Dirt Physics

This weekend is the first time NASCAR Cup cars have raced on dirt since September 1970. The same physics applies everywhere. So why will the racing be so different?

Same Physics, Different Sandbox

Yes, we’re still turning left around the 1/2-mile oval track at Bristol Motor Speedway. But we’re doing it on dirt instead of concrete. And the physics of dirt is very different than the physics of concrete or asphalt.

Dirt vs. Asphalt and Concrete

A graphic showing how sand, silt and clay combine to form different kinds of soil

Dirt is made up of five things: minerals, living organisms, soil organic matter, gas, and water. The most plentiful elements in soil are oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, and potassium.

Particles between 2 to 0.05 millimeters (mm) in diameter are called sand; those of 0.05 to 0.002 mm diameter are silt; and the ones smaller than <0.002 mm particles are clay. These particles are held together by organic ‘glues’ such as glomalin, which is a protein produced by fungi.

Contrast that with asphalt and concrete. They’re made of small rocks (aggregate) glued together by bitumen or tar (asphalt) or cement (concrete). There’s an obvious difference in strength. There are no La Brea Glomalin Pits.

Racecars don’t change asphalt or concrete surfaces much. Sure, a green track will rubber in, but the rubber build-up eventually peels or washes away. Nature eventually takes a toll, with freeze/thaw cycles and general weather. But asphalt and concrete racetracks can easily go 10+ years without needing resurfacing. Atlanta Motor Speedway was last paved in 1997.

Dirt tracks have no such permanence. The type of soil used, how it’s packed down and groomed, and how much water it can hold determine the track’s initial characteristics. Once the racecars are out, they’ll pack dirt down and dig it up as they make laps, skid, and — more likely than not — crash.

The Dirt on Friction

Grip mechanisms are different on the two surfaces. Grip on asphalt is due to the tires grabbing the ruts and exposed aggregate of the track surface, along with the stickiness between tread rubber and the track. Slicks are better for asphalt racing because you want as much tread in contact with the track as possible. That’s for grip, and for heat dissipation.

Dirt tires create grip much differently. For starters, they’ve got tread patterns. But they’re different from the tread on rain tires in form and function. Rain-tire treads provide a path for water to escape, so the tire can make direct contact with the track. You can see in the middle picture below how the grooves all point away from the tire center.

A figure comparing dirt tires, rain tires and normal racing slicks

Dirt tires also have grooves to help move loose dirt out of the way, but dirt grip comes from two mechanisms. The tread interacts with dirt the same way tread interacts with asphalt or concrete. Greg Stucker, Goodyear’s Director of Racing, told me that the tread on the Bristol dirt tire is about the same hardness as a normal Bristol tire. However, the tread material on a dirt tire is optimized to provide better grip in a wet environment.

Even given the best tread material possible, however, the coefficient of friction between rubber and slicked-up dirt is much smaller than on asphalt or concrete. A second mechanism is needed to add grip: The front edge of the tread blocks cut into the dirt’s surface.

These aren’t just regular Bristol tires with a tread pattern.


Dirt tires wear differently than tires for pavement. The main wear mechanism is rounding off the front edges of the tread blocks. That makes it harder for them to cut into the dirt, which decreases grip. Think about how much easier it is to use a garden tool that’s sharp than one that’s dull.

Chunking-type wear on a bicycle tire. Dirt tires for cars can exhibit the same wear patterns.

There’s also the possibility of ‘chunking’. If the forces are large enough, they can break some of the tread blows off the tire. The picture at left is a bike tire, but you get the idea.

Finally, dirt can actually blister tires the same way pavement can. Teams get five sets of tires for the race, but can use their practice and qualifying sets. They’ve actually got nine sets of tires to work with for the weekend.

Keeping Up With Dirt

While asphalt/concrete tracks change with temperature and amount of rubber (and PJ1), dirt tracks change much more profoundly over the course of a race.

Or even from race to race, depending on who’s grooming the track and what their goals are. We had a favorite Indian restaurant in Lincoln where the definition of ‘hot’ depended on who was cooking. Similarly, different people can prepare a track different ways.

