The torsion suspension

the torsion suspension of Peugeot, Citroen

The car torsion suspension is one of the car front or rear axle suspension. In these constructions torsion is used as a resilient member.

In the most cases torsion is a metal bar made from the special kind of steel. Sometimes a kit of long plates or rods, put together and fixed as a fagot to built one side-member, can be used as torsion.

In others more common types of suspension elasticity is achieved with the help of springs or leaf spring, working in compression and bending respectively. Torsion works in twisting, the French word torsion actually means twisting. The basic gearing diagram of the torsion is as follows: one end of the torsion is fixed on the stationary component (rear beam, frame, body), another one is fixed on the movable rear beam lever, which is called equalizing bar. When the movable part is shifting the twisting of the torsion occurs; not bending, but twisting of one torsion end towards another. It can be illustrated on the following example: imagine the treaded end of the bolt viced while one try twisting another end with the screw key.

the torsion

Considering the principle of operation of the spring, working in compression, one may make sure that the spring turns work in twisting, as the torsion does. In other words the torsion is an unwound spring. Deformation of the material is the same in both cases.

The most important advantage of the torsion suspension is its compactness and as follows the possibility of the mass reduction.

The main disadvantage is high cost, caused by the complex technology of the torsion materials processing. The processing technology should secure high elasticity and strength of these units. The special heat processing, called cementation allows reaching of torsion surface hardening. With the help of cementation units get more plastic and elastic because of the hard surface and soft middle of the material.

The unique compactness of the torsion suspension is useful for the constructions with high requirements to the hardness and expenditure, as fighting machines and sport cars do.

Since 1934 torsion beam was used in the light Sweden tank "Landsverk" suspension. Much later the torsion suspension was used in the Soviet tanks. Since 1939 serial tanks Т-40 and KV-1 had the torsion beam. Afterwards the Soviet tank IS-2 had the same torsion beam construction as KV-1.

tank IS-2 had the same torsion beam construction
Pictures and tanks suspension information are from the site

In 20ies they started using torsions in racing cars and in 30ies this construction established a big presence. The torsion beam is used in the suspension of BMW, Citroen, Peugeot, Renault, Alfa Romeo, Lancia since the end of 30ies.

In the civil motor industry the requirements are not that hard, so the modern producers may increase the mass and the unit size for cost saving. But the torsion suspension of Peugeot, Citroen remains popular because of its excellent steerability and comfort. Apart from its compactness, it is achieved with the help of the counteraction to longitudinal and lateral force, which usually influence the suspension.

The torsion beam of Citroen and Peugeot is traditionally used in the most models of PSA concern, the owners of these cars speak well of their running characteristics: steerability, especially on turns – the car is just "stuck" to the road. Besides, because of the torsion suspension compactness, those cars have much room for back passengers and capacious boot. According to these characteristics, these cars are on the same level with more expensive ones.