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Idler
From WikiPedalia
An idler is a pulley or roller that does not produce any mechanical advantage, nor transmit power to a shaft. Idlers are used to lead a chain around a bend (as in short wheelbase recumbents), or to take up slack in a drive chain (as in a tandem chain tensioner, or the pulleys in a derailer. Also see our article on how to make and idler out of a freewheel.
