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Air Suspension Basics
TIt can be sporty, it can be comfortable. Don't cut corners!

 

 

Air Suspension:

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 ACC / Airrunner and Air Ride Technologies if you're in America.

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.(PIC:airsuspensionbag.jpg)(PIC:airsuspensionstrut.jpg)
air bag suspension air strut suspension
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

low riderLove '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. (PIC: Get your own)

 

Sources:
Suspension bible
Unabomber @ NASIOC Forums