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Twist-beam rear suspension
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The Twist-beam rear suspension is a type of automobile suspension based on a large H shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each wheel, 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, or behind, the wheels.
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Encyclopedia
The Twist-beam rear suspension is a type of automobile suspension based on a large H shaped member. The front of the H attaches to the body via rubber bushings, and the rear of the H carries each wheel, 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, or behind, the wheels. 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.
This suspension is used on a wide variety of front wheel drive cars, and was almost ubiquitous on European superminis. It was probably introduced on the Audi 50, which was rebadged as the Volkswagen Polo.
This 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 IRS. This limits the handling of the vehicle, and VW have dropped it in favour of a true IRS for the Golf Mk V in response to the Ford Focus' Control Blade rear suspension.
Advantages
- Can be durable
- Simple
- Neat package, reduces clutter under floor
- Fairly light weight
- Springs and shocks can be light and cheap
- No need for a separate sway bar
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 (steer effects)
- 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
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