Abstract: A drive mechanism is disclosed that is selectably engageable to provide a bicycle that has normally driven and nondriven wheels with either single or dual wheel drive. The drive mechanism includes a power take off means that has: a first power transmission mechanism mounted on said frame; a second power transmission mechanism mounted on the bicycle frame in positive drive power transmitting engagement with a normally non-driven wheel of the bicycle; and a power transfer apparatus connected to receive power from the first power transmission mechanism and transmit it to the second power transmission mechanism. A coupling mechanism is operatively connected with the power take off and is selectively actuatable to place the first power transmission mechanism either in a drive position receiving power from the normally driven wheel or a disengaged non-power transmitting position.