs have very strongly sprung cages that don't bounce around as much over rough ground. This helps prevent chain loss.