Air VS Coils by European Car

WHEN IT COMES TO SUSPENSION TUNING, THERE ARE WORDS PURISTS LOATHE. WORDS LIKE STANCE, RAKE, TUCK, POKE, SLAMMED AND AIRED-OUT. 

And while we appreciate the latest trends and innovations, we’re also traditionalists at heart. That said, we still like to get the car sitting right. The stance is still important to us but perhaps we don’t go to the extremes of notching the frame for clearance, cracking subframes, oil pans and spines just to look cool. 

In what must be the last ten years or so, people started looking towards the lowrider scene to get their cars even lower, first adopting hydraulics but quickly opting for air-ride. And while these systems allowed height adjustment to clear speed bumps and driveways, the cars often rode worse than a Honda on cut springs. Let’s just say I was confident I’d never install such a system on my car, but Editor Emmerson kept telling us things were improving. For years he’s been on a mission to compare air suspension to coilovers to assess exactly how the latest technology stacked up, devising this cunning test in an attempt to get a definitive answer to whether a car on air-ride could handle as well as one on coilovers. 

Before now, it had been hard to find identical cars with the latest systems but AccuAir recently approached us with claims its new “Sport” air system developed on the B8 Audi S4 was revolutionary and suggested it could outperform a coilover-equipped car. Skeptical, we were intrigued to see if this was possible and found a similar S4 on KW coilovers for our ultimate suspension shoot-out.

For this test we had three 2013 Audi S4s at our disposal. There was a bone-stock example, the car from AccuAir and a third with KW coilovers. The two modified vehicles had been built by TAG Motorsports in Escondido, CA and featured similar bolt-on parts – nothing that would give either car an advantage over the other. And to ensure a level playing field, each was fitted with high-performance Continental ContiSportContact 5P tires for consistency. 

“It’s important to note that the capability the AccuAir B8 system demonstrated in this test is something new for air suspension,” stated AccuAir co-founder Reno Heon. “Many people will have legitimate reservations about its performance ability based on previous experience from poorly engineered air systems. Some of those experiences are because many previous air systems were simply designed to make the car go up and down, without considering vehicle dynamics. Many systems were also poorly installed or were a mixture of parts that left too much room for installer error,” he continued. 

Essentially, Reno is suggesting we should toss away our negative preconceptions about air suspension, because no previous kit has been engineered like this one. 

The AccuAir Sport kit is comprised of four air springs, four height sensors, two compressors and an e-Level air management package. It includes everything needed to install it on your driveway and is priced at $4850. Admittedly, this is expensive by traditional coilover standards, although not some of the more advanced systems, as we shall see… 

Anybody with deep pockets can further invest in options like the iLevel Wi-Fi receiver that can be operated by the AccuAir iPhone app, or RF receiver on a key fob to control the system remotely.

The company estimates approximately 15 hours for installation, although it’s relatively straightforward since every piece was constructed to fit the car perfectly, with almost no modification necessary, aside from several small holes in the trunk floor to route the air lines. 

The new Sport system was designed to utilize the factory struts, which means it retains Audi’s Drive Select feature that adjusts damping for comfort or dynamic driving styles. However, the main piece of the equation is the air springs. They sit atop the dampers and have been painstakingly engineered to perform in this environment.

There are a few basic requirements AccuAir has for its air springs. They must fit into the space dictated by the factory spring pockets, so no modification is required to fit them. 

Next, the springs must have enough travel to allow the car to be raised to at least stock height, while sitting low enough when aired out (deflated) without needing inner fender or body modifications. The final criterion is the air spring rate – this is what you feel and is the most important aspect. 

With an air spring, the rates change depending on the overall shape, type of rubber construction and the internal air volume. Once installed, each height setting creates a different spring rate, with the lower settings softer and higher settings harder. So the AccuAir engineers are able to fine-tune the spring rates for a given “driving height.” 

In order to do this, AccuAir went to great lengths to ensure the springs were designed to the factory suspension parameters. To do this, they use a Vehicle Spring Rate Tester (VSRT) that places the car on scales to measure the change in force at the wheels, keeping the tires level, while pneumatic cylinders pull the chassis down. String potentiometers are placed on the fenders to measure the change in displacement at the wheels. 

Once the stock spring rates are known, the air bags are matched to the same rates or stiffened accordingly. Once installed, the AccuAir e-Management control system ensures the springs are correctly inflated to maintain a level ride. 

With three programmable ride heights, the system constantly compensates for the weight of the car to maintain the correct spring rates and ride height. So it’ll ride the same with just you or a car full of passengers, and whether you’re cruising or driving hard. 

Air pressure is provided by two air compressors in the spare wheel well that fill the air tank in the trunk. The tank is essentially a reservoir the bags can draw from when necessary to ensure rapid response, with the compressors topping up the tank as pressure drops.

So unlike many systems that are cobbled together, you’re not getting a makeshift setup designed to dump your car on the floor. This custom built AccuAir system for the Audi S4 was engineered to give you the best of all worlds; from canyon carving to driveway clearance, wheel tucking and family comfort. It’s a no-compromise system, with more specific applications to follow.

To begin, we gained access to the top-secret test facility used by our sister magazine, MotorTrend, at Auto Club Speedway in Fontana, CA. They use a figure-eight test course to accurately measure lateral g-forces as well as lap times. With MotorTrend’s road test editor Scott Mortara at the wheel, he put the cars through the identical test that every new car undergoes. And having driven everything on the road, Scott’s feedback was invaluable to us.

The results of the test in the table (left) don’t tell the complete story, but did fall in line with what we felt through our trouser dyno. The most important numbers were the average lateral acceleration g-forces, which is the measurement of accelerative force directed perpendicular to the direction of travel. So if you’re driving a car and turn the wheel hard in one direction, the force you feel is lateral g. 

And more lateral grip means better handling. So while the differences are very small, it does show that the AccuAir system offered an improvement in grip over both the stock and KW-equipped cars The figure-eight lap times are also very close; within a tenth of a second of each other. To put it in perspective, MotorTrend tested the Audi R8 V10 Plus on the same course and it set a 23.5sec time. More interestingly, its lateral accel was the same 0.99g as the AccuAir S4 on its grippy Continental SportContact 5P tires. 

The KW-equipped S4 ran 0.1sec faster around the track, but we agreed the KW spring rates were too stiff and with another few laps, the air-ride S4 could be driven as fast or faster. “The coilover car was disturbed more by bumps on the course and had similar understeer as the stock car,” said Scott. The stock S4 did exhibit lots of understeer at the limit, and the coilovers exaggerated this trait to an extent, making it difficult to get the car to rotate.

“The air-ride Audi S4 would be my overall choice,” Scott claimed. “The ride felt good and initial turn-in was better than the coilovers, all the way through the corner and especially on the exit. I’d drive the air suspension every day,” laughed Scott. And we would, too… We found ourselves laughing because we weren’t expecting these results, especially when we arrived to find the AccuAir S4 sat on the floor, with its wheels sucked into the fenders. Sure, it might look silly to some traditionalists, but it’s a growing trend in the European tuning scene and something many enthusiasts are after. Yet with the press of a button on the touchpad, the car raises and calibrates itself in seconds to the appropriate driving height where it went on to handle (and look) the best.

So against all predictions and our preconceptions, the AccuAir car was the star of the MotorTrend figure-eight test.