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


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.

Ruoff Mortgage 500
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.”

Corey LaJoie learning in his week with Chase Elliott’s team


Spending this week with Hendrick Motorsports has proved eye-opening for Corey LaJoie.

He will pilot Chase Elliott’s No. 9 car today at World Wide Technology Raceway after NASCAR suspended Elliott one race for wrecking Denny Hamlin during last week’s Coca-Cola 600. This gives LaJoie the chance to drive in the best equipment of his career.

MORE: Corey LaJoie not giving up on his dream 

MORE: Details for Sunday’s Cup race

Working with Elliott’s team also has given LaJoie an inside look as to what makes Hendrick Motorsports so successful.

“I thought that I knew what we didn’t have at Spire Motorsports, but I had no idea,” said LaJoie, who starts 30th after tagging the wall during his qualifying lap. “There’s tools that those guys have, intellectual properties specific to Hendrick Motorsports, that even some of the other teams don’t have.

“But the biggest thing that I noticed was just the people and the attitude of the pursuit of perfection. All the key partner teams across all the (manufacturers) all have the same data, but (Hendrick Motorsports has) an unbelievable way of delegating, taking, compacting and making it just digestible – whether it’s for a driver, an engineer, a crew chief.

“I think the fact that they have four incredibly strong teams individually raises the tide for those guys because when you’re sitting in the simulator and William Byron ran a 33.20 (seconds for a lap) … if you’re running a 33.35 with the same setup, you know you have a tenth-and-a-half under your butt and you have to go find it. And then when I go run a 33.20, William next time is going to want to run a 33.19.

“There’s always a consistently raised watermark on the driver’s end. There’s always a consistently raised watermark on the crew chiefs in trying to build the best setups, and the engineers trying to find the best strategies.

“The inner-team competition is one of the biggest things, and I think there are several teams that have that … the healthy ones are certainly evident. But it’s just the overall structure. We have a Hawkeye (camera-based inspection stations used by NASCAR at the track) … all the things that do the same stuff that Hendrick Motorsports has, but the depth of people, collective focus of the goal and the mission is noticeable and evident. It’s a different world.”

It would be easy for LaJoie to be overwhelmed in this situation. His career has been marked with underfunded rides and trying to make the most of his equipment. He’s having his best season in Cup this year. LaJoie ranks 19th in points heading into today’s race.

LaJoie acknowledges the opportunity he has, but he also can’t let it alter his focus.

“It’s been a wild week,” he said. “I can get all sentimental … (about) my dad subbing in for Ricky Craven in 1998 (for Hendrick Motorsports) and all that sort of stuff. But at the end of the day, when I sit in that thing, I don’t know that NAPA is on it, or the No. 9 is on it.

“I’m going to drive it like I have been driving the No. 7 Chevy and putting that thing 19th in points. It’s been a super fun, successful year so far, and we have a lot of work left to do and things to accomplish over there.”

When he returns to his Spire Motorsports ride after today’s race, LaJoie admits this weekend’s experience with Elliott’s team will help him with his own team.

“How I prepare, how I’m going to engage with my team at Spire Motorsports going forward is going to change,” LaJoie said. “I think I’m going to be able to come in there and just apply and share some of the things I’ve learned over the course of the week with (crew chief Ryan) Sparks and the No. 77 team, as well, and I think we’re all going to be stronger for it.”

Dr. Diandra: Is 2023 the season for a Ricky Stenhouse Jr. redemption?


Coming into 2022, Ricky Stenhouse Jr. had two career Cup Series wins in 364 starts. But both wins — and his career-high 13th-place season finish — happened back in 2017.

Stenhouse was unceremoniously dropped by Roush Fenway Racing in 2020 and landed with JTG Daugherty Racing. He made the news every now and then at a superspeedway but could be counted upon to head up season-ending lists of drivers involved in the most accidents. In the years Stenhouse hasn’t been at the top of the list, he’s been near the top.

DNFs and accidents have plagued Stenhouse throughout his NASCAR career. Jack Roush went so far as to park the Mississippi native in his early days in the Xfinity Series because he tore up so much equipment.

Stenhouse redeemed himself, going on to win two Xfinity championships.

From the way his 2023 season has started, it looks as though Stenhouse might be on a similar mission of redemption this year in the Cup Series.

Finishing races

Stenhouse started the 2023 season in the best possible way – winning the Daytona 500. But drivers from less-funded teams who win early superspeedway races usually settle to the bottom of the rankings by now.

Stenhouse hasn’t. He ranks 13th heading into Sunday’s race at World Wide Technology Raceway.

Standings aren’t as good a ruler this year as they usually are because of drivers missing races and teams incurring penalties. But Stenhouse’s statistics back up his ranking.

Stenhouse has finished every race this year on track, as opposed to in the garage or on the hook. Only Ryan Blaney and Corey LaJoie have achieved the same distinction.

In 11 of those 14 races, Stenhouse finished on the lead lap. That’s the same number of lead-lap finishes as William Byron. Denny Hamlin and Martin Truex Jr. are tied for most races finished on the lead lap with 13 each.

This time last year, Stenhouse had already racked up seven of the series-leading 18 caution-causing incidents he would be involved in for the season. Runner-up Chase Elliott had 15 incidents.

Going into Gateway this year, Stenhouse has been involved in only two accidents (Talladega and Charlotte) and had a tire go out at Darlington.

