|
This article was first published in 2005.
|
When it comes to car suspension, it can all get very complicated bloody
quickly. If you don’t believe us, just open any textbook on car suspension, talk
to any race car engineer about topics like ‘roll centres’ and ‘dynamic weight
transfer’, or see how many bits and pieces are included in a full suspension
makeover. In fact, start even thinking about all this stuff and it rapidly all
gets too hard.
And too expensive.
But there is a simple way to achieve a noticeable improvement in handling
without spending much money at all. The trick is in the sway bars, also called
anti-roll bars or stabiliser bars. The purist will say that changing sway bars
should be the last step in setting up a performance suspension system – but
that’s OK, we’re not purists.
Function
A sway bar is a torsion bar that resists body roll by linking
the suspension movement of the wheels at the one end of the car. During
cornering, this holds the car flatter. Sway bars are placed on the front, rear,
or both front and rear suspensions systems. Most cars have front and rear sway
bars from the factory.
It’s important in understanding sway bars to realise that in
cornering, a sway bar works by trying to lift the inside wheel. So a stiff sway
bar on the front will tend to lift the inside front wheel. A stiff sway bar on
the back will tend to lift the inside rear wheel. And since a lifted tyre (or
one that is unloaded) loses grip really fast, in most cases a thicker sway bar
will reduce grip at that end and increase it at the other end. Not always, but
that’s what usually occurs where you’re modifying a relatively modern car that
already has sway bars.
A sway bar resists bump (ie suspension compression) on just one
side of the car but because the sway bar is mounted on pivots, the suspension of
both left and right wheels can compress together without additional spring
resistance being imposed. Except over one-wheel bumps, this means that a stiffer
sway bar will not cause a stiffer ride, unlike when higher rated springs are
fitted. However, large sway bars cause an increase in NVH (noise, vibration,
harshness) and over-barred cars tend to cause the body to jerk sideways while
passing over one-wheel bumps.
Advantages
The major advantage of changing sway bars is that the behaviour
of the car can be tuned to better match what you want in handling. That applies
to all types of cars – those that understeer, oversteer, rear-wheel drives,
front-wheel drives and four-wheel drives.
Front wheel drive cars tend to understeer. This is the case
because the front tyres are forced to perform two tasks – transferring torque to
the road and also turning the car.
It’s best to fit a thicker anti-roll bar to the rear of these
cars, with stiffness increases of 100 per cent over standard being common. This
helps hold the car flat, counteracting understeer without causing the same
problems as a bigger front sway bar which tends to lift the front inside wheel,
worsening the understeer.
But can the rear roll stiffness be too great? Yes, it can. A
FWD car with an over-stiff rear will have throttle-off oversteer. This can be a
little tricky, especially in wet conditions or with an inexperienced driver.
Someone who lifts off sharply when the car starts to understeer might be a
little surprised when the back suddenly comes out! Very sporty FWD cars (and
those set up for circuit use) frequently lift the inside rear wheel right off
the ground because of the very stiff rear roll stiffness.
But in a front-wheel drive that understeers, upping the
thickness of the rear sway bar is a great place to start. And, as we keep
saying, it’s cheap too.
Because a rear wheel drive car transfers torque to the road
through the back wheels, it’s necessary that they both stay on the ground.
Lifting an inside rear wheel will cause wheelspin (in cars without an LSD) and
more power oversteer (in cars with an LSD). RWD cars therefore tend to use
greater front roll stiffness, provided largely by the sway bar. However, if
understeer is strong, a slightly stiffer rear bar can be used.
If a rear-wheel drive oversteers, or a non-LSD car picks up an
inside rear wheel and spins it, fit a larger front sway-bar. If the car
understeers a lot, fit a bigger rear sway-bar.
Some four wheel drive cars understeer, some four wheel drive
cars oversteer. The solutions are much the same as you apply to a front-wheel
drive (if it understeers) or rear-wheel drive (if it oversteers.) Yep, that’s a
simplification, but remember, we’re not purists!
Sway Bar Stiffness
The stiffness of a sway bar is dependent on its shape and
construction. The longer the lever arm acting on the torsion bar, the softer
will be the effective rate. To reduce unsprung and sprung mass, some sway bars
use a hollow rod, but most are made from solid bar. If all you’re doing is
changing the thickness of a solid sway bar, the change in rate is easily worked
out.
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,
even though it’s only 4mm thicker!
So as you can see, small changes in diameter make for large
changes in stiffness.
So....
- Sway bars resist body roll
- Sway bars can be placed at the back, front or both ends
- Increasing the thickness of a sway bar at one end of the car often has the
most effect at the other end
- Front-wheel drives often benefit from a upsized rear sway bar
- Rear-wheel drives that understeer often benefit from a upsized rear sway
bar
- Rear-wheel drives that oversteer, or pick up an inside rear wheel and spin
it, often benefit from a upsized front sway bar
- In four-wheel drives, combat understeer with a thicker rear bar, or combat
oversteer with a thicker front bar
- It’s easy to work out how much extra stiffness a given increase in solid bar
diameter makes
Buying the Bits
Upsized sway bars can be very expensive, especially if you want a big jump in
size and/or you have a rare car. But there are some cheaper approaches...
