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Anti-Roll Bars and Torsion Beam Rear Suspensions, Part 2

increasing rear roll stiffness - for under $50!

By Julian Edgar

Click on pics to view larger images


This article was first published in 2011.

Last issue in Anti-Roll Bars and Torsion Beam Rear Suspensions, Part 1 we looked at torsion beam rear suspension systems and found that in this design, the beam itself acts as an anti-roll bar. Sometimes the torsional stiffness of this beam (ie, how strong it is an anti-roll bar) is provided by just the open ‘U’ or ‘V’ shaped section, while in other cars the factory adds an internal bar that stiffness it further.

We also looked at how in most front-wheel drive cars, increasing rear roll stiffness results in a better-handling car with greater throttle control and less roll.

So how do you increase the rear roll stiffness of a car with torsion beam rear suspension? Let’s take a look at what is commercially available and then also provide a cheap, DIY solution.

Aftermarket Anti-Roll Bars

The most common approach to increasing rear roll stiffness is to add a traditionally-shaped anti-roll bar. This is a long U-shaped bar where the ends of the arms of the bar are attached to the trailing arms of the suspension at the shock absorber mounts, or by the use of new mounts. The straight part of the bar swivels in plastic or rubber D-shaped bushes that are clamped to the torsion beam.

Depending on where the torsion beam is placed within the ‘H’ that forms the suspension design, and depending where on the torsion beam the new anti-roll bar is clamped, during body roll this design places the new transverse bar in both torsion and bending. Either way, it resists one wheel rising (or falling) to a greater amount than the other and so resists body roll. (The bending loads are often absorbed by bush deflection.)

This type of anti-roll bar has some deficiencies. Depending on the design, these can involve:

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High addition to unsprung weight - the bar is a large, thick piece of steel!

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Lower ground clearance, especially if the bar bolts on under the shock absorber mounts.

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Doubtful strength in the brackets that clamp to the torsion beam – for example, some manufacturers suggests that “for racing use”, this type of clamp-on bracket should be spot-welded to the beam.

However, there are also major advantages to this approach. They include:

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Easy adjustability through the use of multiple bolt positions for the attachment points to the trailing arms.

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No requirement to drill holes during installation, and so seamless removal if required. This approach also places little additional stress on the original suspension arms.

Another, different, approach to aftermarket bars is to fit one inside the torsion beam. If the factory already fitted an internal torsion bar to stiffen the beam, the aftermarket one generally bolts in place parallel to the original.

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For example, one such aftermarket anti-roll bar uses a straight piece of spring steel bar with two tubular mounting bars positioned at each end. Drill four holes in the torsion beam and it can be bolted into place.

Advantages of this approach include:

- Smaller addition to unsprung weight

- No reduction in ground clearance

Disadvantages include:

- Requirement to drill holes (apparently the torsion beam is usually a very hard steel that’s difficult to drill)

- Places greater loads on the welds of the original torsion beam suspension

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In a similar way, if the model that you’re dealing with came out without an internal anti-roll bar, but other factory models had one fitted, the factory bar can be sourced and bolted into place. Here the factory holes were present and an aftermarket bar was fabricated to suit them.

Another way

But there is another way of increasing rear roll stiffness. It uses clamps to bridge the otherwise open section of the U-shaped axle, so stiffening it.

So how does this work? It’s easiest to see by making a simple torsion beam model from cardboard and then adding cardboard strip “clamps” across the open face of the ‘V’.

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When the model axle is torsionally twisted, the cardboard ‘clamps’ no longer stay at 90 degrees to the beam but are forced to adopt angles to it. In other words, if the clamps are strong enough to hold the open edges of the torsion beam’s V parallel to one another, the torsional stiffness of the beam is increased.

Clamps that will achieve this can be made in different designs.

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Here is a V-shaped design bent from 10mm threaded steel rod and using 40 x 5mm steel plate across the ‘flats’ of the torsion beam.

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And here is another design of clamp developed using U-shaped clamps from the mounts used to hold truck and trailer axles to leaf springs.

Testing clamps

I made a trial clamp, bolted it to the rear axle of my Skoda Roomster and then went for a drive. The understeer was clearly reduced.

I bolted on two of the clamps and went for another drive. This time the understeer was markedly reduced, to the point where through a slow speed dirt corner, the car would drift with both the front and back sliding together. Go in more quickly and it would understeer; back-off sharply and it would oversteer. (Obviously with the stability control switched off!) That was radically better than standard.

The advantages of the clamps are clear: low mass, cheap, easy to attach and remove, and their effect can to some extent be made adjustable by adding or subtracting clamps.

But what are the downsides of this approach?

As with the aftermarket anti roll bar that fits within the beam, the clamps place greater stress on the rest of the structure, particularly the welds joining the trailing arms to the beam. In addition, the flexing behaviour of the beam’s metalwork is changed. This may concentrate bending loads in a way that does not occur with the standard beam – for example, bending loads may be concentrated at the clamps. This could potentially result in stress cracks forming at these points.

Conclusion

There are at least four ways in which the anti-roll properties of torsion beam rear axles can be increased.

- External anti-roll bar (added where none existed)

- Internal anti-roll bar (added where none existed)

- Increased diameter internal anti-roll bar (replaces factory bar)

- Clamps placed around beam axle to stiffen it in torsion

So if you want to upgrade rear roll stiffness and you have a torsion beam suspension, you could start by experimenting with something as simple and cheap as some muffler clamps…

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