The car suspension bible - how car suspension works including shocks, struts, springs, raising and lowering your suspension, different types of suspension, all the technologies involved, DIY car maintenance and much more.

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Hydropneumatic Suspension

{Thanks to Julian Marsh, Jonathan Bruce, Simon Byrnand and Pieter Melissen for some updates to this information.}
Since the early fifties, 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 may also be used for the power steering and the brakes, and in the DS for the semi-automatic gearbox. Note - the C5 and C6 only use the high pressure hydraulics for the suspension - brakes and steering are conventnional.
Under the company's new Peugot management, Citroën produced the LN, followed by the Visa and then the LNA and then the BX. The BX was a major turning point in Citroën's history. 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. The BX employed pseudo-McPherson struts at the front with a hydropneumatic unit replacing the coil spring and damper. At the rear a 'conventional' trailing arm was used with the hydropneumatic unit mounted horizontally.
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 a suspension sphere. The accumulator is an additional sphere (which holds a reserve of hydraulic fluid under pressure to even out the load on the pump caused by varying demand) acting rather like a battery. The accumulator is gas (typically nitrogen) under pressure in a bottle contained within a diaphragm. This is effectively a balloon which allows pressurised fluid to compress the gas, and then as pressure drops the gas pushes the fluid back to keep the system's pressure up. In the image here, the nitrogen gas is represented in red and the LHM fluid is represented in green. As the pressure in the fluid overcomes the gas pressure, the nitrogen is compressed by the diaphragm being pushed back. Then as the pressure in the fluid reduces, the gas pushes back the diaphragm which expels the fluid from the sphere, returning gas and fluid to equilibrium. This is the hydropneumatic equivalent to the spring being compressed and then rebounding.
Still with me? We can keep going...
So how can the interaction of compressing gas, hydraulic fluid and a diaphragm form a spring? Simple(ish): The pressure of the gas is the equivalent to the spring weight. The inlet hole at the bottom of the sphere restricts the flow of the fluid and provides an element of damping. By replacing the spheres for ones of different specifications, it's possible to adjust the ride characteristics of these cars.

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 refinements and developments were quite effective. 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.
The following cars were fitted with hydropneumatic suspension: Traction Avant 15 Six H, D series, GS/GSA, SM, BX, some XMs and most Xantias. The following were fitted with Hydractive 1 or Hydractive 2 suspension (the difference between H1 and H2 are mainly concerned with computer parameters): most XMs and some Xantias. The Xantia Activa was fitted with Hydractive suspension. The C5 is fitted with Hydractive 2 suspension and the C6 with Hydractive 3.
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. Since the rear brakes use the rear suspension fluid, the tail is pulled down allowing for level braking.
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 it in the rear suspension of the Silver Shadow. When Citroën was the owner of Maserati some of their cars were also hydropneumatised.

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

Hydraulic Suspension

Hydraulic suspension is an innovation making its way into motor sports, no doubt to trickle down to consumer vehicles eventually. It has been designed by a Spanish company called Creuat and pioneered by the Racing For Holland Dome S101 sports car team. In the image below you can see both the traditional coilover system (the yellow/blue/red units) at the front of the car. This photo was taken before scrutineering for the 2005 24 Hours of Le Mans race. The team had both systems online and when scrutineering passed the car, the coilover units were removed, to race for the first time completely with hydraulic suspension.
Central to their system is a control unit mounted next to the cockpit. They tell me the system can't be compared to the hydropneumatic suspension 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.
This system is so new that the best source of information on it is Creuat's own website. You can find it at this link and you need to look for the Le Mans Project in their menu on the left side of their page. The hydraulic suspension page is a work-in-progress project and its content changes almost weekly at the moment.
Racecar Engineering magazine have a feature article about this suspension system at this link but you need a subscription to read the whole thing. Fortunately Creuat have scanned the article and made it available as a 6.2Mb PDF file which you can read here.
Thanks to Sander van Dijk for sending me this photo, plus a ton of others of their racing car.

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

Picture credits: Bose Learning Center & Bose press kit.

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 linear 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.

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.
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.

Air suspension

In days gone by, air suspension was limited to expensive logistics trucks - heavy goods vehicles that needed to be able to maintain a level ride no matter what the road condition. Nowadays, you can retrofit air suspension to just about any vehicle you like from a Range Rover to a Ferrari. Air suspension replaces the springs in your car with either an air bag or an air strut made of high-tensile super flexible polyurethane rubber. Each air bag or strut is connected to a valve to control the amount of air allowed into it. The valves are in turn connected to an air compressor and a small compressed air reservoir. By opening and closing the four valves, the amount of air sent to each unit can be varied. By letting the same amount of air out of all the units, reducing the pressure in the bags, your car gets lowered, whilst increasing the air pressure by the same amount in each unit results in your car lifting higher off the ground. The rubber bags filled with air provide the springing action that used to be the realm of metal springs, and you have the option to maintain the factory (or aftermarket) shock absorbers for - well - absorbing shocks. That's it in a nutshell.

