Disclosed is a spring for preload in a planetary traction drive designed for a driven turbocharger on an engine. The ring roller of the planetary traction drive has two parts with a spring between the two parts for generating a preload force on the ring roller assembly of the planetary traction drive. The spring provides a spreading force on the ring roller, and allows for setting a desired preload force on the traction surfaces of the traction drive during assembly and operation, even with variations in manufacturing of the different parts of the traction drive.