digitaLmbuL’s FiLes

Apart from your car’s tyres and seats, the suspension is the prime
mechanism that separates your bum (arse for the American) from the road. It
also prevents your car from shaking itself to pieces. No matter how smooth
you think the road is, it’s a bad, bad place to propel over a ton of metal
at high speed. So we rely upon suspension. People who travel on underground
trains wish that those vehicles relied on suspension too, but they don’t
and that’s why the ride is so harsh. Actually it’s harsh because
underground trains have no lateral suspension to speak of. So as the rails
deviate side-to-side slightly, so does the entire train, and it’s
passengers. In a car, the rubber in your tyre helps with this little
problem.
In it’s most basic form, suspension consists of two basic components:

Springs
These come in three types. They are coil springs, torsion bars and leaf
springs. Coil springs are what most people are familiar with, and are
actually coiled torsion bars. Leaf springs are what you would find on most
American cars up to about 1985 and almost all heavy duty vehicles. They
look like layers of metal connected to the axle. The layers are called
leaves, hence leaf-spring. The torsion bar on its own is a bizarre little
contraption which gives coiled-spring-like performance based on the
twisting properties of a steel bar. It’s used in the suspension of VW
Beetles and Karmann Ghias, air-cooled Porsches (356 and 911 until 1989 when
they went to springs), and the rear suspension of Peugeot 205s amongst
other cars. Instead of having a coiled spring, the axle is attached to one
end of a steel shaft. The other end is slotted into a tube and held there
by splines. As the suspension moves, it twists the shaft along it’s length,
which in turn resist. Now image that same shaft but instead of being
straight, it’s coiled up. As you press on the top of the coil, you’re
actually inducing a twisting in the shaft, all the way down the coil. I
know it’s hard to visualise, but believe me, that’s what is happening.
There’s a whole section further down the page specifically on torsion bars
and progressive springs.

Shock absorbers
Strangely enough, absorb shocks. Actually they dampen the vertical motion
induced by driving your car along a rough surface. If your car only had
springs, it would boat and wallow along the road until you got physically
sick and had to get out. Or at least until it fell apart.
Shock absorbers perform two functions. Firstly, they absorb any
larger-than-average bumps in the road so that the shock isn’t transmitted
to the car chassis. Secondly, they keep the suspension at as full a travel
as possible for the given road conditions. Shock absorbers keep your wheels
planted on the road. Without them, your car would be a travelling
deathtrap.
You want more technical terms? Technically they are called dampers. Even
more technically, they are velocity-sensitive hydraulic damping devices -
in other words, the faster they move, the more resistance there is to that
movement. They work in conjunction with the springs. The spring allows
movement of the wheel to allow the energy in the road shock to be
transformed into kinetic energy of the unsprung mass, whereupon it is
dissipated by the damper. The damper does this by forcing gas or oil
through a constriction valve (a small hole). Adjustable shock absorbers
allow you to change the size of this constriction, and thus control the
rate of damping. The smaller the constriction, the stiffer the suspension.
Phew!….and you thought they just leaked oil didn’t you?
coilover suspension
A modern coil-over-oil unit

The image above shows a typical modern coil-over-oil unit. This is an
all-in-one system that carries both the spring and the shock absorber. The
type illustrated here is more likely to be an aftermarket item – it’s
unlikely you’d get this level of adjustment on your regular passenger car.
The adjustable spring plate can be used to make the springs stiffer and
looser, whilst the adjustable damping valve can be used to adjust the
rebound damping of the shock absorber. More sophisticated units have
adjustable compression damping as well as a remote reservoir. Whilst you
don’t typically get this level of engineering on car suspension, most
motorbikes do have preload, rebound and spring tension adjustment. See the
section later on in this page about the ins and outs of complex suspension
units.
Suspension Types

In their infinite wisdom, car manufacturers have set out to baffle use with
the sheer number of different types of suspension available for both front
and rear axles. The main groupings are dependant and independent suspension
types. If you know of any not listed here, e-mail me and let me know – I
would like this page to be as complete as possible.
Front suspension – dependent systems

So-called because the front wheel’s suspension systems are physically
linked. For everyday use, they are, in a word, shite. I hate to be
offensive, but they are. There is only one type of dependant system you
need to know about. It is basically a solid bar under the front of the car,
kept in place by leaf springs and shock absorbers. It’s still common to
find these on trucks, but if you find a car with one of these you should
sell it to a museum. They haven’t been used on mainstream cars for years
for three main reasons:

    * Shimmy – because the wheels are physically linked, the beam can be
set into oscillation if one wheel hits a bump and the other doesn’t. It
sets up a gyroscopic torque about the steering axis which starts to turn
the axle left-to-right. Because of the axle’s inertia, this in turn feeds
back to amplify the original motion.
    * Weight – or more specifically unsprung weight. Solid front axles
weigh a lot and either need sturdy, heavy leaf springs or heavy suspension
linkages to keep their wheels on the road.
    * Alignment – simply put, you can’t adjust the alignment of wheels on a
rigid axis. From the factory, they’re perfectly set, but if the beam gets
even slightly distorted, you can’t adjust the wheels to compensate.

offroadingI frequently get pulled-up on the above statements from people
jumping to defend solid-axle suspension. They usually send me pictures like
this and claim it’s the best suspension system for off-road use. I have to
admit, for off-road stuff, it probably is pretty good. But let’s face it;
how many people with these vehicles ever go off-road? The closest they come
to having maximum wheel deflection is when the mother double-parks the
thing with one wheel on the kerb during the school-run…….

Front suspension – independent systems

So-named because the front wheel’s suspension systems are independent of
each other (except where joined by an antiroll bar) These came into
existence around 1930 and have been in use in one form or another pretty
much ever since then.
MacPherson Strut or McPherson strut

This is currently, without doubt, the most widely used front suspension
system in cars of European origin. It is simplicity itself. The system
basically comprises of a strut-type spring and shock absorber combo, which
pivots on a ball joint on the single, lower arm. At the top end there is a
needle roller bearing on some more sophisticated systems. The strut itself
is the load-bearing member in this assembly, with the spring and shock
absorber merely performing their duty as oppose to actually holding the car
up. In the picture here, you can’t see the shock absorber because it is
encased in the black gaiter inside the spring.
The steering gear is either connected directly to the lower shock absorber
housing, or to an arm from the front or back of the spindle (in this case).
When you steer, it physically twists the strut and shock absorber housing
(and consequently the spring) to turn the wheel. Simple. The spring is
seated in a special plate at the top of the assembly which allows this
twisting to take place. If the spring or this plate are worn, you’ll get a
loud ‘clonk’ on full lock as the spring frees up and jumps into place. This
is sometimes confused for CV joint knock.
[rover 2000 front suspension]

Rover 2000 MacPherson derivative During WWII, the British car maker Rover
worked on experimental gas-turbine engines, and after the war, retained a
lot of knowledge about them. The gas-turbine Rover T4, which looked a lot
like the Rover P6, Rover 2000 and Rover 3500, was one of the prototypes.
The chassis was fundamentally the same as the other Rovers and the net
result was the the 2000 and 3500 ended up with a very odd front suspension
layout. The gas turbine wasn’t exactly small, and Rover needed as much room
as possible in the engine bay to fit it. The suspension was derived from a
normal MacPherson strut but with an added bellcrank. This allowed the
suspension unit to sit horizontally along the outside of the engine bay
rather than protruding into it and taking up space. The bellcrank
transferred the upward forces from the suspension into rearward forces for
the spring / shock combo to deal with. In the end, the gas turbine never
made it into production and the Rover 2000 was fitted with a 2-litre
4-cylinder engine, whilst the Rover 3500 was fitted with an ‘evergreen’
3.5litre V8. Open the hood of either of these classics and the engine looks
a bit lost in there because there’s so much room around it that was never
utilised. The image on the left shows the Rover-derivative MacPherson
strut.

Potted history of MacPherson: Earle S. MacPherson of General Motors
developed the MacPherson strut in 1947. GM cars were originally
design-bound by accountants. If it cost too much or wasn’t tried and
tested, then it didn’t get built/used. Major GM innovations including the
MacPherson Strut suspension system sat stifled on the shelf for years
because innovation cannot be proven on a spreadsheet until after the
product has been produced or manufactured. Consequently, Earle MacPherson
went to work for Ford UK in 1950, where Ford started using his design on
the 1950 ‘English’ Ford models straight away. Today the strut type is
referred to both with and without the “a” in the name, so both McPherson
Strut and MacPherson Strut can be used to describe it.

