There is disclosed a continuously variable ratio transmission unit having first and second discs mounted for rotation about a main axis, a toroidal cavity being defined between the discs by facing surfaces thereof and containing a plurality of rollers which serve to transmit drive from one disc to the other, each roller being coupled to a respective carriage through a respective bearing arrangement and each carriage being coupled at a first extremity to a first linear actuator and at a second extremity to a second linear actuator whereby the carriage is movable along a direction transverse to the main axis, the first and second linear actuators working in opposition to exert an adjustable net force on the carriage, the orientation of the carriage being constrained and the bearing arrangement being such as to enable the roller to rotate about a roller axis to thereby transmit drive from one of the discs to the other and also being such as to enable the roller axis to precess relative to the carriage to vary the transmission ratio.