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
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
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
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
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:
That the car actually does have a rear sway bar
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
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.
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 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.
We paid trade price for the custom-made sway bar from Whiteline
Calculating Sway Bar
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.
you can see, small changes in diameter make for large changes in