Abstract: A drive unit for varying the transmission ratio between two parallel rotatable shafts is provided with a disk mounted on each shaft for rotation therewith, the disks being formed with a plurality of radial grooves in which respective bearings are provided. Each bearing is provided with a wheel, at least one of which on each disk is engaged by an endless chain, the bearing being radially displaceable in the grooves by a nut provided in each bearing which is limitedly radially displaceable relative thereto and is in engagement with a rotatable radial screw provided in each groove for displacing the nut. Between the nut and the bearing there are provided respectively a compression spring and cam means for enabling the nut to be displaced by the rotating screw when the respective wheel of the respective bearing is engaged by the chain without the bearing being displaced, the bearing being displaced upon disengagement from the chain.