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Car Bibles : The Car Suspension Bible

These pages were last updated on 5th August 2005. Copyright Š Chris Longhurst 1994 - 2005 unless otherwise stated. This site and all contents unless otherwise noted are copyrighted. The author will respond expeditiously to any intellectual property infringement. Reproduction in whole or in part in any form or medium without express written permission of Chris Longhurst is prohibited. Copyright info. (20 of 20)8/10/2005 10:46:32 AM

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DISCLAIMER:I am in no way affiliated with any branch of the motor industry. I am just a pro-car, pro-motorbike petrolhead :-) The information on these pages is the result of a lot of information-gathering and research. This website was originally established in 1994 to answer a lot of FAQs from posters on the old transportrelated usenet groups. By reading these pages, you agree to indemnify, defend and hold harmless me (Christopher J Longhurst), any sponsors and/or site providers against any and all claims, damages, costs or other expenses that arise directly or indirectly from you fiddling with your car or motorbike as a result of what you read here. In short : the advice here is worth as much as you are paying for it. One more thing : the Google ads are only at the top of the page here - I need to pay for my site space and bandwidth somehow. The rest of the page is ad-free for your reading pleasure.

What does it do? Apart from your car's tyres and seats, the suspension is the prime mechanism that separates your bum (arse for the American) from the road. It also prevents your car from shaking itself to pieces. No matter how smooth you think the road is, it's a bad, bad place to propel over a ton of metal at high speed. So we rely upon suspension. People who travel on underground trains wish that those vehicles relied on suspension too, but they don't and that's why the ride is so harsh. Actually it's harsh because underground trains have no lateral suspension to speak of. So as the rails deviate side-to-side slightly, so does the entire train, and it's passengers. In a car, the rubber in your tyre helps with this little problem. In it's most basic form, suspension consists of two basic components: Springs These come in three types. They are coil springs, torsion bars and leaf springs. Coil springs are what most people are familiar with, and are actually coiled torsion bars. Leaf springs are what you would find on most American cars up to about 1985 and almost all heavy duty vehicles. They look like layers of metal connected to the axle. The layers are called leaves, hence leaf-spring. The torsion bar on its own is a bizarre little contraption which gives coiled-spring-like performance based on the twisting properties of a steel bar. It's used in the suspension of VW Beetles and Karmann Ghias, air-cooled Porsches (356 and 911 until 1989 when they went to springs), and the rear suspension of Peugeot 205s amongst other cars. Instead of having a coiled spring, the axle is attached to one end of a steel shaft. The other end is slotted into a tube and held there by splines. As the suspension moves, it twists the shaft along it's length, which in turn resist. Now image that same shaft but instead of being straight, it's coiled up. As you press on the top of the coil, you're actually inducing a twisting in the shaft, all the way down the coil. I know it's hard to visualise, but believe me, that's what is happening. There's a whole section further down the page specifically on torsion bars and progressive springs. Shock absorbers Strangely enough, absorb shocks. Actually, 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 (1 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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. (phew!....and you thought they just leaked oil didn't you?)

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 compression damping of the shock absorber. More sophisticated units have adjustable rebound damping as well as a remote reservoir. Whilst you don't typically get this level of engineering on car suspension, most motorbikes do have preload, rebound and spring tension adjustment. See the section later on in this page about the ins and outs of complex suspension units.

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

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

Shimmy - because the wheels are physically linked, the beam can be set into oscillation if one wheel hits a bump and the other doesn't. It sets up a gyroscopic torque about the steering axis which starts to turn the axle left-to-right. Because of the axle's inertia, this in turn feeds back to amplify the original motion. Weight - or more specifically unsprung weight. Solid front axles weigh a ton and need huge springs 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. I frequently get pulled-up on the above statements from people jumping to defend solid-axle (2 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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