Further note: Earle MacPherson should never be confused with Elle McPherson
- the Australian über-babe. In her case, the McPherson Strut is something
she does on a catwalk, or in your dreams if you like that sort of thing.
And if you’re a bloke, then you ought to….
     [suspension]
Double wishbone suspension systems.

The following three examples are all variations on the same theme.
[suspension]
Coil Spring type 1

This is a type of double-A or double wishbone suspension. The wheel
spindles are supported by an upper and lower ‘A’ shaped arm. In this type,
the lower arm carries most of the load. If you look head-on at this type of
system, what you’ll find is that it’s a very parallelogram system that
allows the spindles to travel vertically up and down. When they do this,
they also have a slight side-to-side motion caused by the arc that the
wishbones describe around their pivot points. This side-to-side motion is
known as scrub. Unless the links are infinitely long the scrub motion is
always present. There are two other types of motion of the wheel relative
to the body when the suspension articulates. The first and most important
is a toe angle (steer angle). The second and least important, but the one
which produces most pub talk is the camber angle, or lean angle. Steer and
camber are the ones which wear tyres.
[suspension]
Coil Spring type 2

This is also a type of double-A arm suspension although the lower arm in
these systems can sometimes be replaced with a single solid arm (as in my
picture). The only real difference between this and the previous system
mentioned above is that the spring/shock combo is moved from between the
arms to above the upper arm. This transfers the load-bearing capability of
the suspension almost entirely to the upper arm and the spring mounts. The
lower arm in this instance becomes a control arm. This particular type of
system isn’t so popular in cars as it takes up a lot room.
[suspension]
Multi-link suspension

This is the latest incarnation of the double wishbone system described
above. It’s currently being used in the Audi A8 and A4 amongst other cars.
The basic principle of it is the same, but instead of solid upper and lower
wishbones, each ‘arm’ of the wishbone is a separate item. These are joined
at the top and bottom of the spindle thus forming the wishbone shape. The
super-weird thing about this is that as the spindle turns for steering, it
alters the geometry of the suspension by torquing all four suspension arms.
They have complex pivot systems designed to allow this to happen.
Car manufacturers claim that this system gives even better road-holding
properties, because all the various joints make the suspension almost
infinitely adjustable. There are a lot of variations on this theme
appearing at the moment, with huge differences in the numbers and
complexities of joints, numbers of arms, positioning of the parts etc. but
they are all fundamentally the same. Note that in this system the spring
(red) is separate from the shock absorber (yellow). Click on the image for
a reverse view of the same system (this will popup a separate window).
Trailing-arm suspension

The trailing arm system is literally that – a shaped suspension arm is
joined at the front to the chassis, allowing the rear to swing up and down.
Pairs of these become twin-trailing-arm systems and work on exactly the
same principle as the double wishbones in the systems described above. The
difference is that instead of the arms sticking out from the side of the
chassis, they travel back parallel to it. This is an older system not used
so much any more because of the space it takes up, but it doesn’t suffer
from the side-to-side scrubbing problem of double wishbone systems. If you
want to know what I mean, find a VW beetle and stick your head in the front
wheel arch – that’s a double-trailing-arm suspension setup. Simple.
     [suspension]
Moulton rubber suspension

This suspension system is based on the compression of a solid mass of
rubber – red in both these images. The two types are essentially
derivatives of the same design. It is named after Dr. Alex Moulton – one of
the original design team on the Mini, and the engineer who designed its
suspension system in 1959. This system is known by a few different names
including cone and trumpet suspension (due to the shape of the rubber bung
shown in the right hand picture). The rear suspension system on the
original Mini also used Moulton’s rubber suspension system, but laid out
horizontally rather than vertically, to save space again. The Mini was
originally intended to have Moulton’s fluid-filled Hydrolastic suspension,
but that remained on the drawing board for a few more years. Eventually,
Hydrolastic was developed into Hydragas (see later on this page), and
revised versions were adopted on the Mini Metro and the current
MGF-sportscar.
Ultimately, Moulton rubber suspension is now used in a lot of bicycles -
racing and mountain bikes. Due to the compact design and the simplicity of
its operation and maintenance, it’s an ideal solution.
[suspension][suspension]
Rear suspension – dependant systems

Contrary to the front version of this system, many many cars are still
designed and built with dependant (linked) rear suspension systems.
Solid-axle, leaf-spring

This system was favoured by the Americans for years because it was dead
simple and cheap to build. The ride quality is decidedly questionable
though. The drive axle is clamped to the leaf springs and the shock
absorbers normally bolt directly to the axle. The ends of the leaf springs
are attached directly to the chassis, as are the tops of the shock
absorbers. Simple, not particularly elegant, but cheap. The main drawback
with this arrangement is the lack of lateral location for the axle, meaning
it has a lot of side-to-side slop in it.
     [suspension]
[suspension]

Solid-axle, coil-spring

This is a variation and update on the system described above. The basic
idea is the same, but the leaf springs have been removed in favour of
either ‘coil-over-oil’ spring and shock combos, or as shown here, separate
coil springs and shock absorbers. Because the leaf springs have been
removed, the axle now needs to have lateral support from a pair control
arms. The front ends of these are attached to the chassis, the rear ends to
the axle. The variation shown here is more compact than the coil-over-oil
type, and it means you can have smaller or shorter springs. This in turn
allows the system to fit in a smaller area under the car.
Beam Axle

This system is used in front wheel drive cars, where the rear axle isn’t
driven. (hence it’s full description as a “dead beam”). Again, it is a
relatively simple system. The beam runs across under the car with the
wheels attached to either end of it. Spring / shock units or struts are
bolted to either end and seat up into suspension wells in the car body or
chassis. The beam has two integral trailing arms built in instead of the
separate control arms required by the solid-axle coil-spring system.
Variations on this system can have either separate springs and shocks, or
the combined ‘coil-over-oil’ variety as shown here. One notable feature of
this system is the track bar (or panhard rod). This is a diagonal bar which
runs from one end the beam to a point either just in front of the opposite
control arm (as here) or sometimes diagonally up to the top of the opposite
spring mount (which takes up more room). This is to prevent side-to-side
movement in the beam which would cause all manner of nasty handling
problems. A variation on this them is the twist axle which is identical
with the exception of the panhard rod. In a twist axle, the axle is
designed to twist slightly. This gives, in effect, a semi-independent
system whereby a bump on one wheel is partially soaked up by the twisting
action of the beam. Yet another variation on this system does away with the
springs and replaces them with torsion bars running across the chassis, and
attached to the leading edge of the control arms. These beam types are
currently very popular because of their simplicity and low cost.
     [suspension]
4-Bar

4-bar suspension can be used on the front and rear of vehicles – I’ve
chosen to show it in the “rear” section of this page because that’s where
it’s normally found. 4-bar suspension comes in two varieties. Triangulated,
shown on the right here, and parallel, shown on the left.
The parallel design operates on the principal of a “constant motion
parallelogram”. The design of the 4-bar is such that the rear end housing
is always perpendicular to the ground, and the pinion angle never changes.
This, combined with the lateral stability of the Panhard Bar, does an
excellent job of locating the rear end and keeping it in proper alignment.
If you were to compare this suspension system on a truck with a 4-link or
ladder-bar setup, you’d notice that the rear frame “kick up” of the 4-bar
setup is far less severe. This, combined with the relatively compact
installation design means that it’s ideal for cars and trucks where space
is at a premium. You’ll find this setup on a lot of street rods and
American style classic hot rods.
The triangulated design operates on the same principle, but the top two
bars are skewed inwards and joined to the rear end housing much closer to
the centre. This eliminates the need for the separate panhard bar, which in
turn means the whole setup is even more compact.
[suspension][suspension]
Derivatives of the 4-Bar system

