Abstract: A vehicle seat suspension unit has a base frame mountable in a vehicle and seat support frame on which is mounted a vehicle seat. A scissor-arm assembly, with primary links and secondary links, interconnect the frames overlapping cam tracks, with cam profiles engaged by rollers attached to main springs anchored on the base frame. As the frames move towards each other, the main springs are extended as the rollers move along the cam tracks to apply a restoring force to the frames. A preload assembly has rocker arms, pivotally mounted on the cam tracks, with rollers bearing on the seat support frame and provided with preload springs. By adjustment of the cam tracks relative to the base frame, the preload and the main spring rate can be varied to suit the operator's weight.