The Bishop VARIATRONIC speed sensitive power steering valve system is based on rotary valve technology. The boost characteristic is modulated by causing the outer sleeve member to progressively slide axially along the inner input-shaft member by a small distance (typically A mm) as a function of vehicle speed. The axial displacement of the sleeve is achieved via a solenoid actuated ‘flapper’ valve situated in the return port of the valve housing, which raises the hydraulic pressure on the return side of the valve and displaces the sleeve against a reaction force provided by a spring. This axial displacement causes the slots in the bore of the sleeve to mask off a pocketed portion of the roll-imprinted metering edge on the input-shaft, thereby changing the metering area characteristic as a function of valve angle, and hence the valve boost characteristic. Details of the design and operation of the VARIATRONIC concept are discussed, with particular reference to metering edge design and method of sleeve activation. The advantages of the system are discussed in the context of other speed sensitive valve systems already disclosed in the literature or in production. The VARIATRONIC system offers an excellent degree of modulation between high speed mode (maximum stiffness and feel) and parking mode (minimum effort), yet the system is simple, compact and hence low additional cost.