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.