A torsion bar is a long, alloy-steel bar, fixed rigidly to the chassis or sub-frame, at one end, and to the suspension control arm at the other.
The bar is fitted to the control arm in the unloaded condition, and as the control arm is raised, the bar twists around its center, which places it under a torsional load.
When the vehicle is placed on the road, with the control arm connected to the suspension assembly, the bar supports the vehicle load, and twists around its center, to provide the springing action.
Spring rate depends on the length of the bar, and its diameter. The shorter and thicker the bar, the stiffer its spring rate.
Torsion bars can be used across the chassis frame on the same principle, in a trailing arm suspension, or as part of the connecting link between two axle assemblies, on a semi-rigid axle beam.
After a lot of use, a torsion bar can sag. On many vehicles, it can be adjusted to allow for this.
It is used in light vehicles as a stabilizer, or anti-roll bar, connected between each side of the suspension on the front, and sometimes the rear.
When the vehicle is turning, centrifugal force acts on the body, and tends to make it lean outwards. The anti-roll bar, or stabilizer, tries to use its connections to each side of the suspension, to resist this roll tendency.
