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Torsion beam suspension, also known as a torsion bar or torsion spring suspension, is a vehicle suspension system. 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 the axle of a wheel. Vertical motion of the wheel causes the bar to rotate along its axis and are resisted by the bar's torsion resistance. The effective spring rate of the bar is determined by its length and diameter.
The main advantages of torsion beam suspension are durability, easy adjustability of ride height, and small profile along the width of the vehicle. It provides a longer travel than leaf spring systems, and takes up less of the vehicle's interior volume compared to coil springs. A major disadvantage is that torsion bars, unlike coil springs, usually cannot provide a progressive spring rate, forcing designers to compromise between ride quality and handling ability - progressive torsion bars are available, but at the expense of durability since they have a tendency to crack where the diameter of the bar changes. In most torsion bar systems, especially Chrysler's, ride height (and therefore many handling features) may be adjusted by bolts which connect the torsion bars to the steering knuckles and require nothing more than crawling under the car with a wrench in hand. In most cars which use this type of suspension, swapping torsion bars for those with a different spring rate is usually an extremely easy task.
Some vehicles use torsion bars to provide automatic leveling, using a motor to tighten the bars to provide greater resistance to load and, in some cases (depending on the speed with which the motors can act), to respond to changes in road conditions.
The system was applied to many new armoured fighting vehicle designs during the Second World War. It was used extensively in European cars as well as by Packard in the 1950s. The Packard used torsion bars at both front and rear, and interconnected the front and rear systems to improve ride quality. The most famous passenger-car application was the Chrysler system used beginning with the 1957 model year, although Chrysler's "Torsion-Aire" suspension was only for the front; the same basic system was maintained until the 1981 introduction of the K-car. Light-duty Dodge trucks however continue to use torsion bars on their front suspension.
Some front-wheel drive automobiles use a type of torsion bar rear suspension, sometimes called a twist-beam system, in which the rear wheels are carried on trailing arms connected by a laterally mounted torsion beam. The torsion beam functions both as wheel-locating arm and as an anti-roll bar to resist lateral motion of the wheels as the body leans in turns. Its advantages are that it is inexpensive to manufacture and install, and engages a minimum amount of interior volume, leaving more space for the carriage of passengers, cargo, and other components. Because the torsion bar acts in the lateral plane, not vertically, the twist-beam axle cannot provide ride-height adjustment, and it suffers the same handling limitations as any other beam axle suspension. Twist-beam rear suspensions were pioneered on the Volkswagen Golf in the early 1970s, and remain common on compact cars and minivans.
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