A vehicle comprises a frame and a slide rail pivotally coupled to the frame. A spring member urges the slide rail in a direction away from the frame. The spring member has a first portion mounted to the frame and a second portion supported on the slide rail at a first spring support position in a first configuration. A spring support is coupled to the vehicle at a second spring support position and is arranged to engage the spring member upon deflection of the slide rail in a second configuration. 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.