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This article was first published in 2007.
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Got a small front-wheel drive with a torsion beam
rear axle? If you have, you can make a suspension change that will cost you very
little and can provide great benefits. (The approach will actually work on cars
with all different types of suspension – but it’s easiest with a torsion beam
rear end.) So what sort of benefits then? They can include changed ride height,
reduced understeer and a better ride. Course, if you do it wrongly, you can make
all those things worse, too!
So what are we on about? Well, with most torsion
beam rear ends, removing the springs is dead easy. Unlike cars where the springs
are kept captive on MacPherson struts, you won’t need a pair of spring
compressors (and the danger involved in using them) and you won’t have to take
the suspension assemblies out of the car. In fact, you won’t even have to take
off the wheels!
But hold on – why do you need to take the springs
out? You want to do that so you can replace them. You should be able to source new springs with your desired characteristics
for well under AUD$50 a pair. In fact, I paid $30. And where did I get new
springs for fifteen bucks each? Easy - at a big wrecking yard. To do the same
you’ll need to take along some measuring tools and expect to spend at least a
few hours looking before you find springs from another car that suit the new
purpose.
Of course, there are no guaranties – perhaps no
off-the-shelf springs are available to suit. But the springs from small cars are
often very similar to each other – with just enough variation to allow the
desired outcome. And of course the wrecking yard search doesn’t have to limit
itself to torsion beam rear axle cars – since you have wrecking yard staff at
your disposal, you can always ask to have suitable springs removed from
struts...
Swapping Springs
Getting the springs out of most cars with torsion
beam axles is as simple as supporting the body on jack-stands and then undoing
the bottom retaining bolts of the dampers. The wheels will then droop downwards
and the springs will be loose enough to remove by hand. At the most, a simple
lever should be sufficient to dislodge them.
Note: some cars with torsion beam rears use
struts, where the coils are captive around the dampers. In these cases the
springs may be under a fair amount of preload so the normal safety precautions
need to be taken when removing them. See your workshop manual for how to remove
the springs - you’ll need spring compressors.
Comparing Springs
When sourcing new springs, you’ll need to maintain
the same internal diameter as standard. (And that’s at both ends!)
When comparing springs of a similar diameter, the
stiffness of the spring increases a lot with small increases in wire diameter,
and increases more slowly with a reduction in the number of coils. So a new
spring with the same wire diameter (you must use calipers to measure wire
thickness – accuracy in this is very important), but with more free coils, will
be softer. With the same wire thickness and diameter, but less free coils, it
will be stiffer. A spring of the same diameter with the same number of coils but
thicker wire (even 1mm thicker!) will be stiffer.
The free length of the spring is also very
important. If it is longer than standard, the spring will be under greater
preload when it is held captive by the dampers. This means that the ride height
will be higher than standard. If the preload is great enough (ie the new spring
is much longer than standard), there may be no compression under normal vehicle
weight – ie there will be zero droop but a lot of bump travel. This is not
good!
The following table summarises the difference for
springs with the same diameter as standard.
|
New Spring |
Characteristic when compared with old spring |
|
Thicker wire |
Stiffer – small changes make a big difference |
|
More Coils |
Softer – small changes make little difference |
|
Longer Free Length |
Higher preload so car will ride higher, reduced droop but more bump
travel |
So if you want to lower the back of the car,
you’ll need a shorter, slightly stiffer spring (stiffer because there will be
less bump travel available). That means you’re looking for a shorter free
length, thicker wire and/or less coils. Such a spring selection will normally
reduce understeer as well as lowering the rear of the car.
Another aspect to keep in mind is relative vehicle
mass. If you grab a spring from the back of an 800kg car and put it in your
1200kg car, the car may appear fine in ride height, etc. However, the stress
level in the spring steel will be higher. To counter this, the donor car should
be similar in mass (or heavier) and you should make sure the number of coils
and/or wire thickness are not reduced too much over standard.
Doing It
It’s starting to sound way complex, isn’t it? But
it ain’t – not when you can change springs in 20 minutes!
So how did I get started on this track? Well over
a decade ago, colleague Michael Knowling had a Charade Turbo. At the time he
knew very little about cars so I was amazed one day when he told me how that
weekend he’d swapped the rear springs for some new ones sourced from a wrecking
yard. A fellow Charade owner had told him what model car to take them out of
(the other guy had previously found they worked) and so for little money and
time, Michael soon found himself driving a successful and low cost upgrade.
