The fundamentals of racing tire management can be broken down into three areas: pre-event preparation, at the race track and on the track. Whether the event is an autocross, a time trial/track day, a road race or an oval race, the same important criteria apply. This month’s “Tech Matters” will serve as a primer on the care and feeding of a product with one of the shortest life spans known to man: race tires.
Preparation is always key to success, and tires do require thoughtful preparation. Take tire selection, for example. What is the optimum tire for your car, the track and the nature of the event? In many cases, you must run a spec tire, but if you can select your tires, the type, rubber compound and manufacturer are all considerations. Keep these items in mind when choosing tires: brand, size, tread design, tread compound.
When selecting tires, given the option to choose, keep in mind the weather forecast, the time of day for your race, likely track temperatures and whether you have company support at the track.
For everyone, selecting tires that are close to identical in roll-out (tire circumference) is important. The overall circumference of all four tires should be within about a quarter inch on a tire with a 75-inch roll-out. This is even more important on the drive axle. If a tire is out of spec, use it on the non-drive axle where possible. That tire can be used as a tool for balancing corner weight during setup. The larger tire in effect raises the ride height at that tire, which will increase the corner weight on that corner and the diagonally opposite corner, while decreasing the corner weights on the other two corners. It may not be much, but every little detail makes a difference.
Mounting and Balancing
With modern mounting and balancing equipment, this is not a big issue, but it is best if the wheel/tire can be balanced dynamically, meaning that the tire balance is dialed-in longitudinally as well as laterally. Make sure wheel weights are secure. Using helicopter tape over the weight can provide additional security.
Starting Tire Pressures
Make your best tire pressure estimate for what you will use on the track. Past experience and consulting with the tire manufacturer representative can get you in the ball park. This also is why keeping records is important. Always set up the suspension and cross weight using your race tires at the tire pressures you expect on the race track.
There are several gases that are used to inflate tires. The overriding criteria is pressure stabilization with heat buildup. It almost goes without saying, but never use a flammable gas for tires.
By far the most common gas for tire pressure is compressed air. It’s cheap, easy and quick. But all compressors have some amount of water in the tank. Combined with water vapor in the atmosphere, compressed air contains considerable moisture. The problem with compressed air is that, as the tire heats, the water vapor begins to expand, increasing the pressure inside the tire.
It is fine to use compressed air as a source for airing tires, but keep careful records concerning tire pressure increase from cold (in the pits) to hot (coming in from the track). You want to race or run at a specific tire pressure, so the hot pressure is critical. You want the tire contact patches to be perfectly loaded across the tread width while on the track, not in the pit area. The correct cold pressure to achieve optimum hot pressures on the track can be a moving target. I’ve seen as much as a 8 psi pressure increase from cold to on-track hot pressures.
Nitrogen is a common industrial gas that is stored in high-pressure gas cylinders, and it’s the most common element in the atmosphere. The main reason for using it to inflate tires is that it contains no water vapor, so nothing can boil or expand when heat is applied (at least heat from tires on a race track). This makes nitrogen an ideal medium for race tire inflation. It is also great to run air tools as long as a pressure regulator is used.
CO2 is rarely used in racing applications, but it is also available in high pressure tanks with pressure regulators. The tanks are small and lightweight, so they are more portable than the nitrogen cylinders. Like nitrogen, no water is present in the tanks, so water vapor will not be an issue. But like nitrogen, if the tires were mounted using compressed air, there may be residual water vapor in the tire, so some adjusting may be necessary.
If you do not keep records of settings, how they work, changes and conditions, the above will be difficult. Keep records!
Getting to the Track
Take care of your tires while transporting the car and gear to the track. Preferably, tow with old tires on the car, saving the race tires for qualifying or the race.
At The Track
Always take tire temperatures. Monitoring individual and average tire temperatures can help you make adjustments that will maximize performance. This helps you manage every square inch of the tire contact patch. See the March 2013 Speed News for a full feature on average tire temperatures and taking tire temperatures.
Adjust the suspension so that tire temperatures indicate that each contact patch is being used fully, and that all four tire contact patches are working for maximum performance.
Maintaining optimum tire pressures is important. First determine through tire temperatures what the optimum pressure is for each tire. They may not be the same. Then record the pressures. Let the tires cool. Check the tire and ambient temperatures as well as the new pressure for each tire. This will be the starting point for the next track session although some extrapolation may be required if the cold and ambient tire temperatures vary significantly from the previous cold readings.
Keeping Tires Clean
Tires get dirty in pit areas. Be sure to clean tire treads before doing any setup or taking any measurements. Also clean the tires before going onto the track. Of course they will pick up dirt and debris going to the grid and onto the track, but try to minimize the problem.