Rear Sway Bars: Improving FWD Handling

This article was first published in 2003.

One of the cheapest and easiest ways of improving the handling of a front-wheel drive car is to fit (or upgrade) the rear sway bar. If you do this using factory parts from a wrecker, it’s a modification that will likely cost you less than AUD$75, have OE factory quality, and be a perfect fit. And even an aftermarket rear bar will still be a cheap upgrade. No aftermarket bar available for your car? It’s straightforward to have one custom made.

And the difference to handling? It can be worth a lot more than every cent...

Sway Bars?

Sway bars (sometimes called anti-roll bars or stabiliser bars) reduce body roll. They do this by linking the two sides of the car – either at the front or at the rear. In the case of a rear sway bar, the roll of the car is resisted by the torsional (twisting) action of the sway bar that attempts to lift the inside rear wheel. In other words, a sway bar is effectively an extra spring that connects the left and right wheels together at one end of the car.

Put too stiff a sway bar on and in vigorous cornering, the inside wheel will be lifted right off the ground. (In fact, you can actually see this occurring at the back in front-wheel drive showroom classes of racing.)

However, sway bars can make an enormous improvement to how a car handles. Purists will suggest that sway bar tweaking should be the last step in a handling package that includes springs, dampers (shocks), bushes, etc, but if you’re on a budget, they can be an excellent first step. That’s especially the case if you’ve got a front-wheel drive that feels like it’s leaning all its cornering weight on the front outside wheel. By reducing roll, the outside tyre is less likely to be overloaded and so will grip better.

Even better, in a front-wheel drive car, fitting the sway bar to the rear keeps the cornering weight distribution on the front wheels more even, allowing them to better get their power down and so reducing power-understeer. (You fit the sway bar on the back cos it doesn’t matter much if the inside rear wheel goes light – it’s not doing a whole lot of work anyway.)

Going Too far? Most manufacturers of front-wheel drive cars keep the rear roll stiffness fairly soft. But if stiff rear sway bars are good for handling, why do they do this? Too stiff a rear bar in a front-wheel drive will result in a much higher likelihood of a spin if you lift-off mid-corner. A soft rear bar also results in plenty of understeer – something which is good for safety in the averagely-driven commuter car. However, if you don’t go to extremes, it’s quite possible to gain better turn-in and much improved mid-corner balance with a bigger rear bar. (Or the installation of one where none previously existed!) You’ll also find that the car can be much better throttle-steered – back-off the throttle a little and the rear will come out gradually; get back on the power and the car will follow the cornering line. The acid test is cornering on a wet road with a load in the back – you don’t want the rear so stiff that a slight throttle lift will cause the car to spin. But to get this effect in an otherwise standard car, you usually have to make a radical difference to rear sway bar size. Note that a front-wheel drive with an over-stiff rear sway bar (or more correctly, too high a total rear roll stiffness) will feel great up to 8/10ths – turn-in will be sharp and the car will sit flat. But go that extra step and you can be bitten. In standard cars we’ve only ever experienced this once – but that car could spin literally in its own length.... see the end of New Car Test - Piloting the 1999 Hyundai FX Coupe. We don’t want to frighten you – make the modification with common sense and you’ve nothing to worry about. But there is a reason that 99 per cent of manufacturers don’t go over-stiff with the rear... and it’s something you should know about.

Selecting a Rear Bar

The first step in deciding what to do is to get down and dirty - lift your car onto jackstands then have a really good look at the rear suspension.

If your car runs an independent rear, the sway bar will be the only bar than connects the wheels. It will be mounted in D-rubbers to the body and then have crank arms connecting to the suspension near each wheel. If your car uses a torsion beam rear axle (now almost universal on budget front-wheel drives), you’ll see a big, U-shaped tube connecting the two wheels. The sway bar is the metal rod that’s mounted within the U of the tube.

When making your inspection you’re looking for two things:

1)       That the car actually does have a rear sway bar

2)       The diameter of the bar

The latter can be measured with a cheaply available digital calliper. The sway bar diameter is important because its stiffness goes up very quickly with an increase in diameter. For example, a sway bar with a 26mm diameter is almost twice as stiff as one with a diameter of 22mm. A few millimetres difference can make a dramatic difference to how stiff the bar is.

