Dr. Diandra: The dirt on Bristol’s dirt tires

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NASCAR teams must make minimal modifications to get their Next Gen cars ready to race on Bristol Motor Speedway’s dirt track. The primary changes are a stronger underbody, a diffuser modification, and mud flaps. Goodyear, on the other hand, had to produce a tire that looks and performs like no other tire you’ll see this season.

Racers call it ‘grip.’ Scientists call it friction. Whatever you call it, that force between tires and track is a prerequisite for a car to move, much less race.

A tire creates friction via its contact patch — the technical term for a tire’s footprint. Imagine lifting one corner of your car, putting a piece of paper on the ground, and then lowering the tire onto the paper. The imprint the tire leaves is its contact patch. That’s where friction happens. One reason racing slicks are wider than street-car tires is to create a bigger contact patch.

In addition to grip, friction creates heat. When you sand a piece of wood, both the sandpaper and the wood heat up. The same thing happens to a tire on a track. Tires must dissipate heat to prevent melting or blistering. That’s why the tread on a racing slick is only about 3/16 of an inch thick.

Like sandpaper wears wood, a track’s abrasiveness also wears the tire. Tires present an inherent trade-off: Soft tires grip, but wear quickly. Hard tires don’t wear as fast, but they also don’t grip as well.

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You can tell if a tire is designed for racing on dry pavement, wet pavement, or dirt just by looking at it.

A photo of a dirt tire (left), a slick (center) and a wet tire(right)
Photo by Mike Siberini/Goodyear

The racing slicks used at most events — the center tire in the photo above — are smooth. The lack of a tread pattern creates a big contact patch and maximizes heat dissipation.

Wet-weather tires (right) and dirt tires (left) sport tread patterns. The patterns are different because they serve different purposes.

Water or dirt between tire and track significantly decreases friction.

A wet-weather-tire’s grooves push water away from the tire. In the photo above, note how the channels all lead away from the tire’s center line. Pushing water away from the contact patch increases friction and thus speed. But wet-weather tires do limit a car: It can’t move so fast that the tire doesn’t have time to get water out of the way. If the tire is in contact with a thin layer of water rather than the track, the lack of friction causes the tire to slide instead of roll.

Before we go into how those big, chunky treads create grip, let’s look at how a normal racing slick works.

How tires produce friction

Friction is — to be blunt — weird. It’s such a complicated phenomenon that scientists still don’t completely understand it. But we understand the basics. Tires on asphalt (or concrete) produce friction mostly via the tire deforming around the surface, as I show below.

But there’s a second type of friction at work. Rubber molecules in the tire form atomic-level bonds with rubber on the track. While ‘interatomic bonds’ sounds fancy, it’s basically like you stepping on a wad of gum during a walk. The gum bonds to the bottom of your shoe, and then either your shoe pulls away from the gum/sidewalk, or the gum pulls away from the sidewalk and sticks to your shoe.

This adhesive type of friction creates the gobs of rubber that build up on a tire after a run. Drivers on old tires swerve before a restart partly to scrape off the rubber gunk and expose a clean surface that will provide better friction.

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A crew member burns debris off a tire during during the Ruoff Mortgage 500 at Phoenix Raceway on March 13, 2022 in Avondale, Arizona. (Photo by Sean Gardner/Getty Images)

Dirt tires do friction differently

Rubber creates pretty strong friction with asphalt and concrete. Dirt, however, is different story.

Dirt contains five components: minerals, living organisms, soil organic matter, gas and water. The particles in dirt come in sizes from smaller than a grain of sand to a couple of millimeters.

All dirt is not created equal. Loam — soil with particles of all sizes — is best for growing things. But if you want to race, you want clay. Clay soil is made up of small particles, densely packed with little or no air between them. The organic matter glues the soil particles tightly together. That’s why clay retains water better than other types of dirt.

Different types of clay produce different racing surfaces. Even though Bristol Motor Speedway is using the same dirt as last year, different preparation methods can lead to different surfaces.

Justin Haley ran an open modified car at the Bristol Dirt Nationals the week before the Richmond race. He noted approvingly that the track surface was “like glass” this year.

“They’ve got the preparation down to a science,” he said.

Smooth, slippery glass may not seem like an optimal racing surface, but it means that the surface is well packed and groomed. That, hopefully, translates to fewer problems with blinding clouds of dust on restarts.

And more friction.

The friction between rubber and dirt, even perfectly prepared dirt, is less than the friction between rubber and asphalt. The Bristol dirt tire’s tread compound is just about as hard as the compound in a Bristol asphalt tire, but the dirt-tire compound works better in damp conditions.

The key to dirt friction, however, is those big square blocks of rubber.

A dirt tire’s tread pattern does help push loose dirt away, like wet-weather tires do with water. But the sharp edges of a dirt tire’s tread bite into the surface, much like the edge of a snow ski or a soccer shoe’s cleats.

All of these friction mechanisms allowed cars to make about 20-second laps on dirt at Bristol last year. The 2021 lap times at pavement Bristol were around 17 seconds, but the banking of the pavement track (24-28 degrees) was higher than that of the dirt track (18-19 degrees last year).

Dirt-track worries

Multiple mechanisms for producing friction mean multiple types of tire wear. There’s the usual wear from friction with the track surface. Under some conditions, dirt can blister tires the same way pavement can. But the edges of the tread blocks also wear, which gives them less bite. A sharp hoe cuts into dirt much better than a dull hoe. It’s also possible to tear or break off parts of the blocks.

Crew chiefs and drivers also must keep up with the track, which is a different challenge with dirt. Dirt tracks can take rubber, making the tread compound even more important. Heat from the cars can dry out the dirt and create dust. The opposite problem can also occur: Cars compact the dirt, making it so hard and slick that the tread blocks can’t cut into it.

Although this is only the second Cup Series dirt race for crew chiefs and drivers, Goodyear has a long history of making dirt tires. Greg Stucker, Goodyear’s director of race tire sales, didn’t know off the top of his head exactly how long they’ve been dirt racing.

“But I’ve been working for Goodyear for 40 years,” he said, “and they were making dirt tires when I started.”