modern suspension

8/12/2019 Modern Suspension

1/13

Hydrolastic Suspension

If you've got this far, you'll remember that Dr. Alex Moulton originally wanted the Mini to

have Hydrolastic suspension - a system where the front and rear suspension systems

were connected together in order to better level the car when driving.

The principle is simple. The front and rear suspension units have Hydrolastic displacers,

one per side. These are interconnected by a small bore pipe. Each displacer incorporatesa rubber spring (as in the Moulton rubber suspension system), and damping of the

system is achieved by rubber valves. So when a front wheel is deflected, fluid is

displaced to the corresponding suspension unit. That pressurises the interconnecting pipe

which in turn stiffens the rear wheel damping and lowers it. The rubber springs are only

slightly brought into play and the car is effectively kept level and freed from any

tendency to pitch. That's clever enough, but the fact that it can do this without hindering

the full range of motion of either suspension unit is even more clever, because it has the

effect of producing a soft ride. Pictures and images of anything to do with hydrolastic

suspension are few and far between now, so you'll have to excuse the plagiarism of the

following image. The animation below shows the self-leveling effect - notice the bodystays level and doesn't pitch.

But what happens when the front and rear wheels encounter bumps or dips together?

One cannot take precedent over the other, so the fluid suspension stiffens in response tothe 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 borepipe? 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 hydrolasticsuspension was introduced in 1964 (on the prototype BMC ADO16) and you'd be lucky tofind a unit today that has had anywork done to it.
http://www.carbibles.com/images/hydrolastic_rear.jpghttp://www.carbibles.com/images/hydrolastic_rear.jpg


2/13

The image here 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 thevehicle.

Hydrolastic suspension shouldn't be confused with Citron'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.

There are a lot of resources on Hydragas available at one of the MGF club sites on theinternet:http://www.mgfcar.de/hydragas

Hydropneumatic Suspension

{Thanks to Julian Marsh, Jonathan Bruce, Simon Byrnand and Pieter Melissen for some

updates to this information.}Since the early fifties, Citron have been running a fundamentally different system to the

rest of the auto industry. Its called hydropneumatic suspension, and it is a whole-car
http://www.mgfcar.de/hydragashttp://www.mgfcar.de/hydragashttp://www.mgfcar.de/hydragashttp://www.carbibles.com/images/hydropneumaticsuspension.jpghttp://www.carbibles.com/images/hydragas_displacers.jpghttp://www.carbibles.com/images/hydropneumaticsuspension.jpghttp://www.carbibles.com/images/hydragas_displacers.jpghttp://www.mgfcar.de/hydragas


3/13

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. Citron 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 conventional.Under the company's new Peugot management, Citron produced the LN, followed by the

Visa and then the LNA and then the BX. The BX was a major turning point in Citron's

history. As a direct consequence of the Peugeot influence, the car was somewhat moreconventional 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) underpressure in a bottle contained within a diaphragm. This is effectively a balloon whichallows 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'spossible to adjust the ride characteristics of these cars.


4/13


5/13

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 Citrons 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 twodamper 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 softmode, 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 Citron clientele. The currentadjustable systems (computer controlled) lack this anti roll characteristic, and there areowners who always prefer the "comfort" setting rather than the "sporty" one, because

again, that is not what Citron 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 and C6 are fitted 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 Citron BX, the rear braking effort

comes from the pressure exerted on the LHM fluid by the weight on those struts. This
http://www.carbibles.com/images/hydropneumaticsuspension2.jpg


6/13

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, Citron 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 Citron was the owner of Maserati some of theircars 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 theRacing For HollandDome S101 sports car

team. In the image below you can see both the traditional coilover system (theyellow/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 Citron uses because this

system doesn't use a pump and has less than a litre of hydraulic fluid in the entire

system.
http://www.citroenet.org.uk/miscellaneous/suspension/suspension8.htmlhttp://www.citroenet.org.uk/miscellaneous/suspension/suspension8.htmlhttp://web.actwin.com/toaph/citroen/work/work.htmlhttp://web.actwin.com/toaph/citroen/work/work.htmlhttp://www.tramontana.co.hu/citroen/guide/guide.phphttp://www.tramontana.co.hu/citroen/guide/guide.phphttp://planetlemans.com./http://planetlemans.com./http://planetlemans.com./http://www.carbibles.com/images/hydraulicsuspension2.jpghttp://www.carbibles.com/images/hydraulicsuspension.jpghttp://www.carbibles.com/images/hydraulicsuspension2.jpghttp://www.carbibles.com/images/hydraulicsuspension.jpghttp://planetlemans.com./http://www.tramontana.co.hu/citroen/guide/guide.phphttp://web.actwin.com/toaph/citroen/work/work.htmlhttp://www.citroenet.org.uk/miscellaneous/suspension/suspension8.html


7/13

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 atthis linkand 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 systemat this

linkbut 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 readhere.

