Racers and car geeks love tossing around hackneyed performance benchmarks, from zero-to-60 sprints and top speeds to quarter-mile times. Why is 60 such a big deal, anyway? These facts and figures have fueled many discussions throughout the decades—first in study hall, now online.
But what about numbers that quantify handling? Unfortunately, lap times are dependent upon the driver, conditions and, to a lesser extent, traffic. So far, humankind hasn’t conjured up the standardized autocross site, either. Slalom times come close, but again there’s a bit of driver error to consider.
Enter the skidpad.
Skidpad testing is like a dyno for tires and suspensions. While it only offers insight into a certain aspect of a vehicle’s performance capability, it’s a great way to accurately measure just how much grip a chassis is capable of generating. Unlike a road course or autocross competition, skidpad testing almost completely removes the driver from the equation to simply reveal how well the car sticks.
The basics are fairly easy to grasp: In short, a car is driven at its cornering limit around a circle. The cornering force is then calculated over that distance and time. Once the cornering limits have been exceeded, the car won’t be able to follow the circle without slowing down, and the lap times (and cornering forces) will suffer.
For a unit of measure, we use g-loads. One g is the rate at which a free-falling object will accelerate due to the force of gravity pulling toward the Earth’s surface—it’s that force that keeps us from floating into outer space.
From our high school physics classes, we know that 1g is 9.8 meters per second squared, or 32.174 feet per second squared. Aerodynamics aside, whether you drop a bowling ball or a marble, they will accelerate at this same constant rate.
Lateral acceleration is the same measurement as vertical g, but in a different direction. We’re measuring the rate of change in the velocity of a moving body along a horizontal plane. Using the g as a measure of a car’s lateral acceleration might seem a bit counterintuitive—we’re not driving Hot Wheels cars on a loop-de-loop—but it is the industry standard. It’s also easier to express than meters per second squared.
We hit the 1g mark about 10 years ago using a sporty little Honda and a full complement of the latest in R-compound rubber. We detailed those exploits in our September/October 1998 issue. A decade later, can we match that cornering force in a five-door wagon on true street tires?
Lots of things have changed in the last 10 years. Cars have gotten taller, heavier and softer as demands for safety and luxurious comfort have trickled down to even the smallest of cars. Our Subaru tips the scales at nearly half a ton more than that old Honda, and that weight undoubtedly works against the suspension’s grip. On the other hand, there have been considerable enhancements in tire compounding and suspension design that help even the heaviest of cars turn on a dime.
Like most enthusiasts, when we get a sporty new ride, we can’t wait to take it to the track or autocross course to see how it will fare. When we took our 2008 Subaru Impreza WRX to an Evolution Driving School last spring, we simply mounted up some sticky Nitto NT01 R-compounds and figured we’d be all set for a day of fun and learning.
About halfway through the first day of the school, we realized just how soft and ill at ease the new WRX was on course. Our Nittos were bearing the brunt of the body roll and awkward suspension geometry: By mid-afternoon, we had corded two of the tires. The outside edges were worn down through the tread cap and into the inner rubber liner.
The culprit? Too much body roll, and poor control of the tire’s contact patch. There was still plenty of rubber remaining on the inside shoulders, indicating that we weren’t maximizing the faces of the tires during cornering.
Going Around in Circles
Our testing plan was pretty simple: Maximize the car’s grip through intelligent suspension and chassis tuning. It would take a little thought and a lot of testing to achieve our goal.
We rounded up some specialized equipment that we predicted would address the WRX’s shortcomings. Rather than simply bolt on everything at once and measure the net gain, we planned to go step by step, testing all along the way to see where the biggest improvements could be found.
Everybody knows that sticky tires can increase overall grip, so our strategy would be to first maximize the suspension before making that big leap. We also figured that this plan would save our good tires from much wear and tear.
For our testing grounds, we used one of the 250-foot-diameter skidpads at Gainesville Raceway’s road course. This driver training and vehicle dynamics area is located behind the famous drag strip.
