Eccentric (mechanism)

An attached eccentric rod is suspended in such a way that its other end can impart the required reciprocating motion.

Unlike a cam, which also converts rotary into linear motion at almost any rate of acceleration and deceleration, an eccentric or return crank can only impart an approximation of simple harmonic motion.

The term is also used to refer to the device often used on tandem bicycles with timing chains, single-speed bicycles with a rear disc brake or an internal-geared hub, or any bicycle with vertical dropouts and no derailleur, to allow slight repositioning, fore and aft, of a bottom bracket to properly tension the chain.

[2] They may be held in place by a built-in wedge, set screws threaded into the bottom bracket shell, or pinch bolts that tighten a split bottom bracket shell.

[3] As a standard sized bottom bracket threads into the eccentric, an oversized bottom bracket shell is required to accommodate the eccentric.