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Air suspension is its own science of pounds per square inch that requires some different tools and its own rules. Independent springs and dampers are tossed aside for pneumatically controlled bags connected by a series of valves and hoses. Aftermarket and OE alike provide a different way of approaching the never-ending engineering challenge of designing automotive suspensions. Nowadays, aftermarket air springs are as popular as ever but finding a source of reliable information on them can be a challenge, so we’re here to fix that. Whether you’re airing down your GTI sitting on aftermarket bags for that perfect fitment at a local cars and coffee or airing up your factory air suspension in your Porsche Cayenne to conquer the desert, all air suspension revolves around the same principles, and we’re here to show you how that all works.

 

A Suspension Overview: How It All Works

So how does a vehicle’s suspension work anyway? Well, at its most basic, the suspension’s job is quite simple. It connects the wheels and brakes to the vehicle’s body, absorbing the bumps while maintaining the tire’s grip on the road. Straight and curved lengths of steel or aluminum, with names like wishbones, control arms, and lateral links, keep the wheel assembly in the center of the wheel well. They use rubber bushings or ball joints to keep the wheel in place, allowing the spring and damper to control vertical movement.

Air Struts Explained Porsche Cayenne Suspension

Modern suspensions use either a coil spring or an airbag spring to absorb the bumps in the road, keeping you comfortable as you drive. The dampers, known as shocks or struts, use a piston mounted on a rod in a sealed tube. The piston moves through gas or fluid, controlling the compression and rebound of the spring, preventing the car from bouncing, and working in conjunction with the spring to absorb the bumps and keep the vehicle under control through the corners.

Like you’ll find in BMW’s sedans, traditional coil springs come in two varieties: linear and progressive. Linear springs have a single compression rate no matter how much they’re compressed. On the other hand, progressive springs get stiffer as they’re compressed. As a result, where linear springs work best in a single application, sacrificing comfort for performance or vice versa, progressives aren’t best at any one thing, rather mediocre at everything. The only other option for a modern spring is air. Air suspension throws conventional springs out of the window for a cylindrical rubber bladder filled with air. A compressor in the vehicle supplies the bladders, and they’re both managed by a control module. The module monitors each air spring for pressure and height and adjusts according to whatever the driver or computer wants. 

Air Struts Explained Mercedes AIRMATIC chassis

The bellow or “bag” that acts as the spring is made from rubber or elastomer, strengthened with cords to provide durability along with the elasticity needed to give a compliant ride. Corteco, the OE air suspension supplier to Mercedes-Benz, BMW, and VW, layers their reinforcing cords in parallel and crossing patterns within the elastomer, allowing the bags to have sharp responsiveness and retain torsional stability as intended by the OE design. The top of their bellow is crimped to the upper suspension mount, while the bottom is fixed to a piston on the damper. The piston is mounted on the top of the damper and moves inside the bellow as the spring compresses. The air pressure inside the bellow and the shape of the piston determine the spring’s stiffness. Together, the air spring and the damper it sits on make up an air strut. 

Air Struts Explained Porsche air strut

The air spring and damper are separate on non-air-strut designs, like on the rear of BMW’s X-series SAVs. The air spring is mounted in place of the coil spring while a separate shock does the damping. It delivers the same result as an air strut but from a slightly different suspension design. Whether mounted to a strut or on its own, each air spring is given a target air pressure rating by the control module. When pressure is low, the compressor pumps more air into the specified springs, raising the ride height and stiffness. Excessive pressure in the springs means its air is diverted to a small reservoir or expelled to the atmosphere.  

 

How and Why? The Evolution of Air Suspension

The history and idea of air suspension can be traced back to the early 1900s, though its first real practical application came during the second world war. American trucks and aircraft were fitted with the air suspension to provide a stable ride height when fitted with artillery and supplies. 

Air suspension saw its first practical use on streetcars in 1950, when Air Lift created an air spring insert that worked in conjunction with a traditional coil spring. They quickly caught on in stock car racing and were used successfully for several decades. American vehicles had optional air suspension in the late fifties, while Mercedes-Benz became the first major German automaker to use it in 1961 on their 300SE. It wasn’t until the late nineties that Mercedes debuted their AIRMATIC system, a truly modern air suspension system fitted to a European car. It combined the basic idea of an air spring with Mercedes’ adaptive dampers to create an all-in-one unit that vastly enhanced the comfort and handling abilities of the suspension. The electronically controlled suspension was able to set the ride height at each corner automatically, something they hadn’t really been able to do since their hydropneumatic suspension of the sixties.

Air Struts Explained 3D graphic rear air suspension

Fast forward to the mid-’00s, and the AIRMATIC received a few key updates. Mercedes’ advancement in damper technology allowed their air struts to read and adapt to road conditions in 50 milliseconds. That’s twice as fast as the average human can blink. Drivers could select their damper settings at the push of a button, too. By this time, nearly all of the major manufacturers had caught on to the advantages of air suspension. The Porsche Cayenne Turbo came with it as standard, and it still does today. In fact, you’d be hard-pressed to find a luxury SUV that doesn’t use air suspension in some capacity.

Air Struts Explained 3D graphic front air suspension

Current suspension technology has allowed the manufacturers to use air springs and active dampers to provide their models with several different suspension settings. BMW, Porsche, and Mercedes AMG models can use their adaptive air suspensions to change the ride qualities from ultimate comfort all the way to ultimate performance. 

 

OE or Aftermarket: What’s the difference?

