Abstract: An improved torsion type rear suspension system for a snowmobile. The suspension includes a pair of rails movable with respect to the snowmobile body and a pair of torsion springs disposed between the body and the rails for biasing the suspension toward its extended position. Also included is a cam block having a contoured cam surface which engages a leg of the torsion spring at a variable point of contact. As the suspension moves from the extended to the collapsed position, the effective length of the spring leg is reduced, resulting in a nonlinear change in spring force as a function of deflection. The cam block may be adjusted with respect to the suspension rails with an eccentric to permit additional adjustment of the spring force function to compensate for varying terrain, vehicle speed, and operator weight.