Join Date: Sep 2005
Location: New Orleans
Bikes: Cannondale Road Tandem, Rodbikes Belt Drive Fixed Gear
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From Sheldon Brown
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 wheel base recumbents), or to take up slack in a drive chain (as in a tandem chain tensioner, or the pulleys in a derailer "
Like on a fixed gear bicycle, the timing chain on a tandem must remain fairly tight all the way around. If there is much slack, the chain would fall off. In cheaper, homebuilt, or older tandems, an idler pulley is an easy way to achieve the desired tension. The other option, which most quality tandems have, is an eccentric bottom bracket. This eccentric allows the bottom bracket to be rotated to tighten the chain tension.