I have no love for drag racing. Much like NASCAR, I respect the technology and skill involved, but if straight-line acceleration is what it's all about, I'd prefer being strapped to a rocket sled for g-load testing on my internal organs. Call it a flaw, but it just doesn't inspire. I'm not even going to begin to explain how I feel about street racing.
Still, this job entails spending a lot of time trying repeatedly to get down the 1320 in as short a time as possible. Or watching someone else doing it. After a while, you start to develop a feel for what constitutes a good or bad run. Reaction time aside, 90 percent of it is in the launch. You know within the first 60 feet whether it's worth beating on a car all the way down the strip, or to tuck tail and turn back for another try. It's all a matter of how much wheelspin you get off the line.
Whether you're dumping the clutch at six grand or gingerly slipping the clutch all the way through first, each car requires a different strategy. What doesn't change is how the tires stick or boil into a senseless cloud of smoke.
This is what I don't get from watching amateurs drag race. Why the hell do people perform massive burnouts on asphalt with street tires when they're staging? The only time we do that is for fancy pictures and slow ETs. Just because drag racers on slicks do burnouts doesn't mean its good for tires designed for everyday asphalt.
Just to get it straight, I sat through an hour-long lecture with one of BF Goodrich/Michelin's best engineers. The physics of rubber is so complex, we never even learned it at engineering school. I came away with the following insights.
Vulcanized rubber behaves in strange ways. The best analogy is chewing gum, which is made of the same stuff, except you put it in your mouth. If you tore a piece of gum apart quickly, it would remain brittle and rip. Do it slowly and the material will retain its elasticity and stretch. So rubber becomes brittle the faster you apply load to it. Also, rubber gets soft and elastic with heat, and hard and brittle when cold.
Imagine drifters continually sliding and doing burnouts. The tires are being beaten continually-and at a high frequency-by every raised pebble edge in the asphalt, making them brittle. At the same time, those tires are generating tremendous heat from surface friction, while energy is dissipated as internal carcasses constantly move and deflect, making the tires soft and sticky. Drifters have to find the right compound that can balance temperature and frequency loading without becoming too brittle or too soft. This is where to find the most grip.
Street tires behave the same way, but are designed to operate over a smaller range of temperatures. All-season radials work with tread temperatures ranging from 20 to 100 degrees F. That's just slightly warm to the touch at its hottest. Any hotter and the friction coefficient drops off rapidly. Too cold and the rubber becomes too stiff to conform to road surface irregularities and provide traction. Maximum-grip summer tires raise this range 40 to 180 degrees, while R-compounds and slicks are happiest with tread temperatures over 200 degrees.
Why is this important? Because even a short burnout on a 70-degree day will blast your tire temps past 180 degrees. And if you're doing it on asphalt, the tire is loaded at such a high frequency that it just turns brittle and starts to disintegrate. That's why you see rubber dust on the ground. The softest summer tires only require a second-long burnout to reach this temperature. And although now soft, the effect is diminished by brittleness resulting from the burnout.
In terms of grip, rubber provides traction in two ways: road roughness effect and molecular adhesion. Road roughness effect provides the majority of traction and takes advantage of the miniscule bumps and dips in a surface for the rubber to bite into and push off from. In softer-compound tires and on smoother roads, molecular adhesion helps by using the viscous properties of molecular chains that resist deformation and generate friction. In simple terms, a rough surface gives street tires more bite, since they're not designed to flex as much.
Combine these two ideas and you'll realize that, while a warm street tire is good, a burnout isn't the best way to generate heat. And launching from the same pile of rubber dust you just made from the burnout will only hurt traction, because the rubber is brittle and dusty, and the dust just compromises the road roughness that street tires need. Drag racers do this because the track is smooth and the super-soft rubber that was just laid down is a nice, hot, soft layer on smooth concrete.
If you're truly obsessed with squeezing out that last tenth, do a couple of non-smoking practice launches to warm the tires and check your tire temps. If that fails, bring a propane torch and heat up the launch surface instead.
Jay Chen
Engineering Editor