Torsion beam suspension, also known as a torsion bar, is a vehicle suspension system. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end has a lever, perpendicular to the bar, that is attached to the axle of a wheel. As the wheel travels up and down it "twists" the bar along its axis to provide the spring force. This system was used extensively in European cars as well as by Packard in the 1950's and Chrysler throughout the 1960's.
The main advantages of torsion beam suspension are durability, easy ride height adjustability, and a smooth ride. One major disadvantage is the lack of a progressive spring rate which can lead to sloppy handling.
Some vehicles use torsion bars to implement automatic leveling with sensors and motors to dynamically adjust the torsion in each bar in response to changing loads and road conditions.
A variation on this theme is often called a twist beam suspension, as is found on VW Golfs and other economy cars. It usually consists of trailing arms connecting to a twistable beam that runs from one side of the car to the other. This design's advantages are mainly space and cost, and does not offer the performance or ride height adjustability of torsion bar suspension. You will often see the names for these suspension types used interchangeably in marketing literature and in conversation which can cause a great deal of confusion.
The more commonly used coil spring is merely a torsion bar formed into a helix.
