Twist-beam rear suspension

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The Twist-beam rear suspension or Torsion beam 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 twist beam suspension can be used as the basis for a De Dion suspension, by mounting the differential to the body, and adding two half shafts. This is seen in some small Volvo 300 series made by Volvo in Holland (formerly DAF).^{[citation needed]}

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.

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,^{[citation needed]} 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.

[edit] 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

[edit] 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