There are many variations on the 4-bar systems I’ve illustrated above. For
example, if the four angled bars go from the axle outboard to the chassis
near the centreline, this is called a “Satchell link”. (Satchell is a US
designer, who used the above linkage on some of Paul Newmans Datsun road
racers some years back.) It has certain advantages over the above examples.
Both of the these angled linkages can be reversed to have the angled links
below the axle and the parallel links above. The roll centre will be
lowered with the angled bars under the axle, a function which is difficult
to accomplish without this design. The other variation on the “four bars”
not shown are the Watts and Jacobs bar linkages to replace the Panhard rod
for lateral positioning. Another linkage is the two parallel bars above the
axle and a triangulated link underneath – a design you will find on the
Lotus 7 – where the lower link has its base on the chassis and the apex
under the differential. Then there is the Mallock Woblink, which could be
described as half way between a Jacobs ladder and a Watts link, and makes
it possible to place the rear roll centre quite low without sacrificing
ground clearance.
Watts links are pretty popular with the hydraulic lowrider/truck bed dancer
types. The Jacobs ladder is used almost exclusively on US midget and
sprintcar dirt track rear ends. The Mallock Woblink is used mostly on the
Mallock U2 Clubman cars in Great Britain.
de Dion suspension, or the de Dion tube

[de dion tube]The de Dion tube – not part of the London underground, but
rather a semi-independent rear suspension system designed to combat the
twin evils of unsprung weight and poor ride quality in live axle systems.
de Dion suspension is a weird bastardisation of live-axle solid-beam
suspension and fully independent trailing-arm suspension. It’s neither one,
but at the same time it’s both. Weird! With this system, the wheels are
interconnected by a de Dion Tube, which is essentially a
laterally-telescoping part of the suspension designed to allow the wheel
track to vary during suspension movement. This is necessary because the
wheels are always kept parallel to each other, and thus perpendicular to
the road surface regardless of what the car body is doing. This setup means
that when the wheels rebound, there is also no camber change which is great
for traction, and that’s the first advantage of a de Dion Tube. The second
advantage is that it contributes to reduced unsprung weight in the vehicle
because the transfer case / differential is attached to the chassis of the
car rather than the suspension itself.
Naturally, the advantages are equalled by disadvantages, and in the case of
de Dion systems, the disadvantages would seem to win out. First off, it
needs two CV joints per axle instead of only one. That adds complexity and
weight. Well one of the advantages of not having the differential as part
of the suspension is a reduction in weight, so adding more weight back into
the system to compensate for the design is a definite distadvantage.
Second, the brakes are mounted inboard with the calipers attached to the
transfer case, which means to change a brake disc, you need to dismantle
the entire suspension system to get the driveshaft out. (Working on the
brake calipers is no walk in the park either.) Finally, de Dion units can
be used with a leaf-spring or coil-spring arrangement. With coil spring (as
shown here) it needs extra lateral location links, such as a panhard rod,
wishbones or trailing links. Again – more weight and complexity.
de Dion suspension was used mostly used from the mid 60′s to the late 70′s
and could be found on some Rovers, the Alfa Romeo GTV6, one or two Lancias
a smattering of exotic racing cars and budget sports cars or coupes.
More recently deDion suspension has had somewhat of a renaissance in the
specialist sports car and kit car market such as those from Caterham,
Westfield and Dax. These all uniformly now use outboard brake setups for
ease-of-use, and a non-telescoping tube, usually with trailing links and an
A-bar for lateral location (rather than a Watts linkage or Panhard rod.)
Whilst a properly setup independent suspension system will always win
hands-down on poorly maintained roads, when you get on to the track, the
advantage is not so clear cut and a well set up deDion system can often
match it turn-for-turn now, espeically for flyweight cars.
Rear suspen
sion – independent systems

It follows, that what can be fitted to the front of a car, can be fitted to
the rear to without the complexities of the steering gear. Simplified
versions of all the independent systems described above can be found on the
rear axles of cars. The multi-link system is currently becoming more and
more popular. In advertising, it’s put across as ’4-wheel independent
suspension’. This means all the wheels are independently mounted and
sprung. There are two schools of thought as to whether this system is
better or worse for handling than, for example, Macpherson struts and a
twist axle. The drive towards 4-wheel independent suspension is primarily
to improve ride quality without degrading handling.
The eBay problem

A lot of attention and marketing has been coming out of Ford recently about
their new Control Blade™ rear suspension. Details and engineering facts are
predictably sketchy but the glossy marketing brochures will tell you this
revolution in rear suspension will make your Ford Focus handle better, grip
the road better, and brake better than everything else on the road. It
warrants some investigation when they make claims like that, but it turns
out what they mean is “we’ve got a new suspension system”, and not much
else. It actually started out its life sometime around 1998 in Ford of
Australia and I believe Holden had something to do with it too. Since then
its become far more mainstream.
So “Control Blade™” is the snappy marketing name that Ford use to describe
their new system. It sounds good, looks good on paper, and has an aura of
21st century-ness about it. “Blade”. Ooh. Cool.
The reality isn’t quite so cool though – control blade is basically an
evolution of trailing-arm suspension. However its still an interesting
development and it does serve the purpose for which Ford designed it. The
primary purpose of Control Blade suspension is to increase the interior
space available in the vehicle. Most suspension systems used in daily
drivers have strut towers front and rear. In the front it’s not really a
problem, but in the rear it impedes on boot (or trunk) space. Ford wanted
to give more space in the back and needed to find a good way to remove or
reduce the size of the strut towers. The result is their Control Blade™
system which in essence separates the shock absorber from the springs. To
do this, Ford needed to use a trailing-arm type suspension so that they
didn’t have swingarms up under the wheel arches. The springs were shortened
and moved inboard and underneath. In one variation, the shock absorbers
still sit vertically but the space they take up now is hugely reduced
because they no longer have the coil springs around the outside. In the
second variation the shock absorber is a subminiature unit mounted inboard
of the springs underneath the vehicle. I’m not sure of the merits of the
super-short shock absorber but Ford seem to think it works. The control
blades themselves are basically the trailing arms which give lateral
support and provide the vertical pivot point for the entire unit.
The Ford spiel says this about Control Blade™: “It has the key function of
promoting ride and reducing road noise transmission, while providing the
freedom to let the lateral links define toe and camber by absorbing any
rearward forces and allowing the rest of the suspension to do it’s job
uninterrupted. Effectively isolating the handling components of the new IRS
from the road noise and impact harshness components of the suspension.”. In
English? It means better handling and less road noise. Looking at the basic
design it’s not difficult to see that this system has a much lower centre
of gravity than a Macpherson strut (for example). Lower C-of-G in a vehicle
is always a good thing. The geometry of the Control Blade™ system also
provides significant ‘anti-dive’ under braking force, which means a the car
body will dive less when you jump on the brakes which in turn translates
into more well-behaved braking response. Lower C-of-G, less roll and less
pitch during braking all add up to better handling, althouth whether the
average driver would notice or not is a different matter.
Another function of this system is that they’ve separated the two basic
functions of suspension. With the springs and shock absorbers being mounted
in different places, Ford have managed to optimise the function of these
components. It’s similar in concept to what BMW did with the telelever
front suspension on motorbikes – separating braking from suspension forces,
only in the control blade system, it separates the springing support of the
suspension from the shock reducing functions of the shock absorbers.
The images below are currently from other sources as I’ve not had the time
to render up my own just yet, but they show the basic layout of each
variation of control blade suspension and I’ve annotated them accordingly.
control blade
control blade
Aftermarket work on Control Blade™ vehicles.

There’s one thing worth noting about this suspension system. Because the
spring and shock are in different locations, and because of the reduced or
removed strut towers, it makes it very difficult to bolt-on aftermarket
suspension kits to these vehicles. For the daily driver, that’s probably
not an issue but if you’re looking at spiffing up the suspension on a Ford
Focus for track days or racing, it’s not going to be quite so
straightforward as it is on other cars. Just so you know.
Hydrolastic Suspension

If you’ve got this far, you’ll remember that Dr. Alex Moulton originally
wanted the Mini to have Hydrolastic suspension – a system where the front
and rear suspension systems were connected together in order to better
level the car when driving.
The principle is simple. The front and rear suspension units have
Hydrolastic displacers, one per side. These are interconnected by a small
bore pipe. Each displacer incorporates a rubber spring (as in the Moulton
rubber suspension system), and damping of the system is achieved by rubber
valves. So when a front wheel is deflected, fluid is displaced to the
corresponding suspension unit. That pressurises the interconnecting pipe
which in turn stiffens the rear wheel damping and lowers it. The rubber
springs are only slightly brought into play and the car is effectively kept
level and freed from any tendency to pitch. That’s clever enough, but the
fact that it can do this without hindering the full range of motion of
either suspension unit is even more clever, because it has the effect of
producing a soft ride. Pictures and images of anything to do with
hydrolastic suspension are few and far between now, so you’ll have to
excuse the plagiarism of the following image. The animation below shows the
self-leveling effect – notice the body stays level and doesn’t pitch.
hydrolastic suspension

But what happens when the front and rear wheels encounter bumps or dips
together? One cannot take precedent over the other, so the fluid suspension
stiffens in response to the combined upward motion and, while acting as a
damper, transfers the load to the rubber springs instead, giving a
controlled, vertical, but level motion to the car.
Remember I said the units were connected with a small bore pipe? The
restriction of the fluid flow, imposed by this pipe, rises with the speed

---

of the car. This means a steadier ride at high speed, and a softer more
comfortable ride at low speed.

