PURPOSE: To provide a planetry friction driving device that has a good mechanical efficiency and a simple design.

CONSTITUTION: A planetary friction roller drive 30, 66, 84, 26 is adapted particularly for use in driving automotive accessories such as an alternator. The driving device delivers engine torque to the ring roller element 26 and connects drivably the sun roller element 30 of the friction drive to a driven accessory such as an alternator. A torque flow path characterized by two speed ratios is effected by a compound planetary carrier assembly 50 as a preload at a friction drive contact locations between the planetary rollers 66, 84 and the ring roller element 26 and between the planetary rollers 66, 84 and the sun roller element 30 of the planetary assembly is established by a preload spring 134 acting in a tangential direction. The reaction torque for the planetary friction drive is established by a selectively engageable brake 136 acting on the sun roller element 30 and thus effects a ratio transition from unity.