Torsion beam

The twist-beam rear suspension (also torsion-beam axle or deformable torsion beam) is a type of automobile suspension based on a large H or C shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each stub-axle assembly, on each side of the car. The cross beam of the H holds the two trailing arms together, and provides the roll stiffness of the suspension, by twisting as the two trailing arms move vertically, relative to each other.

The coil springs usually bear on a pad alongside the stub-axle. Often the shock is colinear with the spring, to form a coilover. This location gives them a very high motion ratio compared with most suspensions, which improves their performance, and reduces their weight.

The longitudinal location of the cross beam controls important parameters of the suspension's behaviour, such as the roll steer curve and toe and camber compliance. The closer the cross beam to the axle stubs the more the camber and toe changes under deflection. A key difference between the camber and toe changes of a twist beam vs independent suspension is the change in camber and toe is dependent on the position of the other wheel, not the car's chassis. In a traditional independent suspension the camber and toe are based on the position of the wheel relative to the body. If both wheels compress together their camber and toe will not change. Thus if both wheels started perpendicular to the road and car compressed together they will stay perpendicular to the road. The camber and toe changes are the result of one wheel being compressed relative to the other.

Conceptual model of a twist beam suspension. The green segments illustrate the axle stub centerlines. At rest the axles are in line and the wheels are vertical (Camber = 0 degrees)

The twist beam suspension with the left axle deflected upwards. The deflected wheel now has negative camber. The left and right axles are no longer aligned. The right wheel's camber has changed to positive due to the deflection of the left wheel.

Single wheel deflection (deflection due to roll) vs both wheels up (deflection in bump). Note that when both wheels are deflected the axles remain in line and the wheels have no camber change.

Single wheel deflection shown vs both wheels at rest.

Both wheels shown deflected up (bump) and at rest. Note that the axle halves remain in line and the wheel camber does not change.

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Wikipedia