Abstract: A control system for a chain carrier-idler assembly which positions and controls the drive chain shifting from one drive sprocket to another drive sprocket of the multiple sprocket cluster of a derailleur drive rear bicycle wheel mechanism. A chain carrier-idler assembly has its inboard or outboard motion controlled by the positioning of a "floating" gear from a neutral position to an inboard-drive or outboard-drive position. A movable lever arm (driven by a DC motor) carries the floating gear, and also carries a series of electrical contacts which connect and disconnect with contacts on a baseplate parallel to the lever arm, thus to energize or de-energize the motor. Another movable contact (representative of the position of the carrier assembly and drive chain) moves across a series of spaced fixed contacts representative of the location of each sprocket gear. A power source, rocker switch, and connecting circuitry are provided to upshift or downshift the drive chain.