The twist-beam rear suspension is a type of automobilesuspension 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.
Functioning
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.
Uses
This suspension is commonly used on a wide variety of front wheel drive cars, and was almost ubiquitous on European superminis. The Renault 5, launched in 1971, was the first such car to employ a twist-beam suspension. Three years later, 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. 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 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 Megane and Citroen C4 also have stayed with the twist beam.. Kia Soul is also using twist beam, although the larger Hyundai Elantra and Hyundai i30 models employed either a torsion beam or a true multi-link independent rear suspension depending on market.
Advantages
Low cost
Can be durable
Fewer bushings than multi-link suspension that are less stressed and less prone to wear
No need for a separate anti-roll bar - the axle itself performs that function
Disadvantages
Basic toe vs lateral force characteristic is oversteer
Since toe characteristics may be unsuitable, adding toe-control bushings may be expensive.
Camber characteristics are very limited.
Not very easy to adjust roll stiffness
Welds see a lot of fatigue, may need a lot of development
Not much recession compliance - can be poor for impact harshness, and will cause unwelcome toe changes
Wheel moves forward as it rises, can also be poor for impact harshness
Need to package room for exhaust and so on past the cross beam
Camber compliance may be high
No redress for wheel alignment. Alignment geometry is factory-set and not generally adjustable. Any deviation from factory specifications/tolerances could mean a bent axle or compromised mounting points.
