Disclaimer: While based on facts about drag racing, this page contains the personal views and opinions of one racer - Michael Beard
The experiments I conduct and the laboratory I use are somewhat unorthodox. The lab is a rolling one: a 1974 Plymouth Duster powered by a 300 horsepower 360 cubic inch motor. I compete at NHRA Division 1 bracket races on a weekly basis, usually at Beaver Springs Dragway, Beaver Springs, PA. As for my credentials, I am the defending NHRA Div. 1 Heavy ET Champion and ET Driver of the Year. I have been racing for over five years now, and every race in that time has been a learning experience. While my primary interest of study at the drag strip has been psychological, I have learned more than I thought possible about the mechanical and other physical aspects of racing. I hope to bring some of the automotive technology issues to light that I have discovered for myself. There is no better place to start than on the ground. This week's focus is tires.
While racing tires and street tires bear little resemblance, keep in mind that many of our earliest automotive advances came from manufacturer's involvement with racing. In the early 1960's, high-powered drag cars were developing traction problems because the only tires available then were small and made of a hard compound. The most immediate solution was to spin the tires in a substance called rosin, a sticky powder. The evolution of softer compound tires did away with the necessity of using rosin. Traction was restored. Today, tires have become an engineering science, and there are some special tricks to use to get the most out of them.
Drag racing tires are called slicks. They are made of a soft compound of rubber, which affords better traction, but they tend to wear out very quickly. The sidewalls of slicks are designed for straight-line performance, rather than cornering. They are also made of a soft compound, which are designed to give way to any stresses. Wrinklewalls, as they are called, will look very strange when you mash the accelerator. The power from the engine goes through the transmission and rear-end components, through the axles, and is applies to the rear tires. The slicks try to rotate, but are resisted by the friction of the ground. The tire ends up spinning faster at the center of the wheel than at the outer edge near the ground, which results in the tire wrinkling around a portion of the bottom of the tire.
More than a few people have looked at pictures of my drag car coming off the starting line and asked, "Um...isn't your tire flat?" Nope. That's physics. The physics behind the wrinkle is this: when the slick wrinkles, the tire creates a larger contact patch with the ground, or area where the surface of the tire and the ground are in contact. Looking at it from the ground's perspective, you would see the original contact patch, and then when power is applied, you hold onto what you had, and the tire forces more of itself down onto the ground on the front. In simple terms, the tire does flatten out, creating a larger contact patch, and thus better traction. What if that first application of power is too much for the ground's friction to hold onto, though? Well, that's simple too: the tires spin, and you lose. You have more technology to rely on in the tire, however, than just the wrinklewalls.
The soft compound of the tire is composed of not just a special formula of rubber, but of some traction-aiding chemicals as well. How do you get these chemicals active? That's the fun part. If you have ever seen the Pros on TV smoking the tires half off, then youUve seen a burnout. To do a burnout, you back the car up into a designated area called the burnout box which has been sprayed with water. You tap the gas to spin the tires over in the water, and then pull out just onto the dry asphalt. You hold the brake and crank it up to 5,000 rpm in first gear. This spins the tires hard, and as the water starts to dry off, the slicks start to heat up. As soon as the white smoke starts to come off them, they are as hot as they are going to get, and you let off the brake and approach the starting line. Heating the slicks releases some of the chemicals, and the tires become sticky to the touch. This sure doesn't make them last any longer, but consistent traction is a key to winning races. As I mentioned before, not heating the tires is going to lead to traction deficiency, but over-heating the tires can do the same. If too many of the chemicals are released and get too hot, they can actually make the tire slippery.
In case you were wondering, I made 447 runs down the quarter-mile track on my first set of slicks, or just over 100 miles of racing before they were done. That was over two seasons, and many drivers go through two to three sets of slicks in a single season! A single slick will typically cost $150, so you can see that it is easy to allocate much of your budget to tires for a season. Technology is not cheap. There is obviously a lot more to learn about racing and automobiles!
Copyright © 1996-1999 Michael G. Beard
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