The track of lap 1 will likely not even recognize the track of lap 250. Heat generated by cars can dry the dirt out and produce copious amounts of dust. It’s hard to get good grip on dust. The other extreme, Greg Stucker told me, is that cars can compact the dirt into a hard, slick surface that makes it difficult for the tread blocks to bite into it.

I asked Greg if dirt tracks can ‘take rubber’. He told me that, under some conditions, they can. When that happens, the surface of the tread blocks becomes even more important in terms of grip. I tried to get him to estimate how much of the grip came from the bite of the tread blocks vs. the traditional tire/surface friction. He said there were too many variables that changed too much over the course of a race to even try to estimate that figure.

You can imagine how much of a challenge all of this is going to be — not just for the drivers, but for the crew chiefs, too.

Goodyear, however, has decades of experience designing dirt racing tires. Greg Stucker didn’t know the exact date. But he did note that he’s been working for Goodyear for forty years and they were doing dirt tires when he started there.

Radial vs. Bias-Ply Tires

Dirt-track tires are bias-ply rather than the radials that have become the NASCAR standard. I updated the post about the differences between radial and bias-ply tires recently if you’d like additional information. But here’s a new diagram and short summary of the differences.

A diagram showing the differences between radial and bias-ply tires

A ply is a piece of rubber reinforced with some type of fiber (like Kevlar, say) for additional strength. Plies in a radial tire go from bead-to-bead. A steel belt package over the plies gives the tread surface additional strength and resistance to damage. In contrast, the plies in bias-ply tires run at angles to each other, which creates a stronger body.

The two constructions absorb loads differently. In a radial, the belts are stressed by contact with the track, while the lower sidewalls are stressed by contact with the bead. That causes a lot of flexing in the upper sidewall, which has to mediate between those two stresses.

The bias-ply tire is more uniform, which allows it to distribute loads throughout the tire. Because they lack belts, bias-ply tires conform better to ruts and bumps in the track. That’s why the recommended right-side tire pressures for the dirt race are half those for the concrete racetrack, even though the loads won’t be significantly different. You need as much of the tread making contact with the track surface as possible.

This Car Wasn’t Designed for Dirt

The Cup car is much heavier than the cars that normally race on dirt tracks. NASCAR switched from bias-ply to radials starting in 1989. The last race on bias-ply tires was at Bristol in 1992 — which was also the last race on asphalt at Bristol. Greg Stucker reminded me that, as NASCAR transitioned from bias-ply to radial tires, the car’s design was optimized for radial tires. So the drivers and crews are facing the additional challenge of racing the car on a surface and a tire it wasn’t designed for.

You Drive Differently on Dirt

In dirt racing, you steer more with your gas pedal than your steering wheel. We can break any car’s motion into the motion of the car’s center of gravity plus the motion about the CG. Unless something has gone very wrong, the motion about the CG is a rotation, as if you stuck a giant pin through the car’s CG into the track.

We call the rotation in one direction tight or understeer because the car doesn’t rotate enough. The other direction is loose or oversteer (the car rotates too much).

On a pavement track with a well-set-up car, the front wheels turn the car and the rear wheels power the car. But on a dirt track, you use the gas pedal to rotate the car, which may actually mean steering in a different direction than you intend to go. If you grew up in ice and snow, you know how this works because you were constantly told about steering in the direction the rear of the car is moving if you want to stop a skid.

Turn-Aid (aka Stagger)

A red Solo cup rolling on its side moves in an arc. That’s because the base has a smaller circumference than the lip. Cars that only turn in one direction use the same principle to predispose the car to turn. It’s called stagger and it’s achieved by making the right-side tires larger than the left-side tires.

NASCAR usually runs pretty minimal stagger. For last year’s Bristol race (on pavement), the right side tire had a diameter of 28.2″ and the left side had a diameter of 27.9″. The difference for the dirt tires they’ll run this weekend is almost a full inch: 28.2″ on the right vs. 27.3″ on the left.

Goodyear usually specifies tire circumference because it’s an easier and more repeatable measurement. I explained this in terms of diameter because I think it’s easier to visualize how different the left- and right-side tires would look if you stood one next to the other.


Physics is physics the world around, but a lot of things we know from pavement racing simply don’t apply when the racetrack surface is dirt.

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