An eccentric is a cylindrical plug or bearing cup that fits into an enlarged bottom bracket shell. The plug is machined to accept a typical bottom bracket, but offset from the center of the plug, so that by rotating the plug, the location of the bottom bracket (and hence the chain tension) may be adjusted (fore and aft to tension the chain, the upper or lower eccentric position for a given chain length can be chosen to fine tune bottom bracket height). Once properly adjusted the plug is then fixed in place by a pair of set screws, a clamping bottom bracket shell, an expanding wedge in the plug, or the plug may be manufactured in left and right halves that clamp against the faces of the bottom bracket shell with screws that connect the two halves.