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

MacPherson strut 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 rendered image here, you can't see the shock absorber because it is encased in the blue strut tower, 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 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. Potted history of MacPherson: Earle S. MacPherson of General Motors developed the MacPherson strut in 1947. GM cars were originally design-bound by accountants. If it cost too much or wasn't tried and tested, then it didn't get built/ used. Major GM innovations including the MacPherson Strut suspension system sat stifled on the shelf for years because innovation cannot be proven on a spreadsheet until after the product has been produced or manufactured. Consequently, Earle MacPherson went to work for Ford UK in 1950, where Ford started using his design on the 1950 'English' Ford models straight away. Further note: Earle MacPherson should never be confused with Elle McPherson the Australian 端ber-babe. In her case, the McPherson Strut is something she does on a catwalk, or in your dreams if you like that sort of thing. And if you're a bloke, then you ought to....

The following four types of system are all essentially a variation on the same theme. (3 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

Coil Spring type 1 This is a type of double-A arm suspension. The wheel spindles (purple) are supported by an upper and lower 'A' shaped arms (green). 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 socalled '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. Yes I know they don't look like wishbones here, but believe me, they are. The spindle is a highly complex construction in this (4 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

system, as are the wishbones themselves. This rapidly becoming one of the most favoured suspension types for new cars as it gives excellent road-holding 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 3D rendering 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. 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 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 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 (5 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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 doubletrailing-arm suspension setup. Simple. It's used mostly in older cars and beach buggies now.

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 lower image). 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.

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. (6 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

Solid-axle, leafspring 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. (7 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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 solidaxle-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 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-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 beam supports. 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 left here, and parallel, shown on the right. 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. (8 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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.

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

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 purple structure is the subframe, the green parts are the suspension swingarms, and 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. (9 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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

Hydragas Suspension Hydragas is an evolution of Hydrolastic, and essentially, the design and installation of the system is the same. The difference is in the displacer unit itself. In the older systems, fluid was used in the displacer units with a rubber spring cushion built-in. With Hydragas, the rubber spring is removed completely. The fluid still exists but above the fluid there is now a separating membrane or diaphragm, and above that is a cylinder or sphere which is charged with nitrogen gas. The nitrogen section is what has become the spring and damping unit whilst the fluid is still free to run from the front to the rear units and back. (10 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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.

Hydropneumatic Suspension

{Thanks to Jonathan Bruce and Pieter Melissen for some updates to this information.} Since the late forties, Citroën have been running a fundamentally different system to the rest of the auto industry. They call it hydropneumatic and it encompasses features as diverse as brakes, suspension & steering. As its name may suggest, its core technology and mainstay of its functionality is hydraulics. Superbly smooth suspension is provided by the fluid's interaction with a pressurised gas. They pioneered in the rear suspension of the 15 (Traction Avant) model, and it 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. The system is powered by a large hydraulic pump operated directly by the engine in much the same way as an alternator or an air conditioner is, and 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-auto box.) Because this page is all about suspension, for clarity we'll look at the simplified version of this as installed in the "BX" model. The Citroën BX was a major turning point in the company's history as it was the first car to be produced under the company's new Peugeot management, following the 1970s take-over of Citroën by Peugeot. As a direct consequence of the Peugeot influence, the car is somewhat more conventional than its larger sibling designed earlier - the CX. This Peugeotenforced "normalisation" of the design makes it easy enough to examine as an illustration of how hydropneumatic suspension works. There are two main components you need to familiarise yourself with and to understand. The spheres are like the springs on the car, and the struts are the hydraulic components that make the fluid act like a spring. Lets start with the sphere. The spring in this suspension system is provided by a hydraulic component called an accumulator, which is gas under pressure in a bottle contained within a diaphragm, 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. As you can see in the drawing above the pink gas (nitrogen) is compressed when the pressure in the green fluid (LHM) overcomes the gas pressure, and pushes back the diaphragm which compresses the gas. Then as the pressure in the fluid reduces, the gas pushes back the diaphragm and as the gas overcomes the fluid, it expels the fluid from the sphere, returning gas and fluid to equilibrium. This is the hydropneumatic equivalent to the spring getting compressed (bound) and getting depressed, ie springing back (rebound). Still with me? We can keep going... How can a gas, a diaphragm and a hydraulic fluid compressing, 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 sphere for ones of different specs, it is possible to adjust the ride characteristics with these cars. Rumour even has it that a racing team in Anglesey is customising their car by pressurising their own spheres to custom pressures to make an exact match for the circuit the are on. 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. As you can see in this diagram, the strut has a sphere on top of it and the strut in itself acts like a syringe to inject fluid into the sphere. When the wheel hits a bump it rises, pushes the piston of the strut 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 and its time to let the wheel back down, 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 by the company deciding 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, a trend low riders are now following on with in California, nearly fifty years later. Also, they could link the four corners together to make a system that prepared the car for the bump to keep it even and offer the passengers a smoother ride. Basically they put fancy valves called height correctors on the (11 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