A skidpad’s surface and condition can make a huge difference in the numbers generated, as both asphalt and concrete come in a wide variety of flavors, from super-slick to super-stick. The asphalt at Gainesville is what we’d consider average to good in terms of grip—although the pavement itself is a bit off-camber, which slightly affects the data gathered.
We’ve found that after a few laps, the surface remains consistent throughout the day. During our testing, the weather remained sunny and brisk—for Florida—at about 50 to 55 degrees Fahrenheit.
Testing on a skidpad is not as simple as it seems. It’s really more than just driving around in a big circle: You’ve got to manage the available grip by hearing what the tires are doing. Our pilot, Tom Heath, had to drive at the fine edge of traction, keeping the tires howling at a high-pitched squeal. As a tire moves past its optimum slip angle, this sound transitions to a lower pitch—more of a “squall” than a squeal. Keeping the tires in this narrow zone of maximum grip yields the best results.
We ran five laps for each session, and the activity in each session was recorded using a Traqmate data acquisition system. Traqmate representative Ron Hlozansky mounted the sensor unit on the WRX’s armrest, and it stayed in exactly that spot for the whole day.
That constant position was important, as moving the device could have caused its internal accelerometer to generate inconsistent results. Additionally, the Traqmate gave us highly accurate lap times. These lap times, as well as the diameter of the skidpad, allowed us to calculate the g-forces for each lap. This meant we could compare two streams of data in order to narrow our margin of error.
The suspension work and tweaking was all done at the track by our expert race mechanic, Geoff Thompson of Andre’s Automotive. By keeping all of our testing within a window of a few hours, we minimized weather and temperature variables that could have otherwise affected our results.
1) Gather Stock Baseline Data
Testing with the stock suspension components and alignment settings punished the Subaru's original Bridgestones.
Indicated g: 0.958
Calculated g: 0.865
For our first session, we used the original 17x7-inch wheels paired with the stock 215/45R17 Bridgestone Potenza RE92 tires, the exact setup the car received at the factory. We also used the factory-recommended tire pressures as a starting point.
The Bridgestones quickly took a pounding during this baseline session, and within five laps we already had significant wear and chunking on the outside-front tire. We rotated that tire to the inside-rear position to keep our test going.
The pundits were right: In this configuration, the car wallowed around the skidpad. Even the smallest surface bumps caused big changes in the car’s attitude, making it difficult to keep the car at maximum grip.
2) Adjust Tire Pressure
Indicated g: 0.966
Calculated g: 0.847
The outside corners of the front tires were screaming for mercy at the factory-specified tire pressures—and taking a severe beating in the process. We bumped them up to 40 psi to give the tires a fighting chance of staying alive, then went back onto the pad.
The air pressure increase added some stiffness to the carcass of the tires and proved to be a step in the right direction. The tires were more predictable and controllable at the limits of traction, although they didn’t offer more calculated overall grip. They did show less indication of rolling over onto their sidewalls and suffered much less damage than in the first session, but we still rotated them just in case.
3) Add AST Coil-Overs
The stock suspension allowed for too much pitching and rolling. The AST coil-overs were part of the cure.
Indicated g: 0.981
Calculated g: 0.889
During our first two sessions, we could see and feel the WRX pitching and rolling around the skidpad. Upping the tire pressures was a step toward combatting this behavior, so we figured that increasing the wheel rate would help as well.
Wheel rate is the amount of force that it takes to compress the car’s suspension as measured at the center of the contact patch. It’s affected by the design of the suspension, spring rate and anti-roll bar rate.
For the third session, we installed the Dutch-made AST 4100 coil-overs. These units improved our WRX’s handling in several ways. First, they lowered the car’s center of gravity, helping to better distribute the weight of the chassis across the tires.
These coil-overs also use a significantly higher spring rate than the factory Subaru pieces, helping to better control the body motion and allowing the tires to concentrate on providing grip. The spring rates were 450 lbs./in. front and 550 lbs./in. rear.
Each unit features adjustable rebound—12 levels are easily accessed by a little knob found at the top of each unit. For a starting point, we set the rebound adjustment of all four dampers at the halfway point.
