Abstract: A power drive unit for driving a utilization mechanism such as, for example, a roadwheel of a light vehicle such as a bicycle, tricycle, or quadricycle. The utilization mechanism is driven from the output shaft of a prime mover mounted through a controllable clutch and chain drive train. Preferably, the prime mover is an internal combustion engine driven chain saw from which the saw chain and chain bar have beem removed and replaced by an adapter providing a chain and belt drive from the chain saw engine output shaft to a chainwheel coupled to the utilization mechanism, for example, a bicycle conventional chainwheel such as to drive the bicycle rear wheel through the conventional multi-speed hub or multi-speed sprocket cluster, or alternatively, to drive the front wheel of a bicycle through a chainwheel coupled to the front wheel hub, the front wheel hub being preferably a multi-speed hub with internal gears.