A suspension system includes a torsion bar attached to a vehicle frame member by an adjuster assembly including a spring biasing anchor arm. The biasing member provides an elastic resistance to torsion bar rotation for suspension system vibrations below a predetermined torsional force. However, when the predetermined torsional force is exceeded, the elastic resistance of the biasing member is overcome and the anchor arm rotates into contact with the stop. Further suspension system vibrations are absorbable by the torsion bar with stiffer resistance to provide a stiffer suspension response. The stiffer suspension is thus provided when required, such as when cornering, while allowing a smooth ride when not required, such as during normal straight travelling.