Variable speed transmission systems

A variable ratio frictional drive gear comprising two axially spaced rotatable torus discs rotatable about a common axis, part toroidal surfaces on the respective axially presented faces of said discs respectively, an input connected to one of said discs, an output connected to the other of said discs, a set of circumferentially spaced drive rollers in frictional rolling contact with the part toroidal surfaces on said discs respectively, bearing structures on which said rollers are mounted respectively, each of the bearing structures being tiltable through a ratio angle with respect to the plane of the rollers when the rollers are parallel to said common axis to vary the distances from the common axis of the discs at which the rollers engage the two torodial surfaces of the discs respectively, means for moving the roller bearing structures tangentially with respect to said axis of the discs by appling a force to said bearing structures, said force being in a direction which is non-parallel with respect to a plane perpendicular to the common axis of the discs and means for accomodating effective movement of the roller bearing structures relatively to said common axis of the discs, in a direction parallel to the common axis of the discs.