A vehicle comprises a frame and a slide rail disposed below the frame for guiding a drive track along a road surface. An arm has a first portion pivotally coupled to the frame at a frame pivot location and a second portion pivotally coupled to the slide rail at a rail pivot location. Vertical movement of the slide rail causes a length of the arm between the frame pivot location and the rail pivot location to extend or contract. The vehicle suspension system becomes increasingly stiffer as the deflection of the slide rail increases, to contribute to a more comfortable ride and to reduce the risk of bottoming-out.