Tech guide: air suspension. We get the low-down on what gets you low down. This month we get the lowdown on air suspension systems. Words: Midge. Burr Photos: Various.
Air-ride systems have been around for about as long as cars themselves. Initially conceived to enable carrying heavy or uneven loads in relative comfort, there are examples as far back as the early 1900s. Various aftermarket kits have been on sale since the 1920s. It became extremely popular with bootleggers in the US in the ’30s and ’40s for maintaining stock ride height with a boot full of moonshine. And nowadays you’ll find OEM applications on everything from buses and HGVs to expensive SUVs and saloons. In short, air-ride is far more common than you might think. It’s not some sort of underground dark art and, to be honest, it’s not particularly special either… until it comes to the tuning scene, of course.
But the question still remains: why should you fit it to your modified BMW? Well, if you’ll excuse the pun, here’s the lowdown…
HOW IT WORKS…
There may be a few different configurations out there but the principle of how air-ride works is always the same. Unlike ‘closed’ system hydraulic suspension, which uses a specific amount of fluid pumped (at extremely high pressure) from a sealed reservoir to rams on each corner, air suspension employs an ‘open’ system where air is used, expelled and replaced.
In a street car system this cycle all centres around an air tank. The air is transferred via valves or solenoids to each airbag, lifting the vehicle as the pressure increases. The same air is expelled to the atmosphere when the time comes for lowering.
The idea is that a compressor will keep the tank topped up at all times acting as a reservoir for the bags. Obviously a bigger tank means more air in reserve for numerous rounds of lifting and lowering, while a larger compressor (or multiple compressors) will fill the tank to the optimum pressure faster.
Theoretically it is possible to run a system directly from a compressor, although it would take almost forever to ramp up enough pressure to raise the car. Some race cars and trucks have also been known to employ a type of closed system by doing away with everything bar the bags and using an externally-mounted valve to fill them. On a road car, though, this isn’t exactly what you’d call practical – in car adjustability is kind of the point!
WHAT’S IT ALL ABOUT?
In a word: adjustability. Air suspension is the only truly practical way of drastically changing your ride height on the go but, although many forum ‘experts’ will scoff, it comes with more benefits than simply being able to dump your car into the weeds.
Apart from the ability to create a show-stopping stance, many modern vehicle-specific kits are built with performance in mind and, in most cases, will not only offer better handling than standard but very often they’ll boast greater adjustment than coilover or spring and damper setups, too.
Like performance springs, modern airbags are progressive – the more they compress the stiffer they get – and this dynamic spring rate offers plenty of performance potential, especially combined with an optimal damper setup. In fact, air-ride was popular in drag racing and NASCAR as far back as the 1950s. There are also plenty of race cars and drifters running air suspension right there, more than you may think.
With air suspension the ride can be firm and tight, soft and comfortable, or anywhere in between. You can increase the pressure to firm up the ride for the circuit and then drive home in the lap of luxury, all at the push of a button. Then again you could just want to run your car super-low but with the benefit of actually being able to get on your drive.
An airbag, or to give it its proper name, an air spring, is just that – a simple pneumatic spring. Its job is to replace the standard coil, whether that’s in a coilover-damper configuration or a separate spring and damper setup. As we all know, BMWs have varied in design over the years but the principle is the same – essentially all you’re doing is swapping out a coil for a spring that can be adjusted with air pressure.
There are two types of common bag design – double convoluted bags and sleeves. The former, also known as bellows bags, donuts, and even double cheeseburgers, are the most common nowadays and are nearly always found on the front suspension. These have a shorter stroke than sleeves but a superior load capacity and a more progressive spring rate. Tapered or rolling sleeve designs may turn up on the rear where clearance is an issue or if a higher lift is required. These are smaller in diameter than bellows bags and generally have a smaller load capacity. Nowadays all bags are designed specifically for linear travel and that means they’ll expand and contract upwards and downwards rather than simply blow up like a balloon. They’re also suitably durable, too. Contrary to popular belief they’re not at all easy to burst and will hold well over 100psi – more than you’ll ever need.
The tank is the business end of the operation – it’s the air supply to the bag on each corner. It’ll be no more complicated than the air tank on your average workshop compressor though, albeit with a few more fittings. All you really need to know is which one to choose for your particular application.
In the old days tanks were mostly made from steel and hidden away from view but now we tend to regard them as more of a showpiece. For this reason there are also plenty of alloy items available in a number of bare, polished and painted finishes. Some are even skinned in carbon fibre or have all their welding polished out for a seamless look. Of course, it doesn’t hurt performance when all these are relatively lightweight, too.
