A drive unit for a bicycle with an electric motor or a geared motor that may be affixed to a frame tube of the bicycle by means of a holder and with a toothed belt for transmission of the torque onto a sheave attached on a wheel of the bicycle. On its side that is turned away from the spokes, the sheave is provided with one or several radial projections to prevent the belt from running off. The sheave having a central opening is centered for concentricity with the axle of the wheel and is attached to the wheel spokes of the bicycle.