Thanks to Sander van Dijk for sending me these photos, plus a ton of others of their

racing car.

Digital Suspension Systems

Beginning in 2006 with the Audi TT (see below), the concept of fully independentsuspension systems came into being. Traditional 'analogue' independent suspension is

still connected side-to-side by anti-roll bars. With the advent of computer-controlled

suspension systems that are able to rapidly adapt to changing road surfaces, the anti-rollbar is no longer needed. Its function can be replaced as long as sensors and

electronically-adjustable suspension can be combined together. For example when the

sensors detect body roll in a corner, the suspension components in all four corners of the

car can be electronically adjusted to compensate in real-time. Other vehicles that usedigital suspension now are the Range Rover Evoque and the Audi R8 but the list will

surely grow as it becomes more mainstream.

The next couple of topics deal with two such systems - ferrofluid, and linearelectromagnetic suspension.

Ferrofluid or magneto-rheological fluid dampers - Audi

Magnetic Ride.

With the 2006 Audi TT, Audi launched their innovative magnetic semi-active suspension.

Its a totally new form of damping technology refined from Delphi'sMagneRidesystem.

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 betweencabin 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 thegear-changes (front-to-back weight shift) within milliseconds.
http://www.creuat.com/http://www.creuat.com/http://www.creuat.com/http://www.racecar-engineering.com/content/feature_01.htmhttp://www.racecar-engineering.com/content/feature_01.htmhttp://www.racecar-engineering.com/content/feature_01.htmhttp://www.racecar-engineering.com/content/feature_01.htmhttp://www.carbibles.com/docs/racecar.pdfhttp://www.carbibles.com/docs/racecar.pdfhttp://www.carbibles.com/docs/racecar.pdfhttp://delphi.com/manufacturers/auto/http://delphi.com/manufacturers/auto/http://delphi.com/manufacturers/auto/http://delphi.com/manufacturers/auto/http://www.carbibles.com/docs/racecar.pdfhttp://www.racecar-engineering.com/content/feature_01.htmhttp://www.racecar-engineering.com/content/feature_01.htmhttp://www.creuat.com/


8/13

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 ferrofluiddemonstration.

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 settingsaccordingly. 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 5 Watts, and the entire thing takes up no more

room than a regular coil-over-oil unit. Vorsprung durch Technik indeed.

The diagram here 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
http://www.carbibles.com/images/magneride.jpghttp://www.carbibles.com/images/magneride.jpg


9/13

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 Suspension Systems& Bose press kit.

This is another digital 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
http://www.bose.com/controller?url=/automotive/bose_suspension/index.jsphttp://www.bose.com/controller?url=/automotive/bose_suspension/index.jsphttp://www.bose.com/controller?url=/automotive/bose_suspension/index.jsphttp://www.bose.com/controller?url=/automotive/bose_suspension/index.jsp


10/13

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 andreturns power backthrough 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 pagecan be found hereif 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 declaredBose 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 willtell if Bose manage to succeed where Aura Systems failed.

Air suspension
http://www.bose.com/controller?url=/automotive/bose_suspension/index.jsphttp://www.bose.com/controller?url=/automotive/bose_suspension/index.jsphttp://www.bose.com/controller?url=/automotive/bose_suspension/index.jsphttp://www.carbibles.com/images/aura_ram.jpghttp://www.carbibles.com/images/bose_suspension.jpghttp://www.carbibles.com/images/aura_ram.jpghttp://www.carbibles.com/images/bose_suspension.jpghttp://www.carbibles.com/images/aura_ram.jpghttp://www.carbibles.com/images/bose_suspension.jpghttp://www.bose.com/controller?url=/automotive/bose_suspension/index.jsp


11/13

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 - absorbingshocks. 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'relooking 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

isAir Ride Technologiesif you're in America. In England,Rayvern Hydraulicshave 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 goodchoice, or BSS or GAS in Germany.

In factory fit systems, almost any sports sedan that has variable ride height (like a lot ofthe 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 aretypically 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
http://www.ridetech.com/http://www.ridetech.com/http://www.ridetech.com/http://www.rayvernhydraulics.com/http://www.rayvernhydraulics.com/http://www.carbibles.com/images/airsuspensionstrut.jpghttp://www.carbibles.com/images/airsuspensionbag.jpghttp://www.carbibles.com/images/airsuspensionstrut.jpghttp://www.carbibles.com/images/airsuspensionbag.jpghttp://www.rayvernhydraulics.com/http://www.ridetech.com/


12/13

style strut that still allows the shock absorber to pass through the middle of it.

The two images here 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 makingthe 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 actuatorsthan

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.
http://www.carbibles.com/images/lowrider.jpg


13/13

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.

modern suspension

Documents