Hydrolastic suspension is hermetically sealed and thus shouldn’t require
much, if any, attention or maintenance during its normal working life. Bear
in mind that hydrolastic suspension was introduced in 1965 and you’d be
lucky to find a unit today that has had any work done to it.

The image below shows a typical lateral installation for hydrolastic rear
suspension. The suspension swingarms are attached to the main subframe. The
red cylinders are the displacer units containing the fluid and the rubber
spring. The pipes leading from the units can be seen and they would connect
to the corresponding units at the front of the vehicle.
hydrolastic suspension

Hydrolastic suspension shouldn’t be confused with Citro?n’s hydropneumatic
suspension (see below). That system uses a hydraulic pump that raises and
lowers the car to different heights. Sure it’s a superior system but it’s
also a lot more costly to manufacture and maintain. That’s due in part to
the fact that they don’t use o-rings as seals; the pistons and bores are
machined to incredible tolerances (microns), that it makes seals
unnecessary. Downside : if something leaks, you need a whole new cylinder
assembly.

Hydrolastic was eventually refined into Hydragas suspension…….
Hydragas Suspension

Hydragas is an evolution of Hydrolastic, and essentially, the design and
installation of the system is the same. The difference is in the displacer
unit itself. In the older systems, fluid was used in the displacer units
with a rubber spring cushion built-in. With Hydragas, the rubber spring is
removed completely. The fluid still exists but above the fluid there is now
a separating membrane or diaphragm, and above that is a cylinder or sphere
which is charged with nitrogen gas. The nitrogen section is what has become
the spring and damping unit whilst the fluid is still free to run from the
front to the rear units and back.
displacers

Hydragas suspension was famously used in the 1986 Porsche 959 Rally car
that entered the Paris-Dakar Rally, and today you can find it on the MGF
Roadster.
There are a lot of resources on Hydragas available at one of the MGF club
sites on the internet: http://www.mgfcar.de/hydragas
Hydropneumatic Suspension
[hydropneumatic]

{Thanks to Jonathan Bruce, Simon Byrnand and Pieter Melissen for some
updates to this information.}
Since the late forties, Citro?n have been running a fundamentally different
system to the rest of the auto industry. Its called hydropneumatic
suspension, and it is a whole-car solution which can include the brakes and
steering as well as the suspension itself. The core technology of
hydropneumatic suspension is as you might guess from the name, hydraulics.
Ultra-smooth suspension is provided by the fluid’s interaction with a
pressurised gas, and in this respect, its very similar to the hydragas
system described above. Citro?n pioneered the system in the rear suspension
of the 15 (Traction Avant) model, and it has been fitted to many of their
cars since. Because of the complexity of the system, the rest of this
section gets a bit wordy but hopefully not so much that I’ll lose you half
way through. Because this page is about all types of suspension, for
clarity I decided to concentrate on the simplified version of this as
installed in the “BX” model. If you’re desperate to know every last nut and
bolt of hydropneumatics, just do a google search for it. On we go….

The system is powered by a large hydraulic pump, typically belt-driven by
the engine like an alternator or an air conditioner. the pump provides
fluid to an accumulator at pressure, where it is stored ready to be
delivered to servo a system. This pump is also used for the power steering
and the brakes, and in the DS for the semi-automatic gearbox.
The BX was a major turning point in Citro?n’s history as it was the first
car to be produced under the company’s new Peugeot management, following
the 1970s take-over. As a direct consequence of the Peugeot influence, the
car was somewhat more conventional than its bulkier predecessors like the
CX. This Peugeot-enforced “normalisation” of the design makes it fairly
easy to examine as an illustration of how hydropneumatic suspension works.
Apart from the pump, the two most obvious components in the system are the
spheres on top of each suspension strut, and the struts themselves. The
spheres are like the springs in regular suspension, and the struts are the
hydraulic components that make the fluid act like a spring.
The spring in this suspension system is provided by a hydraulic component
called an accumulator, which is gas (typically nitrogen) under pressure in
a bottle contained within a diaphragm. This is effectively a balloon which
allows pressurised fluid to compress the gas, and then as pressure drops
the gas pushes the fluid back to keep the system’s pressure up. In the
image here, the nitrogen gas is represented in red and the LHM fluid is
represented in green. As the pressure in the fluid overcomes the gas
pressure, the nitrogen is compressed by the diaphragm being pushed back.
Then as the pressure in the fluid reduces, the gas pushes back the
diaphragm which expels the fluid from the sphere, returning gas and fluid
to equilibrium. This is the hydropneumatic equivalent to the spring being
compressed and then rebounding.
Still with me? We can keep going…
So how can the interaction of compressing gas, hydraulic fluid and a
diaphragm form a spring? Simple(ish): The pressure of the gas is the
equivalent to the spring weight. The inlet hole at the bottom of the sphere
restricts the flow of the fluid and provides an element of damping. By
replacing the spheres for ones of different specifications, it’s possible
to adjust the ride characteristics of these cars.
hydropneumatic suspension

Before we go any further it is pretty important that you understand where
the fluid acting on the diaphragm in the sphere gets its force from, and to
do that we are going to have to look at the operation of the other key
component in the Citro?n system – the strut.
The sphere in these systems is actually mounted at the end of the strut.
The strut itself acts like a syringe to inject fluid into the sphere. When
the wheel hits a bump it rises, pushes the piston back and this squeezes
fluid through the tiny hole in the sphere to let the gas spring absorb the
energy of the bump. Then when the car is over the bump, the gas pushes the
diaphragm back out, pushing the fluid down to the strut, pushing the wheel
down to the ground.
Some interesting possibilities were opened up when Citro?n decided to use
this system to spring their cars. One or two of the more obvious ones are
that since the system is hydraulic, the ride height can easily be altered;
Citro?n put fancy valves called height correctors in the system. They are
designed to correct for long-term/static errors in height. To do this there
is a clamp on the middle of each roll bar connected by a linkage to the
height corrector. This linkage varies by model – on DS, CX, GS, BX it is a
simple torsion bar about 8mm diameter and about 400mm long, on the XM and
Xantia it is a coil spring assembly with a double acting override linkage,
but the functionality is the same. By measuring the height at the middle of
the rollbar, it automatically takes the average of the left and right wheel
height on that axle, and therefore cannot detect body roll. This prevents
it from spuriously trying to react to body roll, as it can’t do anything to
counter it anyway – it can only make both sides go up or down together.
Additionally the height correctors have a hydraulic damping chamber in them
which restricts and delays their movement – typically it takes a suspension
movement of at least 20mm in one direction for at least 5 seconds before
the height corrector will respond. Even fully bottoming the suspension
still takes at least 5 seconds for a response.
This works as a simple averaging system and prevents the height correctors
from responding to bumps or road undulations, (which would be undesirable).
The slight exception here is the rear suspension which is subject to squat
due to acceleration because of the front wheel drive. Prolonged heavy
accleration of more than 5 seconds (particularly noticable on an automatic)
will cause a height correction response – an undesirable side effect.
(Hydractive 2 models take steps to try and avoid this response by
stiffening the suspension during heavy acceleration).

