Apart from your car's tires 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 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.
In their infinite wisdom, car manufacturers have set out to baffle use with the sheer number of different types of suspension available for both front and rear axles. The main groupings are dependant and independent suspension types. If you know of any not listed here, e-mail me and let me know - I would like this page to be as complete as possible.
Front suspension - dependent systems
So-called because the front wheel's suspension systems are physically linked. For everyday use, they are, in a word, shite. I hate to be offensive, but they are. There is only one type of dependant system you need to know about. It is basically a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. It's still common to find these on trucks, but if you find a car with one of these you should sell it to a museum. They haven't been used on mainstream cars for years for three main reasons:
* Shimmy - because the wheels are physically linked, the beam can be set into oscillation if one wheel hits a bump and the other doesn't. It sets up a gyroscopic torque about the steering axis which starts to turn the axle left-to-right. Because of the axle's inertia, this in turn feeds back to amplify the original motion.
* Weight - or more specifically unsprung weight. Solid front axles weigh a lot and either need sturdy, heavy leaf springs or heavy suspension linkages to keep their wheels on the road.
* Alignment - simply put, you can't adjust the alignment of wheels on a rigid axis. From the factory, they're perfectly set, but if the beam gets even slightly distorted, you can't adjust the wheels to compensate.
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 spreadsheet until after the product has been produced or manufactured. Consequently, Earle MacPherson went to work for Ford UK in 1950, where Ford started using his design on the 1950 'English' Ford models straight away. Today the strut type is referred to both with and without the "a" in the name, so both McPherson Strut and MacPherson Strut can be used to describe it.
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).
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.
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.
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.
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.
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.
--------------------------
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.
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.
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.
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 balls represent the particles of fluid, and yes I know they're huge - that's artistic licence so you can see them.
-----------------------------
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.
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.
Suspension FAQ - All you need to know and more!
Linear Mode