Why air suspension?

Simple : ride quality. A well set up air suspension system can surpass metal spring suspension in just about any situation. If you want a luxurious, smooth, supple ride that will iron out the deepest of ruts and crevasses in the road, air suspension is what you're looking for. It's why logistics firms have used it in their trucks since the year dot - air suspension transmits much less road vibration into the vehicle chassis. There are literally hundreds of combinations and permutations of air bags and struts that can be adapted to fit just about any vehicle and the big hitter in the aftermarket segment at the moment is Air Ride Technologies if you're in America. In England, Rayvern Hydraulics have a similarly complete range of aftermarket solutions. One point to note: for some reason the imperial fittings used on some American systems are all but impossible to get hold of in the UK, so if you're in England and looking for air suspension, Rayvern would be a good choice, or BSS or GAS in Germany.
In factory fit systems, almost any sports sedan that has variable ride height (like a lot of the current crop of Audis) is using air suspension to accomplish this.

Bags and struts

Air bag systems come in two different flavours - air bags and air struts. The bags are typically used for leaf-spring suspension vehicles, but can easily be adapted (through the use of bolt-on brackets) to almost any swinging-arm type suspension system. Air bags are the most reliable systems because of their simplicity. Air struts are a little more complex and come in two flavours - simple struts and pivoting struts. It used to be that you could only have a simple strut because none of the manufacturers had figured out how to keep the air strut sealed when it twisted - a function that is required if you're going to replace a MacPherson strut. Now though, there are a couple of different options for MacPherson strut replacement, the most complex being the twisting double-doughnut style strut that still allows the shock absorber to pass through the middle of it.
The two images below show an air bag system as applied to the rear leaf spring suspension on a truck, and a simple non-twisting air strut system as applied to a double swingarm unit.

 

Ride height sensors

Simple air suspension is pretty much what I've outlined above, but most systems are far more sophisticated. For example each unit will normally work in conjunction with a ride-height sensor. This is a mechanical lever linked to the suspension arm at one end, and to an electronic resistance pot at the other. The pot is connected to the chassis or frame so that the lever spins the pot as the suspension moves up and down. A computer can use this to read the height of the vehicle in that corner, and with that data, all sorts of wonderful things can happen. For example, if you mash the accelerator pedal, a car will typically squat under acceleration. When this happens, the ride height at the rear of the car gets less. An air suspension system can register this and either send more air to the rear, or reduce the pressure at the front to level off the car again. Same goes for side-to-side roll in corners - air suspension can compensate somewhat for body roll when connected to ride-height sensors. New generation systems also incorporate air pressure sensors to add another level of feedback to the system.

Control panels

In a factory-fit air suspension system, the control panel will either be integrated into the onboard computer (like BMW's i-Drive), or be accessible via a ride-height adjustment control. For aftermarket systems, the control panel is normally a hand-held device with a series of control buttons and LED readouts on it. Either way, the control panel is how you determine what you want the suspension to do, be it hunkered down for sporty driving, or high off the ground for extra clearance.

Low-riders

Love 'em or hate 'em, there's no getting around the fact that some petrolheads just love to slam their rides down to the floor but put air suspension systems in capable of making the cars hop, jump and dance. The only real difference with these systems is that they have a much larger high-pressure reservoir normally in the boot or trunk, connected to valves that can open very rapidly. Instead of the smooth, gentle ride-height adjustment of a factory-fit system, these valves can bang open and discharge huge quantities of air from the reservoir into the air bags extremely quickly. The result is the suspension elongating extremely quickly and with enough force to propel the car into the air.
In truth, the extreme low riders like this tend to go more for hydraulic actuators than air suspension. Hydraulics give far more power, far more quickly and are a lot more robust when it comes to the constant hammering they get from competitions and shows. The principle is exactly the same though - a reservoir, a compressor, a set of valves and a set of hydraulic lifters connected to the suspension components. The downside? No suspension to speak of because the hydraulic actuators have no give in them like the rubber air bags do.

