Video by Paul Nguyen
Edited by Ernie Vigil
Captions by Gargy Cogis

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.

The Mules

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.


(Left) AccuAir-equipped S4 cornered noticeably flatter than KW (middle) and stock S4 (Right), generating the highest lateral-g number


AccuAir

"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.

KW

You know KW. It's a household name among performance enthusiasts, known and respected for making some of the best suspension components in the world, offering both aftermarket and motorsport applications.

For our test, TAG Motorsports fitted the second Audi S4 with KW's Dynamic Damping Control system. This is KW's response to the latest performance cars with factory dynamic damping systems, allowing you to retain suspension adjustment functions but with the height adjustment of a conventional coilover system.

There are three damping modes to choose from at the touch of a button: Comfort, Normal and Sport. Comfort is great for poor surfaces or a highway trek, while Normal feels a little sportier than the stock S4, giving good control with acceptable comfort. The Sport setting is significantly stiffer and would seem most suitable for the racetrack.

The coilovers are part of KW's stainless steel "inox-line," able to withstand heavy use or salted winter roads. To reach the desired stance, TAG lowered the S4 to the point where some driveways and speed bumps were unapproachable. And this was where KW's optional HLS (Hydraulic Lift System) came in handy.

Fitted to the S4, HLS provides additional clearance for lowered vehicles by raising the car with a hydraulic cylinder placed on the coilover body between the spring and its perch. When activated via a console-mounted button, 45mm of lift is provided, gaining clearance for obstacles.

The KW DDC coilover kit retails for around $4000, plus an additional $3600 for the HLS system. This puts it well past the AccuAir system in terms of price, on par in terms of versatility. So which would we prefer?

The Test

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. Now we headed to the hills to assess the handling under the toughest conditions.

Road test

Heading into the nearby mountains, the cars would be in their natural environment. The challenging turns and uneven road surface would be a demanding test for any suspension system, and this is where we expected the coilovers to claw back the lead.

Once in the canyons, we were able to gain speed, tipping into triple digits on occasions, experiencing everything from long sweepers to tight twisties. And yet the AccuAir system remained flawless. Road imperfections were absorbed without any disturbance, and the limit of adhesion was extremely hard to find.

Initially, we were surprised the air-ride wasn't crashing into potholes, and couldn't believe how flat the car cornered, with barely a hint of body roll. It also remained flat under acceleration and braking, giving excellent grip out of corners as the e-Level management ensured all four corners remained at the same height.

It was hard to believe that a car could drive this well on these uncompromising roads at very high speed, and yet would be able to raise itself over a driveway at our destination and drop itself to show off at an event...

In contrast, the KW DDC exhibited slightly more harshness even in its Normal setting - Sport was simply too stiff for these uneven roads, causing the car to become twitchy. However, even in Normal, mid-corner bumps could cause the front-end to transmit the disturbance into the cabin, with the rear-end bouncing along behind.

The coilovers also seemed to give less steering feel, and the overall agility was reduced when compared to the AccuAir system.

We felt the DDC kit might be improved with softer spring rates to reduce its tendency to bounce in corners. That said, the owner had dropped his S4 almost to its lowest setting, giving the suspension less travel than it might ordinarily have. Raising the car would doubtlessly improve the situation, but our test was to compare lowered cars on air and coilovers, so a lower ride height was part of the equation. And while the KW DDC did offer impressive handling compared to the stock car, we found that the choice of air suspension was a no-brainer in any situation, on any road.

These results seem truly remarkable. This day set a new milestone and will be remembered as the moment we learned to appreciate a well-developed aftermarket air suspension system.

It's clear that AccuAir's intention of redefining air suspension has been an overwhelming success. The company has created a system unlike anything else currently on the market, and unlike anything that we, as enthusiasts, have previously experienced. For the price, convenience, looks and dynamics, the AccuAir Sport air suspension system was the clear winner in our test.

The Cars

The two modified 2013 Audi S4s were built by TAG Motorsports in Escondido, CA and share similar components to give them great performance, handling and style.

They both share identical engine modifications, using an AWE Tuning supercharger pulley to increase boost pressure, as well as an S-Flo intake, exhaust and downpipes. The only real differences were that John Holdridge's black car had unresonated downpipes (and sounded better for it) as well as GIAC stage 2 DSG software that made it slightly more responsive.

The black S4 had the AccuAir Sport air suspension kit, and sat on 20x10" HRE P44SC wheels finished in Brushed Gold. Behind them was a 380mm StopTech ST60 brake kit that offered powerful stopping.

Dennis Tang's white S4 was on the KW DDC coilover kit with the KW HLS kit on the front. The fenders were filled with 19x9.5" HRE P43SC wheels finished in satin black as well as the same 380mm StopTech ST60 brake kit. Further differentiating the two was a EuroCode Alu Kreuz drivetrain stabilizer.

The exterior was kept subtle, in true Audi fashion, with both cars getting OEM Euro headlights, black grille and foglight grilles plus TAG Motorsports' blackout package. Additionally, the white S4 had gloss black exhaust tips and white StopTech brake calipers.

Car Lap Time Lateral Accel
Stock S4
25.3sec
0.95g
KW S4
25.1sec
0.98g
AccuAir S4
25.2sec
0.99g

SOURCE
KW Suspension
1075 North Avenue
Sanger
CA  93657
800-445-3767
www.kwsuspension.com
Tag Motorsports
2181 Meyers Avenue
Escondido
CA  92029
760-741-9300
www.tagmotorsports.com
AccuAir Suspension
Grover Beach
CA
877-247-3696
www.accuairsuspension.com
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By Alex Bernstein
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