Another noteworthy feature of Citro?n system is its ability to “pre-set” a
car for bumps in the road, keeping the car on an even keel. This is a
result of the cross-piping between left and right struts on the same axle.
They are connected permanently via a 3.5mm pipe, (except in Hydractive and
Activa systems). The height corrector connects to a T-junction of this
cross piping, but when the height corrector is “closed” (which is nearly
all the time while driving) it represents a dead end, so only the piping
from left to right comes into play. When the wheel on one side hits a bump
some oil will flow into the sphere on that side via the damping valve, and
some will flow across to the other side and extend the wheel on that side,
which gives a slight roll stabalizing response. This tends to make the car
more steady in the roll axis, and reduces the side to side rocking motion
on transverse undulations.
A side effect of this cross piping is that it gives the suspension very
soft compliance for “warp mode” movements, as the suspension spheres
(springing) don’t resist slow roll movements like conventional springs do -
only the rollbar does. (This improves traction a lot at very slow speeds
over very uneven ground) In fact without the rollbars the suspension would
be completely unstable on the roll axis – you could sit on the left and it
would go right down and the other side would go right up…
The downside of the cross connection is the same – the long term roll
stiffness is provided only by the rollbar – and there is no damping control
of the flow of oil from one side to the other, other than some restriction
caused by the small pipe diameter – hence the tendency of older Citro?ns to
have a lot of very slow body roll.
Hydractive 2 overcomes these shortcomings by modifying the side to side
connection – it is increased from 3.5mm to 10mm, but at the mid point there
is a unit with an additional sphere, an on/off valve, and two damper
valves. In the “soft mode” (selected dynamically by computer) this
additional middle sphere is connected in circuit and provides additional
springing, via the two damping valves in the unit. The system effectively
has two parallel paths for the oil to flow for each bump, with different
damping rates. The damper valves in the struts spheres on Hydractive 2 are
very stiff, while the ones in the middle unit are softer, giving a net
result of 3 stage damping in the soft mode, and 2 stage damping in the hard
mode. Any body roll requires oil to either flow into and out of the very
stiff damping valves in the strut spheres – where the opening thresholds
are above that produced by roll movement – or to flow from side to side -
where it must pass through two damping valves in series in the centre unit.
This means roll movements are hydraulically damped in Hydractive systems,
unlike Hydropneumatic. This contributes towards the reduced roll on later
models like XM and Xantia. Because of the large gauge of pipe there is the
potential for greater instantaneous flow when hitting large bumps, so the
roll axis stability of the car is actually improved over older models.
In the “hard mode”, again selected dynamically by the computer based on
inputs such as steering wheel angle and road speed, the central unit is
isolated, completely blocking the cross-flow of oil and isolating the
middle sphere, giving stiffer springing, much stiffer damping, and much
reduced body roll.
The Activa and Hydractive- 2 refinements / developments were quite
effective although only the Xantia has been fitted with it. The main
setback was that ride comfort was even worse than a BMW (although cornering
speeds were fantastic) which did not go too well with the traditional
Citro?n clientele. The current adjustable systems (computer controlled)
lack this anti roll characteristic, and there are owners who always prefer
the “comfort” setting rather than the “sporty” one, because again, that is
not what Citro?n is about.

A further mechanical advantage of hydraulic suspension is that the car is
able to link its braking effort to the weight on the wheels. In the Citro?n
BX, the rear braking effort comes from the pressure exerted on the LHM
fluid by the weight on those struts. This means that as the weight travels
forward under braking, there is less pressure on the back suspension. The
suspension then exerts less pressure on its fluid, and as weight and grip
diminish on the wheels, so does the braking effort, thus the hydropneumatic
system prevents rear wheel lock ups.
In addition to these benefits, Citro?n pioneered computer controlled
suspension in the early nineties by inserting a computer to take readings
from the cars’ chassis and control systems and let the computer make
informed decisions about how to handle the cars suspension. The computer
could then effect these decisions by things like servo valves, and offered
benefits like soft suspension for cruising, but stiffer, sportier
suspension for faster harder driving, allowing the driver to cruise in
comfort and still enjoy a responsive car. It also moves substantially
towards eliminating body roll and if used for a sportier driver will save
tyre wear as well (they claim).

hydropneumatic suspension

Its worth noting that when Mercedes launched their latest 600 SLC version
with a computer controlled anti roll system, Auto Motor und Sport then
proudly claimed that to be the first such anti roll system in world, only
having to correct that one issue later by having to mention a French
invention.
Rolls Royce was the only company ever to buy the patent and they used in in
the rear suspension of the Silver Shadow. When Citro?n was the owner of
Maserati some of their cars were also hydropneumatised.

More in-depth information can be found here:
http://www.citroenet.org.uk/miscellaneous/suspension/suspension8.html
http://web.actwin.com/toaph/citroen/work/work.html
http://www.tramontana.co.hu/citroen/guide/guide.php.
Meanwhile, the rest of us can hopefully feel satisfied with our newly
enriched understandings of hydropneumatic suspension. If you’re still
awake.
Hydraulic Suspension

Hydraulic suspension is an innovation making its way into motor sports, no
doubt to trickle down to consumer vehicles eventually. It has been designed
by a Spanish company called Creuat and pioneered by the Racing For Holland
Dome S101 sports car team. In the image below you can see both the
traditional coilover system (the yellow/blue/red units) at the front of the
car. This photo was taken before scrutineering for the 2005 24 Hours of Le
Mans race. The team had both systems online and when scrutineering passed
the car, the coilover units were removed, to race for the first time
completely with hydraulic suspension.
Central to their system is a control unit mounted next to the cockpit. They
tell me the system can’t be compared to the hydropneumatic suspension
Citro?n uses because this system doesn’t use a pump and has less than a
litre of hydraulic fluid in the entire system.
Instead of springs and dampers, this central Hydropneumatic unit takes care
of each suspension mode in an independent manner. This allows the car to be
tuned to avoid most of the compromises which arise out of the use of
conventional suspension made of springs and dampers.
This system is so new that the best source of information on it is Creuat’s
own website. You can find it at this link and you need to look for the Le
Mans Project in their menu on the left side of their page. The hydraulic
suspension page is a work-in-progress project and its content changes

almost weekly at the moment.
Racecar Engineering magazine have a feature article about this suspension
system at this link but you need a subscription to read the whole thing.
Fortunately Creuat have scanned the article and made it available as a
6.2Mb PDF file which you can read here.
Thanks to Sander van Dijk for sending me this photo, plus a ton of others
of their racing car.
hydraulic suspension

hydraulic suspension
Ferrofluid or magneto-rheological fluid dampers – Audi Magnetic Ride.

[ferrofluid]In 2006, Audi launched the new TT model and one of the
innovations that it came with is their magnetic semi-active suspension. It
is a totally new form of damping technology refined from Delphi’s MagneRide
system. Delphi used to be a division of GM when they developed the first
version of Magneride in conjunction with LORD Corp. (The initial version
was used in the 2002 Cadillac Seville STS). It is designed once again to
attempt to resolve the long-standing conflict between cabin comfort and
driving dynamics. The Audi system is a coninuously adaptive system – ie
it’s a closed feedback loop that can react to changes both in the road
surface and the gear-changes (front-to-back weight shift) within
milliseconds.
[2006 audi tt]So how does this work? Well, the dampers in the Audi system
are not filled with your regular old shock absorber oil. Nope. They’re
filled with (wait for it) magneto-rheological fluid. This is a synthetic
hydrocarbon oil containing subminiature magnetic particles. When a voltage
is applied to a coil inside the damper piston, it creates a magnetic field
(physics 101 – get that old textbook out and check the left- and
right-handed electro-magnetic rules that make electric motors work). Inside
the magnetic field, all the magnetic particles in the oil change alignment
in microseconds to lie predominantly across the damper. Because the damper
is trying to squeeze oil up and down through the flow channels, having the
particles lined up transverse to this motion makes the oil ‘stiffer’.
Stiffer oil flows less, which stiffens up the suspension. Neat.
You might have seen a demo of a similar system on TV in 2005 when an artist
in New York started making living art using a ferromagnetic liquid
(ferrofluid) and electromagnets. The principle is exactly the same – apply
a magnetic field and the fluid lines up along the lines of magnetism. The
image on the left shows a ferrofluid demonstration.
The Audi system has a centralised control unit which sends signals to the
coils on each damper. Hooked up to complex force and acceleration sensing
gauges, the control unit constantly analyses what’s going on with the car
and adjusts the damping settings accordingly. Because there are no moving
parts – no valves to open or close – the system reacts within microseconds;
far quicker than any other active suspension technology on the market
today. And because the amount of voltage applied to the coils can be varied
nearly infinitely, the dampers have a similarly near-infinite number of
settings. The power usage for each strut is around 5Watts, and the entire
thing takes up no more room than a regular coil-over-oil unit. Vorsprung
durch Technik indeed.
The diagram below shows the basic principle of magnetised vs. unmagnetised
ferrofluid, as well as a cutaway of the piston assembly in a Magneride-type
damper. The little blue balls represent the particles of fluid, and yes I
know they’re huge – that’s artistic licence so you can see them.
magneride
Linear Electromagnetic Suspension

