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What does it do?

Apart from your car's tires 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, 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 tire 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 Porshes (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 visualize, 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.



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.

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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, *****. 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.

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 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 sheet 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.

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Double wishbone suspension systems.

The following three examples are all variations on the same theme.



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 tires.




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.




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).

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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.





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.

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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.




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.






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.

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 bastardization 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 equaled 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 disadvantage. 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 drive shaft 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, especially for flyweight cars.

Rear suspension - 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.

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Ford Control Blade™ Suspension

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.




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.

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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.

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 shouldn't be confused with Citroën'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.

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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.



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.

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Hydropneumatic Suspension
[hydropneumatic]

Since the late forties, Citroën 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. Citroën 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 Citroën'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.

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.




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 Citroën 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 Citroën 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; Citroën 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 Citroën 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 Citroëns 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 Citroën 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 Citroën 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 Citroën 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, Citroën 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).



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 Citroën was the owner of Maserati some of their cars were also hydropneumatised.

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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 Citroën 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.




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Ferrofluid or magneto-rheological fluid dampers - Audi Magnetic Ride.



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.



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 ***** represent the particles of fluid, and yes I know they're huge - that's artistic licence so you can see them.



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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.

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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.



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Variable-camber suspension for steering




If 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 straight. 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.





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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.




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.



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.

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.

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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.


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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.



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 **** 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.

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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!



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 **** 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.

Torsion bars



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.




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.



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.

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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 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.

What if I get shorter springs to lower the car? Will my tires rub on my arches?

They shouldn't unless you start messing about with wheel and tire 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 tires would rub. Rubbing against the arches will almost certainly only occur if you lower the car and widen the wheels.

Source: Car Bibles.com

Coolieboi

really good write up...

xudeck

This is some excellent info.

Thanks

2TONE_93GT

iTrader: (3)

thanks a ton for the write up!

repped!

EEngineer

i put on some coilovers, my ride handles much better now, but the damping/ride comfort went out of the window, are there any suggestions to how i can get some comfort back while keeping the handling?