Big horsepower, a perfectly running transmission, a dialed-in rearend, and the correct moment center mean little if you neglect four simple things that every race car has regardless of the division. Obviously, we are talking about tires. They are the last thing between all of your hard work and the racetrack.
While asphalt racers are often left with little choice other than air pressure and stagger, venturing onto dirt is a whole new game. Not only does the racing surface generally change a lot quicker on a dirt track but dirt racers have the very difficult task of maximizing grip while the car is traveling down the track both forward and sideways. Most dirt cars enter the corner traveling across the racing surface in a direction that can best be described as a yaw position, posing some serious issues for the selection of tires. Now if the racer is in a class where the tires are regulated or specified, this places them in an even more difficult position.
The majority of dirt racers are bound by one of three different types of tire rules. There are spec tire divisions, where there is just one tire choice and modifications of that tire in any way, shape or form are not allowed. Then there are semi-restrictive rules such as the IMCA’s Modified Division which mandates that all competitors run 60-15 Hoosier Race Tires with the IMCA logo stamped on sidewall. In that class you can’t groove the tires but you are allowed to sipe and grind. Then there are more diverse rules such as those of the WISSOTA Late Models that mandate a sanction stamped Hoosier WRS2-55 but gives you leeway to run a WRS-55 compound on the right rear. In this division you can groove, sipe and grind. Some sanctions and tracks still allow an open tire rule but they are becoming few and far between. In almost every case, doping (aka chemically treating or altering the tire) is strictly prohibited. So we won’t even address that here. We’ll give you information you can actually use.
Prepping a tire for dirt racing is an art form unto itself, and the rewards for learning and mastering this art form is added grip, traction, and better tire wear which yields more speed.
There are, as we just mentioned, three basic ways to alter a tire for the benefit of enhanced performance; grooving, siping, and grinding. But understanding the when, how, and why of tire prep starts long before you plug in that groover.
It starts with the dirt you race on. In our research for this article, we ran across a book called Selection & Application of Hoosier Late Model Dirt Racing Tires by C.P. Furney Jr. from 2001. It’s chock-full of great information on tires for dirt racing but one thing caught our eye. In the book, Furney gives an in depth analysis of the various soil compositions that lead to different types of track conditions and we thought that was pretty critical for you to know. Obviously, these differing track conditions influence how you prepare your tires. He defines six different types of track conditions that the dirt track racer could face in an evening of racing.
Furney defines a slushy track as one where muddy water is thrown up onto the underside of the car, the hood, and the driver’s helmet. As the tires dig into the track surface they encounter a certain shear strength of that slushy mud. This shear strength is really the only thing that is giving your tire any sort of traction. Therefore, to allow the tire to work to any degree in these conditions you will need a lot of grooves. More than half of the tire (up to 60 percent) should be grooved according to Furney.