G = Gravity
While sheer lateral grip doesn't necessarily equate to a fast car on track, our 200-foot skidpad is still a good measure of most street cars' amount of stick. It's an average of the lateral grip any car generates, calculated from the time it takes to complete a level circle, 200 feet in diameter, in both directions.
We're always striving to break 1g in our street cars. No small feat, considering the tires are supporting the equivalent of the car's weight sideways. Put in perspective, most cars generate just a hair over 1g at peak straight-line braking deceleration, and few can exceed the magic number in acceleration.
Even in all-wheel-drive cars, with all four wheels generating friction to launch, it's harder to accelerate the vehicle in a straight line than it is to hold the car from going sideways. Part of it has to do with the fact that a tire's traction circle is slightly elliptical, since the tire contact patch is wider than it is tall. More critical, however, is weight transfer. Friction is as much a function of the tire itself as the weight that it's supporting. With less weight on the front tire in a launch, less overall friction or grip is available. In the sideways scenario, while there's still weight transfer, a well-sorted suspension will keep weight on all four tires much more than in straight-line acceleration. The forces trying to overpower tire grip and push a car out of a turn are also more powerful than the force an engine can supply to push a car forward.
But the skidpad isn't the ultimate judge of lateral grip. Real race cars suffer mid-throttle asphyxiation and a miserable understeering death on such a small loop where there is no aero grip. A fast constant-radius turn separates the sticky cars from the quick ones. That's exactly what we found in Turn Two at Willow Springs International Raceway in Rosamond, California. The track is considered the fastest in the West, where street cars can see speeds of up to 110mph down the front and back straights. Turn Two's slightly banked profile and large radius gives plenty of room and speed to see just how much difference downforce makes in increasing lateral grip.
For reference, we brought out a bone stock Mazda RX-8, which for our intents and purposes, has negligible aero grip or lift. It averaged 85mph and 1.03g through the turn, compared to the 0.92g it pulled on the skidpad. That extra 0.09g comes just from the higher speed and road camber.
Turn Two averages a couple degrees of banking, which adds to the equation. Normally, on a flat turn, the centrifugal acceleration, (or lateral g) acting on the car is equal to the total lateral friction force of all the tires trying to keep the car from sliding out of the turn. On an angled banking corner, Newton's equal and opposite reaction force to the car's weight, which normally has no effect laterally, now contributes to pushing the car back into the turn, along with friction forces. So at the maximum threshold of tire grip, there's more force available to push the car back into the turn than on level ground. To bring everything back into balance, a car would have to travel faster through the banked turn and generate more centrifugal force. This is why you can round a banked onramp a lot faster than a flat one. -Jay Chen
From Zero to Hero: The 1g Challenge
Getting a car with any semblance of sportiness to pull a g is old news. So we went the opposite way in search of the elusive 1g: find the biggest turd we could get our hands on and see if we can get it to support its own weight sideways. Ironically, finding a reasonable candidate wasn't so easy in a sea of modern high-performance cars. It had to be horrid enough to make the challenge worthwhile, but at the same time new and popular enough that manufacturers are willing to actually make parts for it. Our managing editor's Datsun F10 and our editor's minivan came to mind as potential sacrifices to the skidpad, but we couldn't find anyone sufficiently odd making parts that lend any handling advantage.