A Torsion bar suspension, also known as a torsion spring suspension, is 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, the torsion key, mounted perpendicular to the bar, that is attached to a suspension arm, a 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, cross section, shape, material, and manufacturing process.
A torsion bar is a flexible spring that can be moved about its axis via twisting. Torsion bars are designed and based on the amount of torque used in the twisting of the spring, the angle of the twist, the overall dimensions of the torsion bar and what materials the torsion bar is made from. The most common place to find a torsion bar is in the suspension of a car or truck, in machines used for production or in other precision devices. The flexibility of the spring is the main reason that a torsion bar is used. If a more rigid structure were used such as a steel rod, too much load bearing pressure would be placed on both the wheels and the underbody of the vehicle.
A torsion bar works by resisting the torque placed on it. When one end of the torsion bar is affixed to an object that cannot be moved, the other end of the bar is twisted, thus causing torque to build up. When this happens, the torsion bar is resistant to the torque and will quickly go back to its starting position once the torque is removed. In general, the object that cannot be moved is usually a frame. If there is not any force applied to the torsion bar, it will stay at the same position until force is applied.