Generally speaking, all air tanks are universal items and available in a range of sizes. This offers a trade-off between boot space and a system suited to repeated use. The more air in the tank, the more you can mess about with that ride height without waiting for a top-up! Some companies also offer tanks that are specifically designed to save space by fitting in a spare wheel well.
It’s worth remembering that every kit will come with a tank; some companies will offer a choice of tanks, but custom tanks are also becoming more popular than ever. We’ve seen everything from adapted nitrous bottles, scuba tanks, fire extinguishers and even beer kegs. Anything that can safely hold air at high pressure could be a viable option.
The big advantage of air-ride systems over hydraulics is that standard handling can be improved or, at the very least, be retained. This has a lot to do with air being easily compressible to absorb bumps (unlike hydraulic fluid) but has even more to do with the system being able to retain a proper damping setup.
All modern vehicle-specific air-ride kits come with matched shock absorbers. Many are supplied by well-known aftermarket manufacturers in the form of stripped-down coilover units. Some even have camber-adjustable top mounts and, as you’d expect, come with multi-stage adjustable damping and all the trimmings.
If it’s a retro BMW you’re building, then there may be the rare occasion where a specific kit isn’t available. In most cases universal items can be adapted for your application relatively easily. Some universal kits will come with a range of dampers already installed, others will have bags with a simple provision (like a hole in the middle) to retain a shock absorber. The point is, with air-ride you’ll always keep some sort of damper. And that’s never a bad thing.
The simple job of keeping enough air in the tank is one that falls to a 12-volt compressor. Various sizes are available and many people use more than one for rapid tank filling. After all, the faster the air is replaced the more you can use your spanking new active suspension! Compressors are inherently noisy, too, another argument for using multiple units and keeping them running for an absolute minimum of time. At the very least you’ll want to take this in consideration when you’re looking for a place to mount yours.
Controlling how much pressure the compressor pumps into the tank is also crucial. On the more basic systems a pressure switch between the compressor and tank is used to cut power when the optimum pressure is reached. Systems with digital management will often have the pressure switch incorporated into the manifold and a tank pressure display on the controller. In both cases, though, this will make refilling the tank to the desired pressure automatic.
The valves have the purpose of controlling the airflow from the tank to the bags and, in many cases, they also have the job of expelling the air upon lowering the car, too.
The simplest manual systems come with paddle valves, which look like switches, and are designed to be mounted within easy reach of the driver. On the back they’ll have a feed from the tank, an output for the relevant airbag, and an exhaust port to dump the air. It’s a simple, reliable and cost-effective setup but one that does come with a few compromises.
Because the airlines need to go through the valves it requires ruining them into the cabin during installation. If your tank is in the boot, for example, you’ll have a feed from the tank to the dash and then another back out to the rear bags. Due to the small diameter of the paddles, raising the vehicle can be a little slow, too. Most of all, though, the used air is usually dumped out of the valve and into the cabin which is not only noisy but it doesn’t always smell too fresh either.
These are simply electrically-operated valves designed to eliminate the need for paddle valves in the cabin. They’re a little more expensive, of course, but enable the use of electrical switches, or switch boxes, and don’t require any airlines being routed inside the passenger compartment. In some configurations these are individually mounted directly to the air tank (that’s why you see some tanks with four threaded fittings in the front) and in other kits solenoids are supplied mounted-together in a manifold (or solenoid block) with an exhaust port and single tank feed. These make installation far easier.
DIGITAL MANIFOLDS AND MANAGEMENT
Designed not only to make installation as easy as possible but to ensure day-to-day use is much more user-focused, digital management is fast becoming the norm, especially for daily-driven modified projects. Again a digital management system will incorporate a manifold containing a collection of solenoids to control the airflow to each corner. But they’ll also be designed to work with a simple plug ’n’ play wiring loom to take over management of the compressor functions and the power to the whole system. Many also include a handy feed for a second compressor and, although they’re all essentially universal systems, very often these are engineered so there are only two or three wires that need to be hooked up to the actual car. In other words, for DIY installation, they make life much easier – although that will inevitably come at a premium price. Most professional installers will admit that home mechanics who can fit a set of coilovers and wire-in an amplifier will have little trouble fitting a digital air-ride kit.
