Abstract: A system for adjusting the speed ratio of nutational traction drive transmissions in which speed ratio is dependent on the spacing of oppositely convergent cone members rotatably supported on a nutatable axis. The system includes a thread shaft having oppositely pitched threads engaged by carrier nuts in turn fixed to the cone members. Relative rotation of the cone members and the thread shaft is effected by alternately engagable clutch bodies at opposite ends of the thread shaft, the clutch bodies being controlled for relative and opposite rotational direction with respect to the cone members by gear sets linking the cone members and its support.