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Eccentric
The general term "eccentric" refers to two circles whose center is not at the same point. The primary bicycle usage refers to the special bottom brackets used on better tandems and some other cycles, which allows the bottom bracket to be moved slightly for purposes of fine-tuning chain length.
A tandem synch chain runs between two bottom brackets, and the front bottom bracket is usually mounted in an eccentric. This is a cylindrical part with a large hole running parallel to its axis. This hole is sized and threaded to accept a normal bottom bracket assembly.
The cylinder mounts into an oversized shell located where a normal bottom bracket would be. The outer shell may use pinch bolts or setscrews to secure the moveable eccentric cylinder in position. Some eccentrics use a wedge bolt system, similar to that of a handlebar stem, so secure the cylinder. For more details on this, see my article on Adjusting Tandem Synch Chains.
Eccentric bottom brackets are sometimes used on single bikes too, especially because they permit adjustment of chain tension without moving the rear axle. This is useful if the bicycle is fitted with a rear disc brake or an internal-geared hub.
White Industries makes a rear hub with an eccentric axle, designed to permit use as a fixed-gear or singlespeed without a chain tensioner, on frames with vertical dropouts .
