A torsion bar suspension, also known as a torsion spring suspension or incorrectly torsion beam, is a general term for any vehicle suspension that uses a torsion bar as its main weight bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end terminates in a lever, mounted perpendicular to the bar, that is attached to a suspension arm, spindle or the axle. Vertical motion of the wheel causes the bar to twist around its axis and is resisted by the bar’s torsion resistance. The effective spring rate of the bar is determined by its length, diameter and material.