Picture credit: Wikipedia / Public Domain

Variable-camber suspension for steering

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

 

Picture credits: Matthew Kim, OnCamber LLC

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-sway-bars. Almost all cars have them fitted as standard, and if you're a boy-racer, all have scope for improvement. From the factory they are biased towards ride comfort. Stiffer aftermarket items will increase the road-holding but you'll get reduced comfort because of it. It's a catch-22 situation. Fiddling with your roll stiffness distribution can make a car uncomfortable to ride in and extremely hard to handle if you get it wrong. The anti-roll bar is usually connected to the front, lower edge of the bottom suspension joint. It passes through two pivot points under the chassis, usually on the subframe and is attached to the same point on the opposite suspension setup. Effectively, it joins the bottom of the suspension parts together. When you head into a corner, the car begins to roll out of the corner. For example, if you're cornering to the left, the car body rolls to the right. In doing this, it's compressing the suspension on the right hand side. With a good anti-roll bar, as the lower part of the suspension moves upward relative to the car chassis, it transfers some of that movement to the same component on the other side. In effect, it tries to lift the left suspension component by the same amount. Because this isn't physically possible, the left suspension effectively becomes a fixed point and the anti-roll bar twists along its length because the other end is effectively anchored in place. It's this twisting that provides the resistance to the suspension movement.

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

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

Suspension bushes

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

Variable stiffness anti-roll bars

Some sportier vehicles have the capability to stiffen up the suspension for more aggressive handling by altering how the anti-roll bar behaves. The system itself isn't especially complex. Instead of simple rubber or urethane bushes to clamp the anti-roll bar to the frame of the car, these systems use a motor-driven or electromagnetically clamped bush instead. When the driver decides they want 'sport' mode, the car can increase the friction in the mounting bushes by clamping them more tightly around the anti-roll bar. This better resists the anti-roll bar's ability to twist across the width of the vehicle, which in turn provides more resistance at the ends where it joins the suspension components. The end result is that the suspension components have to take on a lot more load to deflect by the same amount. Or conversely, under the same load, they move less, thus stiffening up the suspension.

The Ins and Outs of complex suspension units.

Generally speaking, this section will be more relevant to you if you ride a motorbike, but you can get high-end spring / shock combos for cars that have all these features on them. The thing to realise is that if you're going to start messing with all these adjustments, for God's sake take a digital photo of the unit first, or somehow mark where it all started out. It's a slippery slope and you can very quickly bugger up the ride quality of your vehicle. If you don't know what the "stock" setting was, you'll never get it back.

Compression damping.

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

Rebound damping.

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

Damping controllers.

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

Spring preload.

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

Sprung vs. unsprung weight.

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

Progressively wound springs

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

Electronic damping force controllers.

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

Picture credit: TEIN

Torsion bars

Torsion bars (or torsion rods) deserve their own section because they are a type of spring which can be used in place of coil- or leaf-springs. It's one of the topics I get the most e-mail on, so instead of continually sending the same answer, I thought I'd cover it on this page.
A torsion bar is a solid bar of steel which is connected to the car chassis at one end, and free to move at the other end. They can be mounted across the car (transverse like the rear suspension on the Peugeot 205 and Renault 16) or along the car (longitudinal, like the front suspension on the Morris Minor) - one for each side of the suspension. The springing motion is provided by the metal bar's resistance to twisting. To over-simplify, stick your arm out straight and get someone to twist your wrist. Presuming that your mate doesn't snap your wrist, at a certain point, resistance in your arm (and pain) will cause you to twist your wrist back the other way. That is the principle of a torsion bar.
Torsion bars are normally locked to the chassis and the suspension parts with splined ends. This allows them to be removed, twisted round a few splines and re-inserted, which can be used to raise or lower a car, or to compensate for the natural 'sag' of a suspension system over time. They can be connected to just about any type of suspension system listed on this page.
The rendering below shows an example longitudinal torsion bar. The small lever at the far end of the torsion bar would be attached solidly to the frame to provide the fixed end. The torsion bar itself fits into that lever and the suspension arm at the front through splined holes. As the suspension at the front moves upwards, the bar twists along its length providing the springing motion. I've left the shock absorber assembly out of this rendering for clarity.

Lift Kits

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

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

Speaking of trouble...

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

Lowering Kits

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

Lowering kit questions.

What if I get shorter springs to lower the car? Will I need to adjust my caster and camber angles and/or my shock absorbers?

Generally the answer would be no for caster/camber angles. Most cars have a good 10-13cm (4-5 inches) movement in their suspension from the factory. As most of the lowering springs you can buy only lower by 2-7cm (1-3 inches), your suspension should still be well within it's designed operating limits. Therefore, caster and camber angles shouldn't need looking at. As for the shocks, see the FAQ page.

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

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

Where can I buy a good kit to lower or lift my car?

Again, a lot of local and internet stores that offer you ready to go suspension kits. Spax and Jamex are two big names for car suspension kits.

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