Each system works around an electronic control module designed to add a whole host of extra features. These digital setups are on the cutting edge of functionality and allow trick touches like automatic levelling and adjustment, pressure monitoring, physical height monitoring (via height sensors), lift-on-start and emergency auto top-up.
Each manufacturer has their own range of controllers designed to work with their specific digital management systems. There’s quite a number on the market nowadays and they range from simple wired touchpads to full-on wireless LCD items incorporating pressure displays for each airbag and tank – pretty useful if you don’t want to run an in-car pressure gauge. Some also offer an emergency key ring override just in case you forget to charge the main controller. In each case the controller negates the need to mount paddle valves or switches in your dash. Some of the higher-end management systems also employ Bluetooth or Wi-Fi technology so you can use your smartphone or tablet as a digital controller (via a downloadable app).
Perhaps the most important feature on the majority of digital controllers, not to mention the main reason why they’re so popular, is the ability to program a number of ride height presets. This means you can reach the desired level either automatically on start up or at the touch of a button, something that can’t be achieved with a paddle or simple solenoidbased system.
Pressure gauges are important in any budget paddle system to keep a check on what’s going on at each corner. These offer the only way of knowing the ride height without physically getting out and having a look… assuming, of course, you know the optimum pressure for each bag. Most air-ride gauges offer a dual readout so it’s rare that you’ll need a separate item for each airbag. I mean, who’s got space for four whole gauges along with four paddles anyway?
HARD LINE INSTALLS
For maximum flash many prefer a ‘hard line’ install which, for the most part, is exactly what it says on the tin. What they don’t tend to shout about so loudly is that replacing some or all of your plastic lines with copper or stainless steel piping is easier than you might think. In fact, it’s not unlike making up a brake or clutch line and, provided they’re the same diameter, they’ll even push into the same fittings.
Arguably hard airlines offer no real performance benefits because there’s no noticeable flex in the plastic airlines anyway. They’ll almost certainly require more fittings (with an increased risk of leaks around the joints) too, but there’s no denying they can make any boot install look amazing.
AIRLINE AND FITTINGS
Connecting all the components together are the airlines, and most air-ride kits include a good few metres made from commercial, DOT-approved plastic. Now, although no one likes to hear the word ‘plastic’ when it comes to holding their pride and joy up off the Tarmac, it’s actually far safer than it sounds. After all, they’ve been using exactly the same stuff on HGVs for years – and that’s usually on the brakes!
Commonly available in 1/4” and 3/8” diameters, the thing to consider is that bigger lines equal faster inflation of the bags, but this may come at the cost of overshooting your target pressure more easily. Whatever size you use, all modern airlines are designed with simple installation in mind. Kits will always include premium-quality, push-fit hardware, making it a simple case of cutting the line to the correct length, pushing each end into the fittings and that’s about it – easy.
The vast majority of air-ride systems, including those with digital management, are based purely on monitoring and maintaining a preset pressure in each airbag. There are a few, however, that rely on electronic height sensors mounted to each corner of the chassis to automatically maintain a constant ride height no matter the load, distribution or amount of passengers. This is a well-established idea that works well in changing vehicle conditions but the next generation systems that can monitor both, have already hit the market. The popular 3H Management from Air Lift Performance is designed for exactly that and uses a complicated algorithm incorporating height and pressure information to keep the level constant. It’s just as clever as it sounds, too!
It goes without saying that air-ride setups can be modular, and by that I mean you don’t have to choose a tank, management and bags from the same brand. You may take a fancy to one type of management, for example, but if that manufacturer doesn’t make direct-fit bags for your application, there’s no reason why you can’t use airbags from another company.
Some brands, like AirRex, D2, and K-Sport, offer all-in-one hardware solutions, too. These are usually a tank, compressor, manifold, and management combo, mounted together (or in a box) for ease of installation. In that way they’re not unlike an active subwoofer in that they take away the need for crafting a custom installation and all the guesswork that comes with it. The idea is you fit your bags, plumb-in the airlines, hook up the power and away you go.
TWO-WAY AND FOUR-WAY SYSTEMS
The first thing to consider is exactly what type of kit you’d like to go for. Nowadays four-way systems are by far the most popular and, as you’ve probably guessed, these allow for precise adjustment of each corner individually.
Two-way systems, in which the bags on each axle are hooked together, were popular in the past, particularly in the US, because of the ease of fitment and the fact you only need two valves or solenoids – one for the front and one for the rear. The down side is that we actually have corners and roundabouts here in Europe and a two-way system can magnify body roll in any bend because the loaded airbag will always try to transfer the air to its unloaded partner. To put it bluntly two-way systems aren’t always the most practical in performance cars and everyday drivers… although, if you’re building a quarter-mile-munching hot rod, they could still be useful.