Aftermarket air suspension systems are easy to come by. Companies like X2 Industries and Air Lift Performance offer complete kits for various BMWs, Mercedes, and Audis. However, OEM air suspension systems are quite different from the aftermarket equivalents. Where aftermarkets use a one-size-fits-all approach, OE supplier Corteco spends its time designing and producing bespoke suspension systems. Their partner company, Vibracoustic, handles all of the design, testing, and engineering of modern suspension components for European manufacturers. 

The way the systems are packaged is different, too. Today’s European luxury vehicles have their compressors and reservoir tanks tucked out of the way in a spot designed for them. Your Mk3 GTI 16V doesn’t have a spot for any of that. Instead, space will need to be made in the trunk somewhere if you want to continue to use the back seats. Airing up takes a lot of air, too, so you’ll need a decently sized tank to hold a large enough supply. The size of the tank affects the response time of the air suspension, with the only real downside being too small of a tank. Lacking in size leads to an overworked compressor, though too big of a tank just takes up extra space.  

Air Struts Explained aftermarket tank mounted in golf

The compressor and air tank need to be mounted to the car, and the airlines run to each corner of the suspension. The easiest way to do that on a vehicle not originally equipped with air suspension is to drill and cut into the body. The holes and mounting can be executed neatly, but not everyone is ok with hacking up their car. And if you aren’t, then all of the components must be mounted under the car, though that isn’t going to work too well with aftermarket systems not designed to take that abuse. 

OE designs further differentiate themselves from the aftermarket by their construction. Aftermarket bellows use an elastomer compound impregnated with cording like the OE Corteco pieces; however, they aren’t built to the same standards. OE systems like those from Corteco have to meet the rigorous standards of the manufacturers, so they design specifically sized and shaped bellows and pistons for the designated application. Aftermarket manufacturers regularly use thicker bellows meant to withstand a range of requirements from several different applications. It’s a money-saving move for the aftermarket that lends to their higher failure rate and poorer performance. Their “one size fits all” approach uses a bellow that can’t always cope with the specific requirements of the chassis that they’re on. 

Air Struts Explained 3D bellows cross-section

Moving on to the dampers, the OE air suspension systems are in a league of their own. Mercedes and BMW use active dampers that use various sensors and cameras to read the road conditions and adjust the damping and air pressure on the fly to keep the car controlled and comfortable. Aftermarket solutions won’t have anything like that. Instead, you’ll typically find a single adjustment knob on each strut to change the compression damping, leaving your air suspension to handle whatever conditions you put it through.  

Lastly, there’s the replacement cost. IAM, Corteco’s largest aftermarket competitor, requires either both fronts or rear air struts/springs to be replaced together. Because Corteco’s springs meet the OE specifications, you can get away by replacing only the failed strut, saving time and money.

 

Maintenance: Keeping Your Strut Up

Maintaining the majority of an air suspension system is certainly different than a coil spring setup. Traditional coil spring designs have the damper shaft covered by a loosely fitting rubber boot with the spring exposed to the elements. It’s a style used by every major automaker, and it works fairly well, though it isn’t perfect. A seal keeps the dirt out of the damper, but that dirt can get caught on the edge of the seal and scratch the shaft, causing it to leak and lose its damping effectiveness. The coil-spring itself will sometimes have a rubber or plastic guard protecting it, but they’re usually left exposed. Constant bombardment from dust and dirt particles will eat through the spring’s paint and cause it to rust.  

Air Struts Explained air strut on the table

Air springs and struts, on the other hand, are sealed units. Dirt and grit can collect on the outside of the spring, but spraying it down with a hose should get most of that off. You’ll only really have to worry about the system springing a leak. A spray bottle filled with soapy water will let you know of any components you may suspect of leaking. Simply spray the soapy mixture on the corner under suspicion and watch for any bubbles that may form. Broken airlines and burned-out compressors happen, but there isn’t too much anyone can do to prevent that other than not overworking or over pressurizing the system. 

That simplicity can be viewed as a positive over OE air suspension systems. The various sensors, modules, and pneumatic components used for the active air suspension on modern European vehicles can break without warning, and replacing them can be offensively costly because of their complexity. The aftermarket systems are simple, relatively speaking, with few electronics to go bad. There is one silent killer to both systems, however, and that is moisture. 

Air Struts Explained air suspension component layout

Moisture inside the pressurized air system is not beneficial to any of the components. More than a few pieces contain exposed metallic pieces that aren’t corrosion-proofed. Enough moisture in the system will cause those parts to corrode from the inside out. The moisture can also wreak havoc on the seals inside the system. If one seal goes from excess moisture, there are likely a few more that will need replacing. However, there is a solution! As they’re so aptly named, water traps filter out debris and moisture from the pneumatic system, keeping everything happy and moisture-free. They’re a must-have for any air suspension in particularly humid climates.

So that’s about all you need to know about air suspension. At the end of the day, stick with the suspension that works for you. Coil springs are beyond developed, proving themselves through everyday use and motorsports. Aftermarket air suspension systems are great for chassis that never received it from the factory, and buying replacement parts from OE suppliers is the best way to ensure a long-lasting factory setup. If you have any questions or comments regarding air suspension, please let us know in the comments below. If you’d like to read or watch more of our content, follow along on our blog and subscribe to our YouTube channel

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Written by :
Christian Schaefer

Owner of a flat-six swapped 1998 Impreza 2.5RS and a 1973 Porsche 914. Horizontally opposed views, only.


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