Approaching his career best

I compare three years in Stenhouse’s career in the table below: the 2017 season — his best to date — along with last year and the 14 races run so far this year.

A table comparing loop data stats for Ricky Stenhouse Jr. showing his path to redemption

Stenhouse’s current average finishing position of 13.5 ties with Christopher Bell for sixth best in the Cup Series. That’s 9.3 positions better than Stenhouse’s 2022 average. He’s even beating his 2017 average by 3.6 positions.

Qualifying results are down a bit from 2017 — but remember that those numbers are from the days when NASCAR allowed multiple practice sessions. Stenhouse is only two positions worse relative to 2017, but 7.6 positions better than last year when it comes to establishing his spot on the starting grid.

Stenhouse’s average running position is comparable to 2017 and 2.8 positions better than 2022. He ranks 20th among full-time Cup Series drivers in average running position. Although it’s an improvement, it’s still more than double William Byron’s series-leading 9.1 average running position this year.

More interesting is the difference between Stenhouse’s average running position his average finishing position. Some drivers run better than they finish. Stenhouse is doing the opposite.

In 2017, Stenhouse finished about 1.4 positions better than he ran. This year, he’s gaining an average of about five positions from where he runs.

One might argue this gain results from the plethora of late-race incidents this year that have removed drivers in the front of the field from contention. But Stenhouse deserves credit for putting himself in a position to benefit from those events.

Stenhouse’s green-flag speed rank is 11th among full-time Cup Series drivers. His 15.3 average, however, is 1.7 positions worse than 10th-place Kyle Busch. Still, it’s impressive that JTG Daugherty is right there in the mix with much better-funded teams. William Byron again has the best average green-flag speed rank at 7.9.

Consistently strong finishes

It’s not uncommon for a mid-pack driver to win a superspeedway race. But Stenhouse’s Daytona 500 win appears to be something more. The table below summarizes his wins and finishes for the same three years.

A table comparing finishes for 2017, 2022 and 2023 showing Ricky Stenhouse Jr's redemption attemptsThe difference between last year and this year is striking.

In 2022, Stenhouse finished in the top 20 in 12 of 36 races. He’s already matched that mark this year. He earns top-20 finishes 85.7% of the time in 2023 compared to 33.3% last year. Top-20 finishes aren’t the same as contending for a championship. But they’re a first step.

Stenhouse finished 2017 with nine top-10 races. With about 60% of the season remaining, he’s already earned five top-10 finishes this year.

What’s changed? The Next Gen car is one factor, but it didn’t make much difference for Stenhouse last year. I would point instead to Stenhouse’s reunion with Mike Kelley as his crew chief.

Kelley co-piloted both of Stenhouse’s Xfinity championships in 2011 and ’12. Although Kelley worked with Stenhouse and previous crew chief Brian Pattie since 2020, this is the first year Kelley is back up on the pit box.

Together, they’re basically halfway to matching Stenhouse’s best year.

And another step closer to redemption.

Portland Xfinity race results, driver points

Portland Xfinity results
Photo by James Gilbert/Getty Images

Cole Custer went from fourth to first on the overtime restart when the top three cars made contact and went on to win Saturday’s Xfinity Series race at Portland International Raceway. Custer is the 10th different winner in 13 races this season.

MORE: Portland Xfinity race results

MORE: Driver points after Portland Xfinity race

JR Motorsports took the next three spots: Justin Allgaier placed second, Sam Mayer was third and Josh Berry was fourth. Austin Hill completed the top five.

John Hunter Nemechek remains the points leader after 13 races. He has a 14-point lead on Hill. Nemechek leads Allgaier by 44 points.

Cole Custer wins Xfinity race at Portland in overtime


Cole Custer held off Justin Allgaier at the finish to win Saturday’s Xfinity Series race in overtime at Portland International Raceway. It is Custer’s first victory of the season.

JR Motorsports placed second, third and fourth with Allgaier, Sam Mayer and Josh Berry. Austin Hill finished fifth.

MORE: Race results, driver points

Custer went from fourth to first on the overtime restart when Parker Kligerman, who restarted third, attempted to pass Allgaier, who was leading. Sheldon Creed was on the outside of Allgaier. All three cars made contact entering Turn 1, allowing Custer to slip by. Creed finished seventh. Kligerman placed 14th.

Custer won the second stage when John Hunter Nemechek made contact with Creed’s car while racing for the lead on the final lap of the stage. The contact spun Creed and Custer inched by Nemechek at the line.

Early in the final stage, Creed gained revenge with contact that spun Nemechek, who went on to finish 10th. A few laps later, Nemechek and Joe Gibbs Racing teammate Sammy Smith had issues. Smith spun Nemechek. After getting back around, Nemechek quickly caught Smith and turned into Smith’s car, damaging it.

STAGE 1 WINNER: Sheldon Creed

STAGE 2 WINNER: Cole Custer

WHO HAD A GOOD RACE: Despite the contact on the overtime restart, runner-up Justin Allgaier managed to score his fourth consecutive top-three finish. … Sam Mayer’s third-place finish is his best on a road course. … Austin Hill’s fifth-place finish gives him four consecutive top-five results.

WHO HAD A BAD RACE: Daniel Hemric finished 33rd after a fire in his car. … Riley Herbst placed 32nd after an engine issue. After opening the season with six top 10s in a row, Herbst has gone seven races in a row without a top 10.

NEXT: The series competes June 10 at Sonoma Raceway (8 p.m. ET on FS1).