BASIC FOUR-WAY SETUPS
So we know it’s likely that your BMW will need a four-way system. We’ve also talked about the fact that some systems incorporate digital management and others use valves or solenoids; however, what’s important is that each configuration takes a different approach to installation. It’s also good to remember that each of these setups can be adapted to your particular application with different size airlines and tanks, or the use of multiple tanks and compressors. In their very purest forms, though, there’s four basic ‘single-tanksingle- compressor’ setups: manual (paddle valve), solenoid-controlled, and pressure-sensing and ride height-sensing digital management systems. Here’s what they look like…
TOP FIVE TIPS
1. KEEP AIRLINES THE SAME LENGTH
The longer the airline from your valve or manifold to the bag, the longer it will take to inflate. Try to keep the airlines the same length on each axle to avoid the corners raising at a different rate. If you’re feeling flash you could try keeping the front and rear axles the same, too, then the whole car should lift as one.
2. INSTALL YOUR DIGITAL MANIFOLD ABOVE THE TANK
Like I’ve just said, water in the manifold isn’t particularly good so try to mount your manifold somewhere above the tank. In theory this should help avoid gravity pushing any excess fluid down the feed line.
3 USE PROTECTION
It makes sense to avoid any sharp edges that may damage your airlines so when passing through holes or bulkheads use suitable grommets to stop any rubbing. Wrapping your lines or using some plastic hose as a sleeve is also a great way of keeping them protected.
4. AVOID HOT STUFF
When running your plastic air lines under the car avoid both moving parts and anything that’s likely to get hot. Plastic melts, enough said. It’s also good practice to use a braided steel leader hose from the compressor to the tank (in many kits it’ll be included) instead of a plastic airline. Compressor outlets will always get hot while they’re running.
5. CHECK FOR LEAKS DURING INSTALLATION
It sounds obvious but the vast majority of leaks are caused by improper installation of the components. Once you’ve completed your fitting, pump 80-100psi in all the bags and leave them overnight. If any have dropped significantly by morning then it’s time to check for leaks around all the joints. The easiest way is to get an old spray bottle full of soapy water. Spray it on there and, if there’s any leakage, you’ll soon see bubbles.
To be honest, air-ride systems don’t require much more maintenance than a set of coilovers – just a quick check every once in a while to make sure nothing is rubbing, worn or broken. Despite what you might hear, leaks and split bags are extremely rare, too. Just think about it, they wouldn’t put air suspension on Range Rovers at the factory if that was the case, right? A properly-installed kit should last a lifetime as long as you remember one little rule – moisture in the system is the enemy. Basically speaking, fluid getting into a digital manifold is bad news for your wallet and things will get even worse if that fluid freezes in the airlines. The worst case is that it’ll expand and split the plastic pipes.
The problem is that all compressors create moisture and it will often accumulate in the tank before being pushed around the whole system. Luckily all modern tanks should utilise a separate valve at the bottom, which should be periodically drained. It’s best to do this after ‘airing-out’ your car and, in most cases, this will shed any fluid lurking in the system. Some kits also come with various water-traps to be installed in between the compressor and tank or the tank and manifold. Every little helps.
The majority of air-ride engineering over the past few years has been focused on expanding vehicle application lists, making controllers more interactive and adding a vast array of extra features to control modules. In fact, technology moves so fast that many firms are now offering digital firmware updates over the web. Suffice to say, it’s moved on somewhat from just a decade ago when universal paddle systems were about all you could find for any BMW! The latest new product from AccuAir is something totally different, though, and it has to be said, this one stormed the latest SEMA show. It isn’t available to the public for six months or so but the idea is bound to be popular as it incorporates all the valves and the compressor inside the tank for one supercompact, simple-to-fit unit. We’d certainly like to get our hands on one of these!
Air Lift Performance: www.airliftperfomance.com
K Sport: www.ksportusa.com
Air-ride Technologies: www.ridetech.com
Plush Automotive: www.plush-automotive.co.uk
Car Audio Security: www.caraudiosecurity.com
Rayvern Hydraulics: www.rayvernhydraulics.com
Bag Riders: www.bagriders.co.uk
Only Charged Dubs: www.onlychargeddubs.co.uk
Broadway Pneumatic: www.broadwaypneumatic.com