We wanted to retain the stock alignment settings for this pass, and that required the installation of the Vorshlag camber plates. These plates let us retain 1.5 degrees of negative camber, and we reset the toe to nearly zero.
Once back on the skidpad, we felt an immediate improvement thanks to the stiffer springs and increased damping. The car was much easier to control and remained composed around the skidpad.
We felt like we were beginning to maximize the potential of the stock tires, as they could withstand higher speeds before rolling over. Still, we condemned another Bridgestone RE92 to death by rotating it to the outside front corner.
4) Dial In a Performance Alignment
The Vorshlag Camber plates allowed us to change camber settings as quickly as Geoff could measure them.
Indicated g: 0.972
Calculated g: 0.878
Now we could dial in some additional grip-adding negative camber. We simply loosened the four hex-head bolts on each Vorshlag camber plate and slid the top of the strut shafts toward the center of the car. This move would allow the tires to grip more efficiently by putting the widest portion of the tread on the surface at maximum cornering loads. Basically, it would harness the same forces that cause body roll and use them in our favor.
Dialing in the exact angle for maximum lateral grip is a game of compromises. Too much negative camber will reduce the available grip for braking, so testing is important to determine just the right number. Based on past experience with Subarus, we figured that about 3 degrees of negative camber on each side would be the happy point for maximum handling; we’d tone it down a bit for street use, however. We also tweaked the toe adjustment on the front tires to 1/4 inch of total toe-out to help sharpen the car’s turn-in response.
Remember how we said that negative camber is a game of compromises? The skidpad results showed an ever-so-slight decrease in overall grip. While the large amounts of negative camber were helping to keep our outside-front tire in full contact with the pavement, it was having the opposite effect on the inside-front tire. We could have set up our Subaru like an oval track racer, but we chose to keep the car’s alignment specifications symmetrical. After all, we’ve never been to an autocross or track event that featured only left-hand turns.
Here’s another benefit from all of that negative camber: It dramatically reduced the shoulder wear on the front tires. We decided to keep these settings in anticipation of increasingly higher cornering forces as the testing continued. With greater speeds and g-loads, the need for more camber seemed inevitable. Despite the decrease in grip, controllability was improved by the larger effective footprint of the front tires.
5) Add a Progress Rear Anti-Roll Bar
The Progress anti-roll bar gave us a big bump in calculated g for minimal cost.
Indicated g: 0.989
Calculated g: 0.891
Larger-than-factory anti-roll bars are a great handling upgrade for the money. A stiffer anti-roll bar works to further reduce body roll by tying together the right and left sides of the suspension. This essentially lends wheel rate from the side that’s in droop to the side that’s being compressed.
We made a sizable but easy upgrade here, trading out the stock 19mm rear bar for a Progress Technology 22mm piece. That’s approximately an 80 percent increase in stiffness since an anti-roll bar’s diameter to the fourth power determines its torsional rigidity. The Progress bar is also adjustable, featuring three levels of stiffness. We started with the medium setting.
We were impressed with the fit and attention to detail in this seemingly simple part; features like an end stop for the bushing to prevent the bar from moving laterally show why Progress has been successful in this industry for so long.
The upsized rear anti-roll bar had a huge impact on our WRX’s attitude in corners. We found the car much easier to modulate with the throttle, allowing us to be more precise when placing the car. It might have been even a tick too stiff relative to the front bar, demanding a slight recalibration of our driving technique, but the end result was a higher level of grip. Our driver came in from this session with a huge smile on his face. As a result of the reduced body roll, the gap between indicated g and calculated g narrowed a bit.
6) Adjust the Front Shocks
Indicated g: 1.017
Calculated g: 0.928
Since the AST/Vorshlag coil-overs offered a 12-step adjustment for rebound damping, we bumped up the front dampers to their firmest setting to counterbalance the larger rear anti-roll bar.
On the skidpad, we didn’t feel much of a difference aside from better balance. However, the data acquisition revealed that our indicated grip was now over the magic 1g barrier. The calculated average, on the other hand, wasn’t quite up there yet.