anti-roll bar. These were mounted in such a way that as the suspension twisted, this operated the valves that controlled the transfer of fluid to the struts. It was possible to isolate the front and rear systems and have the front suspension set at a height which required 'x' litres. So when the front nearside wheel takes a knock compressing its sphere, x/2 L is lost in the sphere, then the height correctors allow another x/2 L in, to inflate the offside strut by that much. This keeps the front of the car level in a horizontal plane. As the car clears the bump, the reverse happens; the sphere displaces that fluid, the strut returns to its own height pulling the anti roll bar back true with it which in turn tells the height corrector to lose that extra x/2 litres of fluid from the other side. As one side extends its strut in reaction to clearing the bump, the other is retracting by the same amount to return the car to its set height above the road. Neat huh? 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 is the able to exert 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).

There was a further refinement / development in this suspension design in the 1990s called the Activa system, designed to compensate for body roll. It was 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. 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 Citroen was the owner of Maserati some of their cars were also hydropneumatised. More in-depth information can be found here - - or - ozway/page5.html. Meanwhile, the rest of us can hopefully feel satisfied with our newly enriched understandings of hydropneumatic suspension. If you're still awake.

Hydraulic Suspension Hydraulic suspension is an innovation making its way into motor sports, no doubt to trickle down to consumer vehicles eventually. It has been designed 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. More news on this development as I get it. Thanks to Sander van Dijk for sending me this photo, plus a ton of others of their racing car. (12 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

Linear Electromagnetic Suspension This is the latest innovation in suspension systems, invented by Bose速. The idea is that (13 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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速 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.

The official Bose suspension page can be found here if you want more info. 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. (14 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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.

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.

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-swaybars. 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. (15 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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.

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

The Ins and Outs of complex suspension units. (16 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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

Spring preload. Some motorbike suspension units, as well as some found on cars, give you the ability to alter the spring preload or pre-tension. This means that you're artificially compressing the spring a little which will alter the vehicle's static sag - the amount of suspension travel the vehicle consumes all by itself. For example, if you ride a motorbike on your own, the preload might work on the factory setup. But if you put a passenger on the back, the tendency is for the bike to sag because there's now more sprung weight. Increasing the preload on the spring plate will help compensate for this. (17 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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

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

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. (18 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible

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 oversimplify, 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.

But What if.......? 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. 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 tyres rub on my arches? They shouldn't unless you start messing about with wheel and tyre sizes. Again, given that most suspension kits lower within the car's normal operating limits, there shouldn't be a problem. If there was, then every time you went over a big hump with standard suspension, the tyres would rub. Rubbing against the arches will almost certainly only occur if you lower the car and widen the wheels. See the Wheel & Tyre Bible for more info on this.

Bouncy Links Here's some links for you to follow. Kinetic Suspension systems Tim Stiles Racing - VW and Audi suspension mods etc Monroe shock absorbers Suspension Eibach Suspension TMS Suspension catalogue for BMWs (19 of 20)8/10/2005 10:46:32 AM

Car Bibles : The Car Suspension Bible