bose suspension This is the latest innovation in suspension systems,
invented by Bose®. The idea is that instead of springs and shock absorbers
on each corner of the car, a single liner electromagnetic motor and power
amplifier can be used instead.
Inside the linear electromagnetic motor are magnets and coils of wire. When
electrical power is applied to the coils, the motor retracts and extends,
creating motion between the wheel and car body. It’s like the
electromagnetic effect used to propel some newer rollercoaster cars on
launch, or if you’re into videogames and sci-fi, it’s like a railgun.
One of the big advantages of an electromagnetic approach is speed. The
linear electromagnetic motor responds quickly enough to counter the effects
of bumps and potholes, thus allowing it to perform the actions previously
reserved for shock absorbers.
In it’s second mode of operation, the system can be used to counter body
roll by stiffening the suspension in corners. As well as these functions,
it can also be used to raise and lower ride height dynamically. So you
could drop the car down low for motorway cruising, but raise it up for the
pot-hole ridden city streets. It’s all very clever.
The power amplifier delivers electrical power to the motor in response to
signals from the control algorithms. These mathematical algorithms have
been developed over 24 years of research. They operate by observing sensor
measurements taken from around the car and sending commands to the power
amps installed with each linear motor. The goal of the control algorithms
is to allow the car to glide smoothly over roads and to eliminate roll and
pitch during driving.
The amplifiers themselves are based on switching amplification technologies
pioneered by Dr. Bose at MIT in the early 1960s. The really smart thing
about the power amps is that they are regenerative. So for example, when
the suspension encounters a pothole, power is used to extend the motor and
isolate the vehicle’s occupants from the disturbance. On the far side of
the pothole, the motor operates as a generator and returns power back
through the amplifier. By doing this, the Bose® system requires less than a
third of the power of a typical vehicle’s air conditioner system. Clever,
eh?

Bose have also managed to package this little wonder of technology into a
two-point harness – ie it basically needs two bolts to attach it to your
vehicle and that’s it. It’s a pretty compact design, not much bigger than a
normal shock absorber.
bose suspension

The official Bose suspension page can be found here if you want more info.

[aura systems]aura ram It’s worth noting that a company called Aura Systems
devised (or at least tried to market) a similar linear electromagnetic
suspension system around 1991. They published an article in the Automotive
Engineering Journal claiming that electromagnetic actuators could be used
for vehicle suspensions and it said that small devices could be designed
with a typical thrust capability of about 2500 Newtons and for a reasonable
power demand. This happened at the same time that linear electromagnetic
rams were being developed for entertainment simulators and full flight
simulators to replace hydraulic systems. In fact, it could be argued that
the Aura Systems ram was a direct descendant of the rams found on Super-X
entertainment simulators.
The units looked very similar to the Bose devices and had the same
limitation – they couldn’t carry the dead weight of the vehicle. Aura
Systems ran into financial troubles in 2000, and filed for Chapter 11 in
2005. The time scales fit quite nicely into the declared Bose time frame
(start of development versus going public). Of course they could have been
parallel developments, but the bigger question is why was Aura not able to
sell their system to an OEM at some time during the previous 15 years?
Could it be to do with mechanical limitations – that the sway bars carrying
vertical loads are very good at transmitting road inputs into the vehicle
structure even if the bar rate is low? Time will tell if Bose manage to
succeed where Aura Systems failed.

Variable-camber suspension for steering

oncamberIf you’ve read the wheel and tyre bible, you’ll know that camber is
the lateral tilt of the suspension (and hence the wheel and the tyre) to
the road surface. Proper camber (along with toe and caster) make sure that
the tyre tread surface is as flat as possible on the road surface. The
problem with regular fixed-geometry suspension is that the camber is set up
to be ideal when driving strai
ght. This means that however much you dislike
the idea, when you corner, less of the tyre’s tread is in contact with the
road surface because the tyre has to tilt slightly when the steering is
turned. In 2006, OnCamber LLC patented their variable camber steering
system which they launched at SEMA in Las Vegas. Matthew Kim, OnCamber’s
founder and president was kind enough to send some pictures of their
development system which you can see here. The idea is simple – as the
steering wheel is turned, the steering input shifts the top mounts of a
McPherson strut type suspension system laterally. In other words, the top
of the strut is no longer solidly bolted to the strut tower. When the top
mount point is moved, the camber of the suspension system changes. Turn to
the left, and the mounting points shift to the left tilting the wheels over
to the left giving a larger contact patch whilst cornering. ie. the inside
wheel tilts and goes into positive camber(almost parallel to the outside
wheel), which in turn contributes to the overall grip of the car. The
variable camber action also gives even tyre wear. Pyrometer readings during
testing have shown that the inside, mid, and outside tyre tread
temperatures are all within 2° of each other. With regular fixed-camber
steering, the inside of the tyre was 20° higher. OnCamber’s development car
is an RSX although they have designs on the table for double-wishbone
variants of their system too. On the RSX testbed the camber plates are
attached together by linear guides which permits them to move freely. The
top connecting rods are mechanically connected to the steering rack. The
degree of camber applied with steering is adjustable by varying the
distance of the rods from the pivot point. ie: when the rods are mounted
closer to pivot point you get more camber with less steering input. On
track, this system has shaved 3 seconds off the development vehicle’s lap
times in race conditions. Whether this sytem will trickle down into
consumer level cars is debatable. It’s doubtful that a manufacturer would
add this as standard but the racing and aftermarket scenes will undoubtedly
welcome this development with open arms. 3 seconds off your lap time for a
change of suspension components? Why wouldn’t you? The images below show a
camber plate at the top of one of the strut towers, and the mechanical
steering linkage.
oncamber steering link oncamber camber plate
Anti-roll Bars & Strut Braces
Strut Braces

If you’re serious about your car’s handling performance, you will first be
looking at lowering the suspension. In most cases, unless you’re a complete
petrolhead, this will be more than adequate. However, if you are a keen
driver, you will be able to get far better handling out of your car by
fitting a couple of other accessories to it. The first thing you should
look at is a strut brace. When you corner, the whole car’s chassis is
twisting slightly. In the front (and perhaps at the back, but not so often)
the suspension pillars will be moving relative to each other because
there’s no direct physical link between them. They are connected via the
car body, which can flex depending on its stiffness. A strut brace bolts
across the top of the engine to the tops of the two suspension posts and
makes that direct physical contact. The result is that the whole front
suspension setup becomes a lot more rigid and there will be virtually no
movement relative to each side. In effect, you’re adding the fourth side to
the open box created by the subframe and the two suspension pillars.
[brace]   [brace]
Simple straight brace(highlighted).      Complex brace (highlighted).
Anti-roll Bars (Sway Bars/Stabilizers)

No, these aren’t the things that are bolted inside the car in case you turn
it over – those are rollover cages. Anti-roll bars do precisely what their
name implies – they combat the roll of a car on it’s suspension as it
corners. They’re also known as sway-bars or anti-sway-bars. Almost all cars
have them fitted as standard, and if you’re a boy-racer, all have scope for
improvement. From the factory they are biased towards ride comfort. Stiffer
aftermarket items will increase the road-holding but you’ll get reduced
comfort because of it. It’s a catch-22 situation. Fiddling with your roll
stiffness distribution can make a car uncomfortable to ride in and
extremely hard to handle if you get it wrong. The anti-roll bar is usually
connected to the front, lower edge of the bottom suspension joint. It
passes through two pivot points under the chassis, usually on the subframe
and is attached to the same point on the opposite suspension setup.
Effectively, it joins the bottom of the suspension parts together. When you
head into a corner, the car begins to roll out of the corner. For example,
if you’re cornering to the left, the car body rolls to the right. In doing
this, it’s compressing the suspension on the right hand side. With a good
anti-roll bar, as the lower part of the suspension moves upward relative to
the car chassis, it transfers some of that movement to the same component
on the other side. In effect, it tries to lift the left suspension
component by the same amount. Because this isn’t physically possible, the
left suspension effectively becomes a fixed point and the anti-roll bar
twists along its length because the other end is effectively anchored in
place. It’s this twisting that provides the resistance to the suspension
movement.

