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Traction - Ultimately determining performance
By Dave Lum


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Traction. The final link to high performance. 800 horsepower is nothing without traction to tame it. Many think of tires when thinking traction, and while they are an important link, they are not the whole story of traction. Weight distribution, weight transfer, and suspension design (and its accompanying movement and angle changes), also have HUGE effects on available traction.

Tires obviously have a significant effect on traction - even a perfect active suspension design is useless without decent tires. Tires are designed to meet various design goals - noise, longevity (tire mileage), rolling resistance (for fuel mileage), ride comfort, wet and dry cornering grip (steady state and transitional - think of a slalom), stopping power, and cost, to name a few. I will be discussing only the high-performance applications in this article. For a further discussion of the various aspects of tire design, look elsewhere on this site . . .

Tires have a given amount of traction available at any given time, if 100% of available traction is being used to slow your vehicle, then as soon as you turn your wheel to steer, you exceed the maximum threshold of traction and you go into a skid - and with little or no change in direction. Also note that when skidding, you are decelerating slower than if you were right at the threshold of a skid (100%). EXAMPLE : Assume you're going 100mph, and slam on the brakes. Your ABS is perfect and you lose 20mph/sec. In 5 seconds you're stopped. Your buddy tries the same thing (same car) but no ABS and skids (tires not rotating at all). He will decelerate at only 15mph/sec - 6 2/3 sec later he will be stopped. I'm not saying ABS will always stop your car better than an experienced driver, but theoretically it should, and it beats locking up your tires. (For more on ABS, look here).

The same applies to cornering at the limit - if 100% traction is used for lateral grip, and you decide to hit the gas (or brakes), it's time to tangent and forget your cornering line. Again tire design comes into play here - some tire designs "break away" gradually, and will give you notice that it's beyond its adhesion limit, while other designs go from maximum grip to sliding much more abruptly. You can guess which one's preferred by most people.

So what effects maximum grip besides the tire? Weight transfer and weight distribution. I include both here, as they affect each other. When you accelerate, your weight transfers to the rear, increasing available traction in the back and decreasing it up front - no mystery why drag racers are rear drive. When you corner, weight transfers to the outside tires, increasing available traction to the outside, and decreasing available inside traction. Don't get too smart here and think "HEY, I'll just add 100lbs to each corner and increase traction all over!!", available traction will go up, but the inertia generated by the extra weight will MORE than offset the gains in traction. Weight distribution also comes into play here, as it will determine how much load will be distributed to each axle, along with how quickly the car transitions from a straight line to cornering. If your car has a 60% front 40% rear weight distribution, then when you corner, your outside front tire will be the most heavily loaded. A 50/50 split is desirable for many reasons, one of them being that when cornering, BOTH outside tires - not just one - will have the bulk of the load. Going back to available traction, you can add all 4 tires for total available traction, so you can see 2 tires using 100% is better than one using 100% and the other using 60%. Acceleration works the same way, and this is where rear wheel drive shines in the dry and front wheel shines under adverse conditions (and we all know all-wheel-drive excels under BOTH, but isn't always implemented optimally). Under nasty road conditions (slick roads - thus low coefficient of friction), front wheel drive works great because there isn't much weight transfer, and the majority of the vehicle weight is on the front (driven) wheels. MOST rear wheel drive cars also have most of their weight in front, so there is less available traction at the driven wheels. Exceptions are rear engine (Porsche 911) and some mid-engine cars (Toyota MR-2).

Suspension geometry also effects traction. If a tires designed contact patch isn't positioned correctly (meaning in full contact with the road), then you're missing out on potential grip. Suspension geometry can create this situation by creating tire angles that aren't optimal. There are many suspension designs, and there are also many different measurements that are used. These include : Camber, lean angles, steering angles, caster, and toe, to name a few. Suspension designers have many of the same design goals that tire designers have (noise, ride comfort, handling, and cost, along with weight, ease of manufacture, adjustability), and like everything else in car design, compromises have to be made. Short of an active suspension, there is no "perfect setup" for every road condition or use. As a car starts into a left-hand corner, the right side suspension compresses due to weight transfer, and the left side extends. With most suspension designs, this travel is accompanied by changes in camber and toe, which effects how the tire is contacting the road, and therefore traction.