Method and apparatus for controlling an infinitely variable transmission of a motor vehicle

In a motor vehicle system having an infinitely variable transmission which includes an endless member running over primary and secondary pulleys each formed of a pair of conical discs at least one of which is axially movable relative to the other disc of that pair and said primary pulley being driven by motor vehicle engine:

• apparatus for controlling the transmission ratio of said transmission comprising:
• hydraulic cylinders for moving an axial movable disc of each pulley;
• a hydraulic valve for operating said cylinders, said valve having an internal axially movable spool the axial position of which determines the transmission ratio of said transmission;
• means responsive to engine speed applying an axial force to said spool in one opposite direction;
• a compression spring applying a biasing force to said spool in an opposite direction, the biasing force being varied by axially movable actuating means engaging said spring;
• and control means responsive to an operating variable of the engine and to the actual transmission ratio of said transmission for controlling the position of said actuating means in a mode to increase and decrease the biasing force of said compression spring in accordance with the actual transmission ratio during at least a portion of the range of operation of said control means.