Air suspension system

What It Is
An air suspension replaces conventional springs with air springs similar to those on big 18-wheel semitrucks. Conventional coil-spring suspensions are the easiest to upgrade, with the airbags slipping right in place of the coil springs and often bolting to proprietary bolt-in brackets supplied by the aftermarket manufacturer. Air-spring design has progressed to the point that pioneer companies like Air Ride Technologies, TCI, Air Lift, and others have developed bolt-on kits for most popular coil-spring cars and trucks. There are even air-spring solutions for leaf-spring and torsion-bar suspensions.

Why You Want One
An air suspension offers at least five major benefits:
Tunability: Air suspensions have a wide tuning range for spring rate and load capacity. Weeks of conventional spring and shock tuning can be painlessly compressed into a few minutes via in-car adjustability. Getting the ride height, load, and rate on a coil spring right on the first try is a hit-and-miss affair, but an air suspension provides a much broader envelope, so precise selection isn't as critical as on a conventional spring.

Handling: Most air springs are progressive. The more they compress, the stiffer they get. Combine this inherent progressive spring rate with in-car adjustability, and the result is enormous performance potential. Tuning for conditions is quicker and faster. In a sophisticated handling application, the air spring should be coupled with shocks adjustable for both rebound and compression and carefully selected antisway bars.

Performance customization: Everyone has his own personal idea of how his car should ride and handle. With an air suspension, these wishes can be accommodated with little or no component changes. By adjusting air pressure and shock valving, you can make the same car be soft and comfortable, firm and tight . . . or anywhere in between. You can drive the car comfortably to the track, firm up the air pressure and shock valving to go racing, then readjust the pressure and valving to return home in comfort.

Stance: Air suspensions make it easy to lower the car so you can look cool. At the far end of the coolness spectrum are spark-throwing minitrucks and lowriders, but today, they represent only a small segment of the market. Far more typical is the guy who just wants to lower his car or truck a reasonable amount for better looks without sacrificing any driveability or durability. Most kits come set up to deliver a normal ride height that's several inches lower than the stock springs. Regardless of how low the car is, air suspensions make it easy to raise the car back up for normal cruising, getting into gas stations, or even rolling onto the trailer.
Load carrying: This is the original commercial application for air suspensions: helping 18-wheelers carry heavy loads while improving driver comfort. Although probably not the main reason for switching over on a pure hot rod, it definitely is one solution for making your dualie tow truck more driveable under varying-load

Air-Spring Design
Today, most air springs are made by Firestone, which pioneered their use on big trucks. Three basic types of air springs are available: the double-convoluted, the tapered-sleeve, and the rolling-sleeve. The double-convoluted design looks like a large double cheeseburger and generally has more load capacity, a shorter stroke, and a more progressive spring rate that's best suited for use on most front suspensions where the spring sits considerably inboard of the suspension's load point, which has the effect of multiplying load-capacity requirements while dividing travel requirements. Tapered- and rolling-sleeve air springs are smaller in diameter with a longer stroke and a more linear spring rate; they're best suited for most rearend applications because there are more travel requirements and fewer load-capacity requirements.

Front Suspension
Shock relocation was usually required with most first-generation air-spring setups because there was no hollow area in the air spring's center to accommodate the stock shock-absorber mounting location that typically ran through the center of the coil spring. Kits designed for conventional air springs come with shock-relocation mounts and new control arms as needed. This is an affordable and straightforward solution, but in some instances, relocating the shock can result in wheel/tire clearance issues, especially with today's huge wheel-and-tire packages.
As air suspensions evolved, a higher-end installation was developed based on coilover shock/spring combos but with an airbag replacing the coilover's coil spring. These setups tend to be more expensive but offer the advantage of easier installation, better looks, and more wheel-and-tire clearance.
On the other hand, depending on the chassis, a coilover-like design could yield less overall wheel travel compared with a separate air spring and shock setup. Reduced wheel travel requires a stiffer bag to avoid bottoming out the suspension at a given ride height. Under these circumstances, if ultimate ride quality is paramount, a separate bag and shock setup might be preferable.

