Torsion-beam Rear Axle

The torsion-beam suspension shows, as far as the economics are concerned, the last construction of a rear-axle front wheel drive, which actually, cannot be surpassed. It is developed from the trailing arm axle. The first cars to have this axle were the Audi 50 and the Golf I. Not only is it more lightweight than almost all of the others, but also makes fewer demands on the stability of the bodywork. In the meantime, it has been adopted by almost all the manufacturers of cars in the compact range.

Two stable, pressed-steel trailing arms are connected to the car body at only two pivot points. Welded between the two is an, in itself, slightly twistable cross-beam which can appear with vastly differing cross-sections, even inside of one model range. Shown above in figure 1 is the most advanced version, with a U- or V- profile (see also figure 5). Although this cross-beam replaces the stabilizer, in sport versions it can additionally, still be built into the cross-beam (see figure 5).

The VW-Golf, originally equipped with this construction, now has a considerably more complicated multi-link rear axle. The question being asked is whether, because of the costs and the weight, the development should be reversed. In any event, the fact is, that the, currently (2009) leading, Toyota-Prius-Hybrid, with it's relatively low weight, despite the large battery, is also going back to the torsion-beam.

Since it's first appearance, this relatively simple construction has gone through a number of changes. To keep the wheel-housings as small as possible to enable easy loading, the dampers and coil-springs are mounted separately (see figure 6). Because of the fuel tank, the cross-beam has been moved further to the rear. Rubber bearings have been inserted into the two pivot points with different rigidity for the front and for the rear (see figure 2). Thus, when the springs compress, the entire axle can countersteer a little. This must be observed when replacing the bearings.

Actually, it was a little revolutionary, when in 1974, the first torsion-beam rear axle appeared. Thereby, VW, who was anything but the leader as far as front wheel drive was concerned, placed themselves on the pole position from the word go. The axle did in fact, like a number of VW inventions at that time, come from Audi. Here, because of the shorter wheel-base and the position of the tank, the cross-beam is shifted a little to the rear, this is why we speak of a twist-beam suspension.

It evolved from the trailing link axle, as was used, e.g., in French front wheel drives and of course, even earlier, it was the standard in British cars. The principle of the cross-beam itself, but not the shape, comes from a predecessor rear axle from Audi, this was used at the time of the DKW (F-series), indeed, there it was mounted diagonally into the rear axle parallelogram.

This cross-beam replaces not only the internal storage of the two trailing arms, but also, at least in the lower performance models, the stabilizer. It is essential in a front wheel drive because, in contrast to the engine-weighted front axle, it artificially weakens the rear axle. Thus, a well balanced driveability and, in addition, a lower tendency to tilt is achieved. The cross-beam reacts to the eneven spring compression of the two rear wheels. By the way, as simple as the construction looks, in the beginning there were difficulties with the manufacture, because the various possible coilings had to produce the identical spring strength, otherwise the car would react differently in right-hand- and left-hand curves. 06/11