Vehicle suspension systems

A variable spring rate in an air suspension system is achieved by cross-linking the air springs 24, 25 on either side of an axle and connecting them via a variable flow valve 38. By partially opening the valve 38, the spring rate of one air spring 24 can be reduced by a controlled amount due to the increase in air volume provided by the air in the opposite spring 25. The variable flow valve 38 is controlled by a control unit 32 which computes an optimum air flow for maximum driver comfort based on inputs from driving condition sensors such as wheel height sensors 42, 43, 44, 45 vehicle speed sensor 46 and terrain detection sensor 52.