There’s a lot that goes into getting your car to launch at the strip. The moon and stars don’t necessarily need to align, but your suspension parts, their geometry, and all other aspects of your car should at least work together. One of the easiest things that you can control is tire pressure. If you’re running too much air pressure, this can cause the tire to bulge (crown), and wear the center of the tread faster than the rest of the tread surface. Not only does this cause premature tire wear, but you’ll be losing a lot of traction—and races. Having too little air pressure may cause the reverse to occur (excessive wear to the outside of the tread), but you’ll still lose races.
Wheel spin loses races, and if you have too much air pressure, your tires can’t react (wrinkle), and your tires will encounter what is called initial spin. Initial spin is when the tires spin almost immediately when you are trying to launch the car. Another type of spin is called reaction spin. Reaction spin usually occurs when the tire is underinflated. What happens is the underinflated tire reacts (wrinkles), but the lack of proper air pressure allows it to overreact, and that force releases itself with a suspension-overcoming force. In other words, it initially hooks, but then bounces as they unload.
You should always check tire pressures when the tire is cold. Simply stated, this means check your tires when they are at an ambient air temperature, not after doing a five minute burnout. There are also differences in required tire pressures depending on the type of tire you’re running. When equipped with high performance street tires—which have stiff sidewalls, a common misconception is that if you lower the tire pressure significantly, you’ll have more traction. With a bias-ply “slick” this is true to a point, but a street radial does not “react” (wrinkle) like a slick does. Remember what we said happens when the pressure is too low?
What is important to keep in mind is knowing how your tires respond to air pressure. Unless there is an established technique that is getting 60-foot times below 2.0 seconds, try starting with pressures listed below in your tires, and work your way up or down—it depends on what your car wants. Racing slicks are able to use lower tire pressure to soften the sidewalls—they are designed to react (wrinkle), and increase the size of the contact patch. See chart below for basic starting tire pressures.
So how do you know that you’re running the correct tire pressure? One way to test for proper tire inflation is to find a clean piece of pavement, preferably off the highway. Next, make sure that both rear tires have the same pressure. Now, do a short burnout and examine the tire marks. An over-inflated tire will leave a mark that is dark in the center, but light on the edges. An under-inflated tire will show dark edges and a light center indicating that the tire is cupping. This procedure can also be used on radial tires.
Once you have gotten your 60-foot times to improve, you might be noticing that the car wanders on the big end of the track. Reducing air pressure in your slicks has improved traction, but oftentimes, the trade-off is reduced stability at speed (this is not usually an issue with a radial tire). The low pressure not only allows the tire to react at the starting line, but at speed, the tire can flex, and make the car seem to “wiggle” at speed. Those, 8-10 pounds of tire pressure in a slick is great at the starting line, but those same low pressures are holding up the back end of the car while you are speeding down the quarter mile. Use your head, it’s better to give up a couple of hundredths of a second at your 60-foot time than it is to have a catastrophe on the big end.
The folks at Mickey Thompson strongly suggest investing in a good tire pressure gauge, and that you keep accurate and complete records of each pass. Recommending air pressure is not easy; there are so many variables that affect it: i.e. weight distribution, transmission, chassis set-up, wheel size, etc. To help you, we have broken down the types of vehicles into the following categories with a recommended starting point for the correct air pressure. These pressures are for off-highway use only, and based on optimum chassis and drivetrain setups. Keep in mind that these are purely starting points.
Vehicle Weight Tire Size Air Pressure
Under 2,500 pounds under 32-inch diameter 7 p.s.i. and up
Under 2,500 pounds over 32-inch diameter 5 p.s.i. and up
2,500 to 3,000 pounds under 30-inch diameter 10 p.s.i. and up
2,500 to 3,000 pounds 30-33-inch diameter 8 p.s.i. and up
2,500 to 3,000 pounds over 33-inch diameter 5-1/2 p.s.i and up
Over 3,000 pounds under 30-inch diameter 12 p.s.i. and up
Over 3,000 pounds 30-33-inch diameter 8 p.s.i. and up
Over 3,000 pounds over 33-inch diameter 6 p.s.i. and up
2,500 to 3,000 pounds under 30-inch diameter 16 p.s.i. and up
2,500 to 3,000 pounds over 30-inch diameter 8 p.s.i. and up
Over 3,000 pounds Under 30-inch diameter 16 p.s.i and up
Over 3,000 pounds Over 30-inch diameter 12 p.s.i. and up
*When working with Drag Radials, use 1 pound adjustments to see change.
*A radial will perform better the more air you can run.
The lower the tire pressure that you can use, the more a tire will conform to the surface of the concrete/pavement. This conformity causes friction, and friction is traction. More traction equals quicker 60-foot times. After you have set the initial pressure by using the above outline, slowly keep reducing or increasing air pressure until there’s just a hint of tire spin. It will take a few passes to figure out the right pressure, but your 60-foot times will tell you when you have it right.