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Five, six, or even seven hundred horsepower is worthless unless you can put that power to the track. We get that power to the track through a complex and integrated system of components; transmission, driveshaft, rearend gears, shocks, springs, the list is virtually endless, but one of the most important facets of that whole equation lies within those four pieces of rubber that are mounted to your wheels...the tires.
While asphalt racers are often left with little choice other than air pressure and stagger, venturing onto dirt is a whole new game. 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.
This is a heat race at Central Arizona Raceway in Casa Grande, Arizona. Check out the chunks of sticky mud flying everywhere. The track is characterized as "heavy" at this stage of the night, requiring a lot of grooves in the tires to gain any traction. Photo by Vyn Polmanteer
The majority of dirt racers are bound by one of three different types of tire rules. There are restrictive rules such as the IMCA's Modified Division which mandates that all competitors run unaltered G60-15 Hoosier Race Tires with the IMCA logo stamped on sidewall. In classes like those there is no grooving, siping, or grinding, nada. This is a pure spec tire rule. Then there are more diverse rules such as those of the National Late Model Series which mandates one manufacturer (Hoosier) but allows three different models, the Spec. D21, Spec. D55 and D70, plus the aforementioned alterations are allowed. Then there is the open tire rule such as the World of Outlaws Late Model Series where any brand is permitted as long as the tire meets WoO's maximum width and circumference specifications.
It's these last two worlds where we are going to delve into in this article. 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.
Dirt
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.
This is the same track, Central Arizona Raceway, but this photo was taken during the feature. Note how different the track is. While still wet, the chunks of flying mud are gone and the track surface is on its way to being black slick, requiring less grooves and some sipes. Photo by Vyn Polmanteer
Slushy
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.
Heavy
One step down from the slushy track is the heavy track, where the tires send chunks of soil flying through the air. These chunks of mud, up to 3/4-inch in diameter have the consistency of soft modeling clay. While most of the traction still comes from the shearing of the soil (a.k.a. tire digging into the surface), some begin to come from the surface of the tire itself. To gain maximum performance you'll want anywhere from 40 to 50 percent of your tires grooved.
Tacky
A tacky track is characterized by stringy shaped chunks of soil whose consistency resembles stiff or hard modeling clay. Traction on tacky tracks comes approximately 50 percent from shearing of the soil and 50 percent from the tires. Therefore, Furney says you'll want approximately 30 to 40 percent grooving.