Tom Heath
Tom Heath UberDork
11/24/09 2:11 p.m.

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.

The 2008 WRX was released amid much fanfare, but frankly it went over like a lead balloon with Subaru fans. Thankfully, its soft and flabby nature got a considerable tune-up for 2009. Of course, that leaves early adopters—like us—contemplating what to do with the 2008 model year duds. Hope’s not lost, however. We think that we can modify our WRX to outcorner that lightweight vintage Honda and remain completely streetable.

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