What does the suspension do?

Apart from your car's tyres and seats, the suspension is the prime mechanism that separates your bum 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, really 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 Peugot 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 imagine 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.
Shock absorbers
Strangely enough, absorb shocks. Actually, shock absorbers are one of those great misnomers in life. They're really called dampers, because they actually 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. Technically, they are actually dampers. Even more technically, they are velocity-sensitive hydraulic damping devices phew- 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. (phew!....and you thought they just leaked oil didn't you?)

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 independant suspension types.

Front suspension - dependent systems

So-called because the front wheel's suspension systems are physically linked. They are, in a word, useless. I could be more obusive about this but I had better not!. There is only one type of dependant system you need to know about. It is basically a sold 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 cars for donkey's years for three main reasons:

Front suspension - independant systems

So-named because the front wheel's suspension systems are independant of each other (except where joined by an antiroll bar. These came into existance around 1930 and have been in use in one form or another pretty much ever since then.

Macpherson strut

This is currently, without doubt, the most widely used front suspension system in cars of European origin. It is simplicity itself. Unfortunately, the image on the left doesn't show you all the components but you will get the idea. However, the system basically comprises of a 'coil-over-oil' 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. The steering gear is either connected directly to the lower shock absorber housing (purple in this image), or to an arm from the front or back of the spindle (in this case). When you steer, it physically twists the 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.

Note: The following four types of system are all essentially a variation on the same theme.

Coil Spring type 1

[suspension1] This is a type of double-A arm suspension. The wheel spindles (purple) are supported by an upper (green) and lower (blue) 'A' shaped arm. If you look head-on at this type of system, what you'll find is that it's a very basic lever 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 which the levers scribe around their pivot point. 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. Also note that the springs/shocks in this example are in a so-called 'coil over oil' arrangement whereby the shock absorbers (yellow) sit inside the springs (red).

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 single solid arms. The only real difference between this and the type 1 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.

Double Wishbone

So-called because the lower and upper arms are the shape of wishbones. The spindle is a highly complex construction in this system, as are the wishbones themselves. This rapidly becoming one of the most favoured suspension types for new cars as it gives excellent roadholding capabilities whilst taking up very little room under the car. This allows for smoother lines on the bodywork, and less intrusion in to the engine bay. A 2D diagram such as that on the right does not do this system any justice. To really appreciate it, you need to get your head in a wheel well and have a look. And I know a few mechanics who've still not been able to figure it out even then.

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. 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 few variations on this theme appearing at the moment, with differences in the numbers of joints, numbers of arms, positioning of the parts etc. But they are all fundamentally the same.

Trailing-arm suspension

[suspensionX] Unfortunately, no image for this yet. The trailing arm system is literally that - a shaped suspension arm is joined at the front to the chassis, alowing the rear to swing up and down. Pairs of these become twin-trailing-arm systems and work on exactly the same principle as the arms in the coil spring type systems described above. The difference is that instead of the arms sticking out from the side of the chassis, they travel back along it. 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. It's used mostly in older cars and beach buggies now.

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 (purple in this image) is clamped (green) to the leaf springs (red). The shock absorbers (yellow) are also attached to the clamps. The ends of the leaf springs are attached directly to the chassis, as are the shock absorbers. Simple, not particularly elegant, but cheap. The main drawback with this arrangement is the lack of lateral location for the axle.

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 'coil-over-oil' spring and shock combos. 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. A variation on this has the shock absorbers separate from the springs, allowing much smaller 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. Also at the ends, the springs and shock absorbers are attached. 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 as shown here, or the combined 'coil-over-oil' variety. One noteable feature of this system is the track bar (or panhard rod). This is a diagonal bar which runs from the rear corner of the beam to a point either just in front of the opposite corner, or in this case, above the opposite spring mount. 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 this system, the axle is designed to twist slightly. This gives, in effect, a semi-independant 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 beam supports. These beam types are currently very popular because of their simplicity and low cost.

Rear suspension - independant 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 independant 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 independant suspension'. This means all the wheels are independantly 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 independant suspension is primarily to improve ride quality without degrading handling.

Hydropneumatic Suspension

This little wonder was invented by Citro�n in the late 50's and has been fitted to many of their cars since. I've had to separate it into it's own category because it is quite different from any other type of suspension system. Typically, this system works off a one-cylinder high-pressure pump driven from the oil pump. It's a separate hydropneumatic circuit which is connected to all four suspension units (which are usually 'double-A' arm units on the front, and a beam or twist axle on the rear) via a system of high-pressure hoses (similar to brake lines). Instead of separate shock absorbers and springs, each suspension unit (red) has a hybrid shock absorber-type piston with a reservoir (yellow) attached to the top of it. At 'rest', the car sits very low on the ground. When the engine is started, the hydropneumatics pump fluid into all the suspension units, raising the car to operating height. The system boasts automatic and manual regulation of ride height, which in turn allows for self-levelling suspension and suspension that will actually roll a car body into a corner. Other features allow the driver to manually select a ride-height suitable for the type of road they are driving on. With the latest technology (again, as in the Activa) the onboard computer can adjust the settings and ride height of all the suspension units several times a second to give a very smooth ride. The hydropneumatic suspension is exclusive to Citro�n, beware second hand vehicles with this system unless you are rich.

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 physical link between them. A strut brace bolts across the top of the engine to the tops of the two suspension posts and makes that 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]
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 roadholding 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. In doing this, it's actually compressing the suspension on that side which basically counters some of the roll in the chassis by lowering that side of the car.

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 addon. 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.

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

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.

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. So for normal driving, you'll be using mostly the lower 3 or 4 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 roadholding. 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!

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