Eight Things to Know About Dirt Tires

Which part of your race car talks to you more than those four black doughnuts? They may not look like much, but they have a lot to say about how your car is handling. Teams that know how to take advantage of the feedback the tires provide have a distinct edge over those teams that do not.

Do you start a race with lower pressure in your tires, in hopes of building heat after a few laps? If so, will you experience excessive wear in the process, and risk a failure? Understanding the tires’ role and how to manipulate it to your advantage is critical to the setup and overall performance of your race car. Circle Track spoke with Shanon Rush and Craig Cowan of Hoosier Tire, and Drew Banas of Race Tires America to find out their top tips on what every dirt track racer must know about dirt tires.

AIR PRESSURE

This tip may seem obvious, but a lack of attention to air pres-sure can significantly affect your race car’s performance. According to Cowan, air pressure significantly determines dirt tire characteristics.

“Since they are bias-constructed, air pressure determines the majority of the spring rate within a tire. Therefore, it is imperative that tires are treated just as any other part of the car setup,” he explains. “Too much air pressure generally makes the car loose off the corner, while too little air pressure gives the car a tight feeling on entry. Unlike asphalt tires, it is not as crucial to balance tread wear across the face of the tire with air pressure. Concentrate on using air pressure to affect the behavior of the car, not the tread wear.”

Compound Selection The task of compound selection is complicated when you consider track conditions and driver preference for a particular compound. “This is by far the hardest part of tire management to master. Selecting the proper compound depends so much on reading the racetrack and knowing what your driver likes; it often takes years to find the proper combination,” says Cowan.

According to Cowan, it’s smart to begin with the basics: one soft, one medium, and one hard tire.

When the track is greasy, wet, or cold, run the soft tire. Switch to the medium tire when the moisture disappears and gives way to dry, slick conditions. When the dust is gone, so is the medium tire, so bolt on the hard one. Once you have a handle on the three types, check out the in-between compounds for certain clay types or wear patterns. Most importantly, take good notes on your choices to make the selection process easier the next time.

“When in doubt, don’t be afraid to ask the local tire dealer or track champion for their recommendations,” he adds.

STAGGER

This can be built into a bias-ply tire by adding air pressure and increasing the circumference, which aids in turning the race car. Or, buy a tire with a certain amount of stagger already built in. Hoosier Tire features a chalk-mark system that enables racers to choose, from a pile of tires the stagger they need without mounting three or four sets.

“The key is to remember that the chalk marks are not what the tire will actually measure,” advises Cowan. “They are guidelines to assist in picking the proper tires. As a general rule, right-side tires will rollout 2-2¼ inches smaller than the mark, while left-side tires will go 3-3¼ inches below the chalk size. Air pressure is the main factor that determines how much size differential you see from the chalk-mark size.”

KEEP YOUR EDGES FRESH

This simple tip will have a significant impact on the amount of bite your race car will get.

“A sharp edge is always more effective, no matter the track condition,” Cowan says. “Most of a tire’s traction capabilities come from leading edges of blocks (tread) grabbing the track surface and pushing the car forward. If an edge is sharp, it bites harder. If not, the edge slips and limits forward traction.” He adds that to get fresh edges, grinding tires is a smart alternative, because it reduces loose rubber and dull edges that break contact with the track. Another advantage is that it freshens the top layer of rubber on the tire, so it fires quicker on restarts.

TIRE CONSTRUCTION

Tire construction is an area in which you have to figure out what tire characteristics will benefit you the most, depending on your race car’s setup.

“Our rib tire (only circumferential grooves are molded in) is constructed softer than our LSB (Large Stagger Blocks molded in), so you always know which tire you have,” explains Cowan. “We have found over the years that softer tires make the car tighter through the middle of the corner while stiffer tires loosen the car at the same point.”

TIRE STORAGE

Tires, as tough as they may look, are quite sensitive to weather changes. Keep your tires out of direct sunlight and excessive moisture, as these may cause cracks in the sidewalls and ruin the tread compound.

GROOVING AND SIPING

Although choosing the right tire compound is important, the ability to manipulate the tread pattern to suit your needs for a particular track or set of conditions is just as crucial. When done correctly, grooving can help you get better traction and allow your tires to dissipate heat, which allows you more control over tire temperatures. Cowan and dirt Late Model driver Scott Bloomquist compiled the following.

• Grooving Soft Tires: While not common in longer races, grooving soft tires may be effective for qualifying or short heats on a wet track. Softer tires are generally used on tracks that have a lot of moisture but not a lot of abrasion. Loose dirt can be cleaned off by cutting more grooves, which improves traction because of the increased number of edges available to dig into the track’s surface. To prevent weakening the structure, only groove about halfway across the blocks of a soft tire.

• Hard Tires: If you plan to run the high line or cushion and need to move some dirt, grooving the tread shoulders is helpful. Grooves on the shoulders help clean off some of the loose dirt to get at moisture beneath it. Siping the shoulders is recommended if you are rolling the tire under when running lower tire pressures on a slick surface. The sipes can help prevent the shoulder area of the tire from glazing over and losing traction.

• Groove Widths: On some tracks, you can groove the tires twice as much with a narrow groove, or half as much with a wide groove, and accomplish the same thing. It depends on the track surface and how abrasive it is, or if it contains rocks that will tear up the tire. You don’t want a lot of grooves on a rocky or highly abrasive track. Wider grooves stand up to these harsher conditions better.

• Groove Shapes: There are three basic shapes used in grooving: square, V, and sipes. Square grooves are the same width through its entire depth. V grooves start out wide at the top and taper to nothing at their bottom. Sipes are thin slits cut by installing the blade upside down in the holder and using the separate ends of the blade to cut slices in the tire.

• Groove Angles: The angle at which grooves are cut determines how much of the edges are exposed to the track when the car is in various degrees of slide. The more the car will be sideways, the more angle should be put into the grooves. As a dirt car travels around the track on a given line, the body is actually pointed toward the infield in varying degrees. If the driver is consistent, you can determine an angle for the tire grooves that will work best for your combination.

• Grooves and Heat: Some tracks naturally put a lot of heat into a tire and can actually cause the tire to melt or blister. On these kinds of tracks, grooving helps cool the tire. The grooves (and sipes) help move air across the tire’s surface, which keeps the tread temperatures down. Grooving also produces more surface area, which helps transfer heat out of the tire to the air. To help a tire dissipate heat without weakening the tread blocks too much, cut grooves in the center of each block instead of cutting all the way across it.

CHEMICALLY ALTERED TIRES

Drew Banas of Race Tires America strongly discourages this practice as a method for reducing lap times.

Here is the warning issued by Race Tires America: “(1) chemical alteration of the tread carcass and or tread compound, such as tire “soaking”; (2) use of tread softener; or (3) the physical defacement (removing, altering, or covering) of tire-sidewall markings in any manner. Failure to comply with this warning could result in premature or catastrophic tire failure that may result in serious injury or death.”

Tire manufacturers are obviously sensitive to the liability of this practice, but we know of the existence of certain products that claim to help you reduce a few tenths off your lap times by softening the compound of your tire. Some tracks and sanctions have outlawed “soaking,” while others still allow it or turn a blind eye. While the practice of soaking tires is common, you should evaluate the level of risk you’re willing to endure to possibly pick up a few tenths versus the chance of an untimely tire failure.