The suspension design requirements for vehicles that need both good on-road and off-road capabilities as well as vehicles where the payload changes significantly during operation is quite challenging. To overcome the ride comfort vs. handling compromise that most vehicles suffer from some suspension systems offer adjustability and control to overcome this conflict. The uptake of controllable dampers on commercial vehicles, allthough cost effective and technically feasible, have been low mainly because dampers on their own cannot significantly reduce the compromises involved with fully laden vs. empty or on-road vs. off-road use. The University of Pretoria developed a Four State Semi-Active Suspension System (4S4) that allows for the control and adjustability of the stiffness and the damping of the suspension. The 4S4 makes use of solenoid valves of which the response time unfortunately increases with an increase in flow, especially when implemented on larger vehicles, reducing the effectiveness of the control.

This paper presents an alternative to the solenoid valves currently used in the 4S4 in the form of a magneto-Rheological (MR) valve that acts as a normal continuously variable MR damper, but also has the ability to virtually block the flow of fluid, thus switching between the two different spring characteristics. Experimental results show that it is indeed possible to replace the solenoid valves with a MR valve.

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