[antiroll]

If you’re loaded, you can buy cars with active anti-roll technology now.
These sense the roll of the car into a corner and deflate the relevant
suspension leg accordingly by pumping fluid in and out of the shock
absorber. It’s a high-tech, super expensive version of the good old
mechanical anti-roll bar. You can buy anti-roll bars as an aftermarket
add-on. They’re relatively easy to fit because most cars have anti-roll
bars already. Take the old one off and fit the new one. In the case of rear
suspension, the fittings will probably already be there even if the
anti-roll bar isn’t.
[swaybar]

Typical anti-roll bar (swaybar) kits include the uprated bar, a set of new
mounting clamps with polyurethane bushes, rose joints for the ends which
connect to the suspension components, and all the bolts etc that will be
needed.

Suspension bushes
[bushes]

These are the rubber grommets which separate most of the parts of your
suspension from each other. They’re used at the link of an A-Arm with the
subframe. They’re used on anti-roll bar links and mountings. They’re used
all over the place, and from the factory, I can almost guarantee they’re
made of rubber. Rubber doesn’t last. It perishes in the cold and splits in
the heat. Perished, split rubber was what brought the Challenger space
shuttle down. This is one of those little parts which hardly anyone pays
any attention to, but it’s vitally important for your car’s handling, as
well as your own safety, that these little things are in good condition. My
advice? Replace them with polyurethane or polygraphite bushes – they are
hard-wearing and last a heck of a lot longer. And, if you’re into
presenting your car at shows, they look better than the naff little black
rubber jobs. Like all suspension-related items though, bushes are a
tradeoff between performance and comfort. The harder the bush compound, the
less comfort in the cabin. You pays your money and makes your choice.

The Ins and Outs of complex suspension units.

Generally speaking, this section will be more relevant to you if you ride a
motorbike, but you can get high-end spring / shock combos for cars that
have all these features on them. The thing to realise is that if you’re
going to start messing with all these adjustments, for God’s sake take a
digital photo of the unit first, or somehow mark where it all started out.
It’s a slippery slope and you can very quickly bugger up the ride quality
of your vehicle. If you don’t know what the “stock” setting was, you’ll

---

never get it back.
Compression damping.

This is the damping that a shock absorber provides as it’s being
compressed, ie. as you hit a bump in the road. It’s the resistance of the
unit to alter from its steady state to its compressed state. Imagine you’re
riding along and you hit a bump. If there is too little compression
damping, the wheel will not meet enough resistance as the suspension
compresses. Not enough energy is dissipated by the time you reach the crest
of the bump and because the wheel and other unsprung components have their
own mass, the wheel will continue to move upwards. This unweights or
unloads the tyre and in extreme cases, it can lose contact with the road.
Either way, you briefly lose traction and control.
The opposite is true if compression damping is too heavy. As the wheel
encounters the bump in the road, the resistance to moving is high and so at
the crest of the bump, the remaining energy from the upward motion through
the shock absorber is transferred into the frame of the bike or the chassis
of the car, lifting it up.
compression and rebound
Rebound damping.

Go on – have a guess at what this is. Well in case you’re not following
along, this is the damping that a shock absorber provides as it returns
from its compressed state to its steady state, ie. after you’ve crested the
bump in the road. Too light, and the feeling of control in your vehicle is
minimised because the wheel will move very quickly. The feeling is the
soft, plush ride you find in a lot of American cars. Or mushy as we like to
call it. Too heavy, and the shock absorber can’t return quickly enough. As
the contour of the road drops away after the bump, the wheel has a hard
time “catching up”. This can result in reduced traction, and a downward
shift in the height of the vehicle. If that happens, you can overload the
tyre when the weight of the vehicle bottoms-out the suspension.
Damping controllers.

High-end kit has controls on the shock absorber for both compression and
rebound damping. Typically the rebound damping will be a screwdriver slot
at the top of the shock absorber, and compression damping will be a knob
either on the side or on the remote reservoir. Ultra-high-end kit has
separate controls for high- and low-speed damping. ie. you can make the
shock absorber behave differently over small bumps (low speed compression
and rebound) than it does over large bumps (high speed compression and
rebound). Of course you could buy yourself a nice big TV, a DVD player,
dark curtains, a new couch and a year’s supply of popcorn for the same cost
as four of these units.
Spring preload.

Some motorbike suspension units, as well as some found on cars, give you
the ability to alter the spring preload or pre-tension. This means that
you’re artificially compressing the spring a little which will alter the
vehicle’s static sag – the amount of suspension travel the vehicle consumes
all by itself. For example, if you ride a motorbike on your own, the
preload might work on the factory setup. But if you put a passenger on the
back, the tendency is for the bike to sag because there’s now more sprung
weight. Increasing the preload on the spring plate will help compensate for
this.
Sprung vs. unsprung weight.

Simply put, sprung weight is everything from the springs up, and unsprung
weight is everything from the springs down. Wheels, shock absorbers,
springs, knuckle joints and tyres contribute to the unsprung weight. The
car, engine, fluids, you, your passenger, the kids, the bags of candy and
the portable Playstation all contribute to the sprung weight. Reducing
unsprung weight is the key to increasing performance of the car. If you can
make the wheels, tyres and swingarms lighter, then the suspension will
spend more time compensating for bumps in the road, and less time
compensating for the mass of the wheels etc.
The greater the unsprung weight, the greater the inertia of the suspension,
which will be unable to respond as quickly to rapid changes in the road
surface.
As an added benefit, putting lighter wheels on the car can increase your
engine’s apparent power. Why? Well the engine has to turn the gearbox and
driveshafts, and at the end of that, the wheels and tyres. Heavier wheels
and tyres require more torque to get turning, which saps engine power.
Lighter wheels and tyres allow more of the engine’s torque to go into
getting you going than spinning the wheels. That’s why sports cars have
carbon fibre driveshafts and ultra light alloy wheels.
Progressively wound springs

These are the things to go for when you upgrade your springs. In actual
fact, it’s difficult not to get progressive springs when you upgrade – most
of the aftermarket manufacturers make them like this. Most factory-fit car
springs are normally wound. That is to say that their coil pitch stays the
same all the way up the spring. If you get progressively wound springs, the
coil pitch gets tighter the closer to the top of the spring you get. This
has the effect of giving the spring increasing resistance, the more it is
compressed.
The spring constant (stiffness) of a coil spring equals:
k = compression / force = D^4 * G / (64*N*R^3)
where D is the wire diameter, G an elastic material property, N the number
of coils in the spring, and R the radius of the spring.
So increasing the number of coils decreases the stiffness of the spring.
Thus, a progressive spring is progressive because the two parts are
compressed equally until the tightly wound part locks up, effectively
shortening the spring and reducing its compliance.
So for normal driving, you’ll be using mostly the upper 3 or 4 ‘tight’
winds to soak up the average bumps and potholes. When you get into harder
driving, like cornering at speed for example, because the springs are being
compressed more, they resist more. The effect is to reduce the suspension
travel at the top end resulting in less body roll, and better road-holding.
Invariably, the fact that the springs are progressively wound is what
accounts for the lowering factor. The springs aren’t made shorter – they’re
just wound differently. Of course the material that aftermarket springs are
made of is usually a higher grade than factory spec simply because it’s
going to be expected to handle more loads.
Note:Make sure you get powder-coated springs! This means they’ve been
treated with a good anti-corrosion system and then covered in powdered
paint. The whole lot is then baked to make the paint seal and stick and
bring out it’s polyurethane elastic properties. It’s the best type. If you
just get normally painted springs, the paint will start to flake on the
first bump, and surface rust will appear within days of the first sign of
dampness. Not good. Besides – powder coated springs look cool too! [smiley]

[springs]
Electronic damping force controllers.

