A planetary traction drive for a driven turbocharger utilizes angular contact ball ramps to provide variable clamping depending on torque throughput. The ball ramps are located between ring rollers and a ring gear, and function to locate the ring gear concentrically to the ring rollers. The angled contact axes of the ball ramps allows use of a low conformity contact between the balls and ball races in the ball ramps to provide efficient movement, while simultaneously locating ring gear concentrically to the traction rings. A variation is shown where only a single angular contact ball ramp is used between the ring gear and a clamping traction ring, and the other traction ring is rigidly fixed to the ring gear to reduce part count and complexity.