If your car was produced in a number of different versions – different
engines, different body configurations, different degrees of sportiness – then
it’s very likely that the sway bar thicknesses varied across the different
models.
For example, a commercial version of your car (a ute or tray-top, for
example) is quite likely to run a thicker front sway bar to counteract the
bigger rear loads. A luxury long wheelbase version might run different bar
thicknesses, as might a sports-pack version. Also think about whether the
suspension system of your car underpinned other body shapes from the company –
it might be worth checking them out as well.
Of course, you might find that your car is already running the thickest sway
bars that came from the factory – but you won’t know unless you go look! In some
cases, the information will be in workshop manuals but for a definitive
judgement, visit a big wrecking yard with a pair of digital calipers in hand.
The other reason it’s good to go to a wrecking yard is that you might find
some larger, aftermarket sway bars that have already been fitted to a car. As
with buying anything from a wrecking yard, it’s best if you can examine the
whole car and so assess whether accident damage is likely to have bent the bar –
although most wreckers will offer a warranty on parts like these. Note that in
some cars the front-end castor is influenced by the sway-bar, so it’s important
that it isn’t distorted.
Now this is the real bargain approach. What you do is to parallel another
sway bar with the original. While that might seem like way overkill, it isn’t
necessarily. Let’s take a look.
Take car that runs a 27mm front sway bar. It’s a rear-wheel drive car with
plenty of torque and without an LSD, is prone to lift an inside rear wheel in
cornering. A bigger front bar will keep the car sitting flatter, providing
better traction at possibly the expense of a little loss in front-end grip.
No upsized bar is available from other models of the same car (ie 27mm bar
was the largest size used) but what if we add a second 27mm bar to the front?
All other things being equal (which they aren’t), that would double the front
sway bar stiffness.
Doing the maths above shows that to double the stiffness of the 27mm bar,
we’d have to increase its diameter to 32mm. Hmmm, doesn’t sound so great a jump
now, does it? Or, by adding a 25mm bar (easily sourced from an earlier model) to
the standard 27mm bar, the stiffness increase would be approximately like
upsizing the standard bar to just under 30mm.
So how do you add a second sway bar? The exact approach depends on how the
sway bar is mounted but one method involves cutting off the mounting ends of the
second bar and then clamping it to the original. Strong clamps can be easily
gained if you buy a whole bunch of original sway bar mounts – they’ll have the
right strength and also come with D-rubbers to suit the sway bar diameter. All
you’ll need in addition are some high tensile nuts and bolts... and even including
the sway bar, the total of that lot can be under fifty bucks!
Here’s how to do it:
Go to a wrecker and obtain a second standard sway bar. Compare it to another
standard bar to check it’s not bent, and make sure it’s not rusty or otherwise
stuffed. When you’re doing your comparison, make sure that there’s room to fit
it below the standard bar, without it hitting anything even with suspension and
steering movement.
Buy eight (yep, eight!) sway bar mounts and rubber bushes. These don’t have
to be from the same make and model of car, but make sure the rubber bush is for
approximately the same size bar (eg within a few millimetres). These rubber
bushes can get chewed-out over time, so inspect the rubber before plonking down
the cash.
Hold the new sway bar up against the original and work out how the two are
to be attached. A couple of approaches can be taken but easiest is to do as is
shown here. In this a diagram the black sway bar and red mounts are the
originals. The green sway bar has had its ends cut off and is then is clamped to
the original using four clamps. Unlike in this diagram, the new sway bar is
usually mounted directly below the original.
Each clamp is made by using a pair of sway bar mounts bolted together,
mounting faces of one clamp against the mounting faces of the other. The
original rubber bushes are retained but the bar no longer swivels within them.
To join the clamps make sure that you use high tensile bolts (available from
bolt stores and industrial suppliers) and not cheap low quality hardware
store nuts and bolts.
The original sway bar is most easily shortened by being cut with a friction
saw. Shorten the arms only by the minimum amount to provide clearance to the
original suspension mounts. If you shorten the arms too much, you’ll find it
hard to retain enough space for adequate clamping. The greater the distance
between the clamps on each side, the better will be the security of the system.
Again check that nothing touches at full steering lock (front) and with full
bump and rebound (front and back).
- Buying New Aftermarket Bars
If it’s impossible to fit a second bar in, and there aren’t any other models
that use bigger bars, the only other alternative is to buy aftermarket bars. But
even here you can economise a bit: buy only the single sway bar and fit it where
it can give the best outcome.
|
Warning!
Upgraded sway bars will almost always make the car feel better in cornering –
turn-in can be improved and the car will feel more settled. But too
stiff anti-roll will often make the car slide more easily in wet
conditions. So be really careful in your testing, on both dry and wet roads,
until you have established with confidence the new handling behaviour.
|
Conclusion
Especially if you’re on a tight budget, upgrading the roll stiffness of one
end of the car with a bigger sway bar can make a real and noticeable difference
to handling without costing an arm and a leg.
Did you enjoy this article?
Please consider supporting AutoSpeed with a small contribution. More Info...
|
More of our most popular articles.
|
|
|
Email a friend
Print article
|