That approach stayed in my mind so when I bought a
Honda Insight (a car with a torsion beam rear end) and found that the rear
suspension was inadequate, I thought of simple spring swaps.
So what was the problem with the Honda? In short,
the rear suspension travel in Insights is terrible and the result is that on my
rough secondary bitumen roads, the standard rear suspension kept hitting the
bump stops. With a heavy passenger in the car, the ride could become very harsh;
placing a big car battery in the boot for the trip home from an auto parts
dealer was enough to cause the rear suspension to bottom on big bumps.
What I wanted was a slightly stiffer spring. Or
maybe more preload. So, either a longer spring of the standard rate, or a spring
of the same length but a little stiffer. There are important differences in the
outcomes of either approach - and I wasn’t sure which was best to go for. That
uncertainty ruled out getting springs made (even the cheapest custom
manufactured spring costs well over double the wrecker route!) and so I figured
some swaps might be the go.
Toyota Springs
I removed the rear springs from the Honda and then
wandered off to take a pic before heading to the wrecker. But right in front of
me lying on my workshop floor was a spring that looked about right! It was from
the strut rear of a Toyota Corolla – I’d bought the whole rear suspension when
sourcing rear discs for my NHW10 Prius. The Corolla springs were very similar
diameter to the standard Honda springs but about 25mm longer – a fair amount of
extra preload. The wire was also slightly thicker. (The Corolla spring is at
left and the standard Honda spring at right.) But since they were there and the
swap was so easy, I bunged them in.
The rear of the car rode higher but since it is
normally very low, that wasn’t a huge problem. But by God the ride sure was a
problem! I’d wondered what a car would be like with nearly no rebound (droop)
travel, but lots of bump travel, and I soon found out. The ride was appalling –
incredibly hard and then on the big bumps, completely undamped as the energy
stored in the coil quickly overcame the standard dampers. Even if you want more
bump travel, I’d not suggest that you go for a longer spring...
So I needed to take a trip to a wrecker.
Wrecker Springs
I took with me to the wrecker the top rubber
spring mount from the Honda, a pair of calipers, a tape measure and one standard
Honda spring. The Honda is a light car (just 850kg) so I looked at cars like the
Mazda 121, Ford Ka, Daihatsu Sirion and Suzuki Swift. (And while I was browsing,
I saw plenty of springs that looked suitable for cars up to about 1200kg – so
it’s not just the ultra littlies that this applies to.) In the end I settled on
the rear springs from a Daewoo Matiz.
The Matiz springs were the same diameter, the same
length, had one more coil but the wire thickness was 1mm greater (the Matiz
spring is on the left). That put my rule of thumb calculation as suggesting the
new ones slightly stiffer – just what was wanted. I physically pushed down on
the original Honda sample and then on one of the Matiz springs and the rule of
thumb calculation seemed about right – the Matiz springs were a bit stiffer.
Thirty bucks later, they were mine. (Incidentally,
when buying, check on the wrecker’s refund or exchange policy.)
Installation
Twenty minutes after arriving home, the Corolla
springs were out and the Matiz springs were in. (I went to the wrecker in
another car!) And the result was near perfect. Over the same bumps the car rode
more compliantly – despite the stiffer springs. That’s because the suspension
travel wasn’t all being used up, so causing a meeting with the bump stops. And,
as you’d expect with stiffer rear springs, understeer was reduced. (However, the
taller rear ride height gave more rear weight transfer, so more than one thing
was happening when cornering!)
But the springs were noisy, making ‘metallic’
sounds that echoed through the all-aluminium body. Now I know why the standard
springs have a rubber compound over the first few coils! But this was easy to
fix – up on jack-stands again, springs out and the rubber protectors slipped
over the first few coils.
With the springs silenced, I went for a longer
drive. Obviously the total suspension travel is unchanged but on the road you’d
swear there are inches more travel. Even over speed humps, where previously the
car used to bottom-out harshly, full travel is not used. And the car feels far
more poised in hard cornering – you’d swear a custom-made rear sway bar had been
bolted into place!
All pretty bloody good for thirty bucks....
Conclusion
Look, I can’t say that you’ll find it easy to whip
out the rear springs, even easier to find some excellent on-paper replacements,
and then find they work beautifully on the road. But I know one thing: if you
don’t try it, you’ll never know!
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