We were closing in on our goal, and we hadn’t even used up the stock tires yet. We were confident that the performance-oriented rubber would get us over the last hump, but we still had some room to optimize our setup and maximize our results.
7) Add a Progress Front Anti-Roll Bar
Indicated g: 1.012
Calculated g: 0.919
We decided that it was now time to add more wheel rate to the front end with a thicker front anti-roll bar. Where swapping anti-roll bars on past Subarus required dropping the front subframe, that’s no longer the case with the latest cars—the job is now quick and pain-free. We set the front Progress bar at the softer of its two settings, hoping to keep some of the steer-by-throttle control while providing just a bit more roll stiffness in the front.
Back on the pad, the front of the car was noticeably flatter, although ultimate grip wasn’t quite as good as before. The car had become a tad more stable, but we lost a bit of the edge we’d enjoyed with the previous setup.
At this point, our tires were starting to look like they’d gone nine rounds with Mike Tyson, complete with bitten-off chunks. Still, the car was acting like a totally transformed beast, as that cushy Subie from the opening rounds had been replaced with a high-performance hooligan. The WRX was back.
8) Switch to Nitto NT05 Tires
Our Nittos immediately improved both control and stability.
Indicated g: 0.955
Calculated g: 0.868
Now it was time to add sticky rubber. Fortunately the market is nearly flooded with options: Nitto’s brand-new NT05 is a shot across the bow of the Bridgestone RE-01R and Dunlop Direzza Sport Z1, and we couldn’t resist trying a set.
Our 235/40R17 size retails for less than a lot of the other performance brands—figure about $150 each. And while the claimed width is only 20mm wider than the factory Bridgestones, the Nittos appear much larger, making them look very meaty on 17x7.5-in Rota wheels.
Unfortunately, we saw a slight jump backward. While overall grip was down a tick for the first run, controllability and consistency shot way, way up thanks to the NT05’s shorter, stiffer sidewalls. We still missed the quick rotation that the soft front/stiff rear anti-roll bar combination provided, but overall the car felt very good on the new rubber.
Why, then, the decreased grip? Our theory is that the tall tread blocks of our fresh tires weren’t helping. At this point, our stock Bridgestones were nearly slicks—less tread squirm generally equals more grip.
9) Stiffen Rear Anti-Roll Bar
Indicated g: 1.032
Calculated g: 0.936
After hearing our driver whine about anti-roll bars for a few sessions, we fully stiffened the adjustable rear bar on our WRX to improve the chassis’s balance. Stiffening this bar forces the inside-rear to share the load of the outside-rear.
The effect is similar to a higher rear spring rate, but it’s only apparent when the car is turning. The effect is negligible when braking, accelerating or encountering bumps. This arrangement allows for a comfortable ride that’s still capable of wicked performance.
After scuffing in the tires some more, we could feel them coming up to temperature and providing increased grip. We also noticed the car’s balance settling into a very nice compromise of sharpness and stability; we found it very easy to adjust the car’s attitude and ride the fine line of traction that provided the best grip around the skidpad.
Our setup was close to ideal, but still not as good as we knew it could be. Indicated g-loads were again beyond the 1g barrier, but the calculated forces hadn’t quite reached our goal—yet.
10) Adjust Tire Pressure and Shocks
Indicated g: 1.067
Calculated g: 0.969
To extract the last little bit of performance from this configuration, we decided to lower the rear tire pressures a bit in an effort to increase grip at that end of the car. We went from 40 psi to 38 psi; we also fully softened the rear shock absorbers. Typically these adjustments don’t affect steady state cornering, but we suspected that some uneven surface conditions might have been upsetting the car’s balance around the skidpad.
After conducting this session, we found that our changes were on the mark. The g-forces were up a solid three tenths, bringing us within striking distance of the 1g barrier—on genuine street tires.
The car must have been working well, as our driver was no longer griping about the WRX’s handling. His big complaint was about his sore back. Holding himself in the stock WRX seats all day was starting to take its toll.