Rear Suspension
For cars already equipped with rear coils, changing over to airbags is straightforward. They often go right where the old springs used to sit, although in some cases, different spring seats may be needed (if so, they come in the kit). Shock-within-airbag designs are available as well, but additional mods may be required to install them.
Leaf-spring cars have two options. The simplest conversion is to remove several leaves from each spring pack and install air springs between the axlehousing and the vehicle frame. Residual leaves are still needed to locate the rear axle, but airbags now perform the primary load-bearing function. On an old car with worn-out leaf springs of uncertain durability and capability, the plan (if you can afford it) is to replace them entirely by moving up to a four-link suspension. Major aftermarket outfits offer premade conversion kits that bolt right into popular leaf-spring cars with only minor welding required for new brackets on the axlehousing. Universal weld-in setups are offered for race cars and custom applications.
Depending on the physical constraints of the chassis layout and body clearance, the four-link kits' upper links may be either triangulated or parallel in relation to the lower links. Generally, parallel four-links fit better in trucks, while triangulated setups are more suitable for cars. For some chassis, both parallel and triangulated four-link configurations are available. If you have a choice, a handling or primarily street-driven vehicle usually performs better with a triangulated four-link; a parallel setup is generally the ticket for a pure drag-racer/straight-line application.

Compressors
At their most basic level, air springs can be inflated using external shop air just like an old air shock, but that obviates one of the main benefits of this type of suspension: in-use adjustability to compensate for changing road conditions, vehicle loads, or intended use. Every time you add load without on-board air-be it fuel, people, or cargo-you have to track down an air hose.

Realizing full benefits from the adjustability offered by air springs really requires an on-board air source. With air suspension, ride-quality tuning is accomplished incrementally, with very small air-pressure changes. Air springs have relatively low volume, so it is difficult to inflate or deflate them with external air in small enough increments to fine-tune ride quality.
An on-board air system consists of at least one air compressor, probably a storage tank, and some sort of control system. A cost-effective and relatively simple solution that provides reasonable ride and handling benefits would be a 2-gallon tank kept full by a single compressor. On the other hand, if you want the car to go up and down in two seconds, on a heavy car it could take as much as a pair of 150-psi compressors and two or more 5-gallon tanks, huge industrial air valves, and 31/44-inch feeder lines. Yet such an extreme setup could create just as many fine-tuning hassles as a basic external-fill setup: On a fast-acting system, every push of the button may result in a 20-psi change in air pressure.

Air Lines
Commercial, DOT-approved, plastic air lines originally developed for big trucks are standard in most kits. They provide an easy, affordable solution to connecting the compressors to the air springs. Typical operating pressures range from about 75 to 150 psi, well within the capabilities of such tubing.

MacPherson strut? No problem. Air Ride Technologies offers AirStruts that directly replace

For the custom look, you can fabricate stainless hard line, just as in a brake or fuel system, connecting it using typical AN flare nuts and pipe adapters. At suspension travel points, flexible hose would be needed in an otherwise hard-lined system, just as it is in a brake system. Also like in a brake system, braided stainless steel Teflon-core hose is preferred over braided synthetic rubber-core fuel/oil-line hose.

Two-Way Versus Four-Way
When air suspension was first introduced to hot rodders in the mid-'90s, a two-way control system was the standard method of inflating and deflating the suspension. In other words, both air springs on each axle were hooked together. This kept things simple, requiring only one control valve per axle. However, it quickly became apparent that during cornering, the outside (or loaded) air spring tried to transfer air to the inside (unloaded) spring, magnifying body-roll issues. Sometimes the problem can be minimized on a light car by installing big antisway bars, but nowadays, nearly everyone uses a four-way system, which plumbs and controls each airbag separately. Although this requires an individual control valve and air line for each spring, it cures all air-transfer problems, plus it allows precise control over vehicle ride height to compensate for varying weight, suspension bind, or alignment changes. Think of a four-way setup as somewhat akin to weight-jacking on an oval-track car.

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