Torsion bar

2.5.2. Torsion Bar / Torque Rod

Torsion bar, rod or bar that resists twisting and has a strong tendency to return to its original position when twisted.

In automobiles a torsion bar is a long spring-steel element with one end held rigidly to the chassis-frame and the other end twisted by a lever, the torsion key, mounted perpendicular to the bar, that is attached to a suspension arm, a spindle, connected to the axle.

Torsion bars are long steel rods of either circular or square cross section. The springing action is generated by the torsional forces when the torsion bar is twisted.

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. 

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.

Fig: Torsion bar / Torque rod

Torsion bar suspensions are used on combat vehicles and tanks like the T-72, Leopard 1, Leopard 2, M26 Pershing, M18 Hellcat, and the M1 Abrams (many tanks from World War II used this suspension), and on modern trucks and SUVs from Ford, Chrysler, GM, Mitsubishi, Mazda, Nissan, Isuzu and Toyota.

The main advantages of a torsion bar suspension are durability, easy adjustability of ride height, and small profile along the width of the vehicle. It takes up less of the vehicle's interior volume than coil springs.

A disadvantage is that torsion bars, usually cannot provide a progressive spring rate. In most cars with this type of suspension, swapping torsion bars for a different spring rate is usually an easy task. Longitudinal torsion bars extend under the passenger compartment, cutting into interior space by raising the floor, while in transverse systems, torsion bar length is limited by vehicle width.