edfc Remember way back at the top of the page I mentioned that some dampers
allowed you to change the damping rate by altering the size of the
constriction hole? That’s all very well and good but you have to stop your
car, get out and twiddle a knob or screw on the top or side of the strut
each time you want to make a change. In 2005 the aftermarket saw the first
appearance of an EDFC – electronic damping force controller.
The premise is really simple. Four servo motors (the four smaller boxes in
the picture here), one for each strut, each one designed to replace the
manual screw adjuster. A control unit mounts inside the car and allows you
to change the damping force of the shocks front and rear without leaving
the drivers seat. The way it works is dead simple. When you first install
the system and power it up, all the servos spin clockwise for a few
seconds. This ensures the adjusters are screwed all the way in on all four
struts. From that point, you can dial in any number from 0 to 20 on the
control unit. When you do, the servo motors spin a certain amount – the
same as you getting out of the car and spinning the adjuster with your
finely calibrated fingers. The units currently have three memory settings
so you can store motorway, city and track-day settings (for example), and
recall them at the push of a button.
Installing the current-generation EDFCs is pretty simple – about the most
difficult thing you’ll face is running the wires from each servo back to
the control unit inside the car.
There’s a few different companies selling EDFCs right now. This link will
take you to a googlesearch for further info.
Torsion bars

torsion bar Torsion bars deserve their own section because they are a type
of spring which can be used in place of coil- or leaf-springs. It’s one of
the topics I get the most e-mail on, so instead of continually sending the
same answer, I thought I’d cover it on this page.
A torsion bar is a solid bar of steel which is connected to the car chassis
at one end, and free to move at the other end. They are almost always
mounted across the car, one for each side of the suspension. The springing
motion is provided by the metal bar’s resistance to twisting. To
over-simplify, stick your arm out straight and get someone to twist your
wrist. Presuming that your mate doesn’t snap your wrist, at a certain
point, resistance in your arm (and pain) will cause you to twist your wrist
back the other way. That is the principle of a torsion bar.
Torsion bars typically have splines on one end so that they can be removed,
twisted round one spline and re-inserted. This can be used to raise or
lower a car, or to compensate for the natural ‘sag’ of a suspension system
over time.
torsion bar    torsion bar
Lift Kits

Because of the mechanical nature of suspension, all sorts of mods are
available. Lifting suspension is a popular mod used to try to increase
ground clearance. This is often a source of misunderstanding. A lift kit
doesn’t really give you more ground clearance. What it does is increase the
height between the axle and the underside of the body. Whilst this does
give more ground clearance for the bodywork, the lowest point on the
vehicle is still the axles – or on a 4-wheel-drive, the bottom of the
transfer case. For this reason, you’ll often see trucks and SUVs with lift
kits and larger wheels and tyres. The lift kit boosts the clearance under
the bodywork whilst the larger wheels and tyres result in the axles being
lifted higher off the ground. Technically of course, in a 4-wheel-drive,
you don’t really need a lift kit – bigger wheels and tyres would do it. BUT
lift kits typically end up being required because adding on the larger
wheels and tyres can often mean they will no longer fit in the wheel
arches. The lift kit will help solve that problem.
Lift kits come in literally hundreds of shapes and sizes, all dependent on
the final application as well as the design of the vehicle the kit is going
to be used on. For street cars, typically with independent suspension, the
kit will basically be longer struts, longer springs and remounted shocks.
For off-roaders with beam axles and transfer cases, the suspension system
is typically leaf-spring, so the kit will be a set of blocks that fit
between the beam axle and the bottom of the leaf spring. Alternatively,
some kits have blocks which lower the spring mounts themselves so that the
spring-to-axle joint isn’t changed. The image below shows an example of a
typical leaf-spring beam-axle suspension system along with two examples of
how it can be raised.
[lift kits]

Fitting a lift kit is pretty basic engineering but it’s really difficult to
do without access to a hydraulic lift, so its best to either get a garage
to do it, or to find a mechanic friend who has a decent sized hydraulic
lift. Trying to mess with the suspension whilst a vehicle is on the ground
is just asking for trouble.
Speaking of trouble…

Lifting a vehicle is going to affect its handling. Most obviously, you’re
going to add height to the centre of gravity, which in turn is going to
make the vehicle more prone to roll in corners. At the extreme, an already
roll-happy SUV or truck will become even more likely to turn over in the
event of an accident.
Similarly, just because you’ve lifted your truck, don’t think you can
instantly go off-road with it like a pro. If you’re doing it for off-road
functionality rather than just pose value, spend the extra cash and get a
one-day off-road course. You’ll have a blast and it will make you
infinitely safer when you do take your vehicle off the beaten track.
It’s also worth pointing out that putting larger wheels on simply to
increase ground clearance can come with all its own problems including the
legality of it, changes to the steering and suspension geometry and
steering load. It’s also a possibility on some types of 4WD vehicle that
larger tyres and steering load can result in tearing the steering box off
the chassis. Other things which tend to fail quicker when this is done are
items like pitman arms, track rods, knuckle and ball joints – all of these
get stressed beyond their normal design limits when you stuff massive tyres
and wheels on a truck.
One other point to consider when doing this: if your speedometer is based
on a mechanical link to the gearbox, your speedo will become so innacurate
that it will basically be useless. You’ll be driving at an indicated 30mph
but could be doing 40mph if the tyres are big enough.
Just be warned.
Lowering Kits

The opposite of lift kits – lowering kits. These are designed to (wait for
it….) lower your car. Also at the other of the scale – lowering kits are
almost exclusively used on cars, whereas lift kits are almost exclusively
used on trucks and SUVs. (Having said that, the number of pimped-out
low-rider trucks on the road does seem to be increasing by the day.)
Lowering your car will similarly affect the handling, just like a lift kit.
But again it’s the opposite end of the spectrum – a lowered car will
typically handle much better than factory suspension, and it will lower the
centre of gravity, making it less likely to tip or roll in an accident. I’m
a European, and as far as I’m concerned, if you’re going for pose value,
lowering your car is the quickest way to do it, hotly pursued by larger
wheels and tyres to make the car appear even more ground-hugging.
Lowering kits typically consist of shorter, stiffer springs and gas shocks
- often nitrogen-filled. Don’t do it by halves. Get a matched kit from
someone like Spax or Jamex. Matched kits have springs and shocks designed
to work together. If you get shorter springs, your factory shocks will be
under a lot of stress because they’ll be operating a much shorter throw
than they were designed for, and ultimately, they’ll normally fail much
quicker. Similarly, don’t get shorter shocks and the cut the springs.
Cutting the springs is the epitome of A Really Bad Idea. You’re weaking the
spring’s structural integrity and the chances are that when you’ve finished
a ham-fisted attempt at hacking off all 4 springs with a grinder, the
result will be 4 springs all slightly different lengths.
There’s something else worth mentioning here – do not try to disassemble a
shock absorber. Ever. Those things are like little bombs, and unless you
have all the right tools, you could easily loose a hand as the shock
explodes into its component parts when you get that last twist off the
collar. Please – just don’t. I know your mate Guido might have told you
it’s a “sure fire” way to shorten the shock, but he’s lying.
Matched lowering kits typically assume you’re going for sportier handling,
so a lot of times, you’ll get a whole slew of new adjustments which you
never had before. Spring height, rebound damping, compression damping etc.
My recommendation is to leave everything as it is to start with. Right out
of the box they’re normally set up pretty well. The following renderings
show an example “before and after” of a lowering kit fitted to a car:
[lowering kits]
Lowering kit questions.

What if I get shorter springs to lower the car? Will I need to adjust my
caster and camber angles and/or my shock absorbers?
    Generally the answer would be no for caster/camber angles. Most cars
have a good 10-13cm (4-5 inches) movement in their suspension from the
factory. As most of the lowering springs you can buy only lower by 2-7cm
(1-3 inches), your suspension should still be well within it’s designed
operating limits. Therefore, caster and camber angles shouldn’t need
looking at. As for the shocks, see the FAQ page.
What if I get shorter springs to lower the car? Will my tyres rub on my
arches?
    They shouldn’t unless you start messing about with wheel and tyre
sizes. Again, given that most suspension kits lower within the car’s normal
operating limits, there shouldn’t be a problem. If there was, then every
time you went over a big hump with standard suspension, the tyres would
rub. Rubbing against the arches will almost certainly only occur if you
lower the car and widen the wheels. See the Wheel & Tyre Bible for more
info on this.

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