Even more dramatic is what happens when you fit a rear bar where before none existed!

If the car already has a sway bar, you’ll be looking at upsizing it. A wrecker is a good place to check the diameters of the standard rear bars used on other versions of the model, or you can dive under parked cars on the street with digital callipers in hand. Also don’t forget that an identical suspension may be used on other models within the manufacturer’s range, even if the bodywork looks different. Bigger bars can be most commonly found on sports versions of the car.

Factory standard swaybars are usually very cheap, so if you can find one that is a little bigger in diameter, buy it and fit it. If nothing is available ex-factory, check what’s available aftermarket. As this is the only suspension change that you’re making, don’t go ballistic in diameter increase – check the ‘Calculating Sway Bar Stiffness’ breakout box for some figures that can guide you.

And if your car doesn’t have a rear ‘bar? Again look very carefully at other versions of your model for factory bars that may have been fitted. If you can find a bar on a suspension that looks the same, it almost certainly will bolt straight up to your car. (One giveaway is the presence of holes in your suspension... holes that look perfect for the mounting of a ‘bar!) Finally, you may need to turn to the aftermarket for a ‘bar that will fit your machine or as we did, have one custom-made for the application. However, be warned that this last approach is by far the most expensive of the three alternative methods of sourcing a new ‘bar.

Doing It

This Japanese-import Toyota Prius uses a torsion beam semi-independent rear suspension design. No rear sway bar is fitted to this version although research reveals that sway bars were fitted to the Australian-delivered cars. However, buying a new Prius sway bar from Toyota was never going to be cost-effective and nothing is available aftermarket, so it was off to the wreckers. Toyota Prius cars in wreckers aren’t numerous (understatement!) but looking under lots of Toyotas soon showed that the current ZE122 Corolla uses suspension that while not identical, is very close indeed in design. However, careful measurement showed that the Corolla bar would not fit the Prius.

But even though the Corolla bar didn’t fit, it was still purchased for AUD$50. Why? Well, it could be used to produce a sample of a bar that would fit. That is, it was close enough to the required shape that with some cutting-and-shutting, the Corolla bar could be used to show what the desired Prius bar should look like. If you need to have a custom-made bar produced, it’s a good approach – get a cheap bar as close as possible to the required shape and then modify it to produce a sample from which the new bar can be made.

The Corolla bar was too long for the Prius and so the first step was to chop out a section from the middle of the bar.

A short length of tube (salvaged from a discarded clothes rack) was then used to join the two halves of the bar back together. The tube was a tight fit over the sway bar (it was hammered on) and so the mock-up sway bar could be adjusted for length but was still rigid.

Trial fits of the mock-up bar were then carried out, with length and ‘twist’ adjustments made with the judicious use of a hammer.

This is the result – a mock-up bar that exactly fits the application. Making a sample like this is far easier than trying to get accurate measurements. This sample bar was then mailed to Whiteline Suspension with instructions that a bar just the same (but missing the clothes rack tube!) be made. But what thickness should it be?

The standard Corolla bar is 21.5mm – and it’s hollow. We elected for a 22mm solid bar for the Prius.

The sample bar was sent off to Whiteline and 5 weeks later back came the newly-made sway bar. It had a retail price of $285 and bolted straight into position. (If you haven’t got factory bolts, make sure that you source high tensile bolts, available from bolt and machinery suppliers.)

The Results

The new sway bar results in far less body roll and much less understeer. The rear ride is a bit firmer (over one-wheel bumps the effective rear suspension rate is now higher) but the overall improvement is absolutely dramatic.

Truly, this is the best bang for your bucks handling mod that you can make to your front-wheel drive.

Contact: www.whiteline.com.au

We paid trade price for the custom-made sway bar from Whiteline Suspension.

Calculating Sway Bar Stiffness Solid sway bar stiffness increases as the fourth power of the diameter. For example, a sway bar might have a diameter of 22mm and you are considering changing it for one which is 26mm in diameter.  224 (22 x 22 x 22 x 22) give a stiffness factor of 234,256 units.  The second bar’s stiffness is 264 which is 456,976. Divide one by the other and you can see that the second bar’s stiffness is almost twice (1.95 times) as high. As you can see, small changes in diameter make for large changes in stiffness!