The twist-beam rear suspension (also torsion-beam axle, deformable torsion beam or compound crank) 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 forming a coil-over. In many cases the damper is also used as a restraint strap to stop the arm descending so far that the coil spring falls out through being completely unloaded. This location gives a high motion ratio compared with most suspensions, improving performance.
The longitudinal location of the cross beam controls important parameters of the suspension's behavior, 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. With twist-beam if both wheels compress together their camber and toe will not change. Thus if both wheels started perpendicular to the road and are 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.
This suspension is commonly used on a wide variety of front wheel drive cars (mainly compacts and subcompacts), and was almost ubiquitous on European superminis. When Volkswagen-Audi changed from rear engined RR layout cars to front wheel drive FF layout cars in the mid-1970s, it adopted the system across not just its Audi 50 / Volkswagen Polo supermini, but also the compact-hatchback Volkswagen Golf and Scirocco models.
This type of suspension is usually described as semi-independent, meaning that the two wheels can move relative to each other, but their motion is still somewhat inter-linked, to a greater extent than in a true independent rear suspension (IRS). This can mildly compromise the handling and ride quality of the vehicle. For this reason, some manufacturers have changed to different linkage designs. As an example, in 2004, Volkswagen dropped the twist-beam in favour of a true IRS for the Volkswagen Golf Mk5, possibly in response to its rival, the Ford Focus's "Control Blade" multi-link rear suspension introduced in 1999 - a first use of multi-link suspension in the segment. General Motors in Europe (Vauxhall and Opel) continued to use twist- or torsion- beam suspension up to the end of GM's ownership of the brand. The twist-beam provided a cost saving of €100 per car compared to multi-link rear suspension, although the version used in the 2009-2018 Opel Astra also employed a Watts linkage at a cost of €20 to address the drawbacks and provide a competitive and cost-effective rear suspension.
Other competitors, the Renault Mégane, Peugeot 308 and Citroen C4 also have stayed with the twist beam.
The sportiest models of its brands, such as the Renault Mégane RS and the Peugeot 308 II GTi, have proven that twist-beam rear suspension can provide a high level of performance on a compact car, on the racetrack, but also during the Moose test.
The Peugeot 308 II was able to outperform its competitors equipped with multi-link rear axles by passing this test at 82 km/h. Only 3 km/h less than the Citroën Xantia Activa's world record of 85 km/h in 1999.
Kia Soul is also using twist beam, although the larger Hyundai Elantra (HD) and Hyundai i30 models employed either a torsion beam or a true multi-link independent rear suspension depending on market.