11) Switch to Nitto NT01 Tires
For that last bit of adhesion, we mounted up a set of Nitto race tires.
Indicated g: 1.117
Calculated g: 1.016
To complete our testing, we pulled out the big guns: a freshly shaved set of 225/45R17 Nitto NT01 race tires. This tire performed well at the Evolution school, and we were very eager to try them with a proper suspension. The additional grip provided by genuine R-compound tires was sure to put us over the 1g barrier, we figured.
The difference in seat-of-the-pants feel between a calculated 0.97g and 1.01g was very apparent, and we’d describe any length of time sustaining this level of grip as a very literal pain in the neck. Right up to the limit of traction, the car was easy to drive. However, once pushed past that limit, it became difficult to bring the tires back into shape without completely backing out of the throttle and settling back onto our target line.
One Last Experiment: Make Biased Shock Adjustments
Indicated g: 1.117
Calculated g: 1.024
We were very happy with our suspension at this point, but we had time for one little science experiment. For our last session, we fully stiffened the rebound setting on the inboard shock absorbers while leaving the outboard dampers fully soft. This would bias the suspension in our favor, essentially allowing the car to “lean into” the counterclockwise direction of our skidpad test. Science is fun.
While our Traqmate results didn’t improve, the calculated g-load was up nearly a hundredth of a point. Why? Because the offset damping reduced the effective body roll even further, bringing our two test methods closer to parity. While this setting is only good for a skidpad, it gave us an insight into the variables that can be manipulated to provide an artificially high reading.
G, We’re About Done Here
Not only did we save money by not trading in our Subaru for a newer model, but we made it stick like glue. The handling now rests solidly in 1g territory, ready to tackle street, autocross and track duty.
What better way to test that new grip than some real competition: We’ll be taking the car to an SCCA Solo National Tour event soon to see how we fare against the Street Touring Unlimited field.
Control Your Roll
Lowering a car equipped with MacPherson struts—like our WRX—is as simple as installing the right springs. However, there are other factors at work. While the handling benefits that can be gained from lowering the center of gravity are easy to understand, there’s more to the picture than just shorter springs.
By lowering the suspension within its range of motion, we affected the movement and relationship of the control arm. To make matters worse, this new geometry forced the lower arm to go farther in its range of travel than it had on the stock car, causing the wheels to toe in as the suspension compressed.
Since our suspension was now already partway through its range of motion thanks to the lower ride height, even small bumps would cause dramatic toe changes. Of course, this didn’t do any favors for the handling. Lowering the car was a net gain, but we wanted to avoid the bumpsteer compromise.
The factory components mitigate this bumpsteer effect by keeping the centerline of the ball joint and the centerline of the tie rod aligned along a horizontal plane. This spatial relationship contributes to the location of the vehicle’s roll center, an invisible but very real point in the space between the suspension components that affects the way the car turns. By relocating the ball joint and the tie rod closer to their original positions, we could enjoy the benefits of our lower center of gravity while avoiding the drawbacks caused by poor suspension geometry.
Aside from living with the compromised handling, there really isn’t a easy fix for this dilemma besides ponying up the money for taller ball joints and rod ends. The good news is that Whiteline Automotive offers heavy-duty tie rod ends and ball joints that would move our roll center back into place in one simple package. These are specialized components, so using a pre-engineered kit like this makes good sense. At less than $200 for the set, they’re even more affordable than their Subaru factory counterparts.
Removing and installing the ball joints requires some specialized tools, so we weren’t able to sample the roll center adjustment kit during our test day. Instead, we took the car to the trusted mechanics at Andre’s Automotive in Ormond Beach, Fla., to have the new components installed.
During our follow-up testing, we noticed that the steering had a more direct, responsive feel that was almost out of character for the traditionally vague feedback we’ve grown to accept from Imprezas. This improved feel and the resulting peace of mind easily justified the cost of these components. The only downside is that we'll have to take the tie-rod ends back off to remain legal for STU-class SCCA autocross competition.
