A NASCAR Engineer Tells You Everything You Need To Know About Tire Temperature And Grip - The Autopian (2024)

Sunday of Memorial Day weekend is celebrated by race fans as the best day of the year. It’s motorsport Christmas, if you will. Three Crown Jewel races take place all in one day. The morning starts off with Formula 1’s Monaco Grand Prix. At lunchtime, you have Indycar’s running of the Indianapolis 500. Finally, as the late afternoon sun begins to set, NASCAR’s co*ke 600 gets underway.

Three very different races, but three excellent tests of man and machine.
The aforementioned co*ke 600 is run annually at the Charlotte Motor Speedway. With almost every NASCAR team being located in and around the Charlotte-metro area, this race weekend also serves as a home race for drivers and crew members. For those of us who spend so many weekends away from home, it’s a nice treat being able to bring your friends and family to the racetrack.

A Little History

Charlotte Motor Speedway was originally dreamed up by race promoter Bruton Smith in the mid-1950s. Inspired by the construction of Darlington Raceway in South Carolina and spurred on by the success he was having promoting stock car races throughout the southeast, building his own racetrack was the next logical step. Smith’s original business partner John Probst Jr backed out of the project after suffering a heart attack in 1958, choosing to focus on his health rather than another business venture. Smith then partnered with moonshiner, turned timber businessman, turned racecar driver Curtis Turner. Together, the two broke ground on July 29, 1959 and on June 19, 1960 the first rendition of the World 600, as it was then known, was run. It had originally been scheduled for May 29, the day of the Monaco Grand Prix and the day before the Indianapolis 500, but was delayed by persistent construction issues.

The racetrack struggled financially in its infancy, but stability was found in leadership from car dealership mogul and PR legend H.A “Humpy” Wheeler. Together, Smith and Wheeler rapidly modernized and expanded the facility, bringing in large crowds with their over-the-top promotions and pre-race festivities. By 1980 they had expanded the facility to a capacity of 150,000 from the original 30,000 and built 40 premium condominium units overlooking the speedway. In 1987 they added 20,000 more seats and introduced the Speedway Club restaurant.

In 1991, Smith partnered with Musco Lighting to install lights around the entire facility. At the time, it was the largest lighted racetrack suitable for nighttime operation. The 1992 All-Star Race was the first night race on an intermediate track and was an immediate success. There is an excellent documentary “One Hot Night: 1992 NASCAR All-Star Race” about that race and all that went into it.

Simply hosting NASCAR races was not enough. In May of 2000, a dirt track was added across the street from the NASCAR track which is now home to the World Finals at Charlotte event for sprint cars, late models and big block modifieds. Plans were then announced in 2007 to add an NHRA drag strip to the facility. In typical Charlotte Motor Speedway fashion, this too was over the top. Instead of the traditional two-lane drag strip, Charlotte’s was to be four-wide.

There has been a lot of talk lately about NASCAR’s new charter deal and the disagreement between teams and track owners on how TV revenue should be split up. Teams want more, but track owners don’t want to take less. The race teams have been very vocal about costs in their pursuit of a larger piece of the pie, and public sentiment has been on their side. Let’s take a break from this disagreement and look at things from the track owner’s perspective because it’s a crucial piece of the puzzle to understand both how Charlotte Motor Speedway became the way that it is and how we got to this impasse.

Repaving The Track Would Cost How Much?

Repaving a racetrack is a wildly expensive proposition. While it’s hard to track down specific numbers, we can get to an accurate ballpark with a little research. Road courses are on the low end of the cost spectrum, Watkins Glen International was repaved in 2015 for around $3m/mile. Paving a banked racetrack is a more difficult proposition and the price reflects it. Kansas Speedway was repaved in 2012 for roughly $6m/mile. Using $6m/mile from 2012 and adjusting for inflation we can safely estimate that it would cost around $12.5m total to repave Charlotte Motor Speedway today.

As stated earlier, Charlotte Motor Speedway was opened in 1960. The facility was then repaved in 1973, partially repaved in 1979 and 1987, fully repaved again in 1994, and by 2005 it was ready for a fresh surface again. Using the 2024 price and counting 1979/87 as half repaves, we can work out that from 1960 to 2005 the asphalt surface of the racetrack cost around $1,086,000 per year to maintain. Even using lower-end numbers from the Watkins Glen repave it would still have cost $544,000 per year to maintain.

Clearly, racetrack operators have an incentive to spread out the intervals between repaves. When the track surface needed to be replaced again ahead of the 2006 season a decision was made to utilize recent advancements in roadway paving technology to extend the life of the racing surface. In that aspect, it certainly worked as we are now in 2024 and the racetrack is in fantastic shape. Even if the track was repaved after this season, the cost per year has already been cut to between $345,000 and $690,000 using the low and high estimates.

While this was great for the speedway’s bottom line, it had an adverse effect on the racing product. So, what was different about this magical asphalt? Well, let’s break it down. Asphalt is a mixture of an aggregate (crushed rocks, gravel and sand) and a binder (a tar-like petroleum mixture). By volume, asphalt is typically 90-95% aggregate and 5-10% binder. When paved in the traditional way, cars traveling around the track would pull up the top level of binder bit by bit, slowly exposing more of the aggregate and making the surface more abrasive. While the abrasive surface caused more tire wear and led to better racing, the surface integrity of the asphalt would diminish and the racetrack would need constant repairs. Cracks would need to be patched constantly, and sections would need touch-ups before eventually, the entire surface would have to be replaced again.

You can see the numerous cracks that had been repaired on the surface of Atlanta Motor Speedway before it was repaved.

The track surface at the Atlanta 2021 July race was basically loose gravel being held together with a hope and a prayer I think they’ll be fine pic.twitter.com/0hNGf30qzK

— kyle (@atlutd_kyle) October 6, 2023

As asphalt technology evolved, new mixtures were developed to make road surfaces last longer. These new mixtures lowered the percentage of aggregate material and replaced it with rubber polymers that were mixed into the binder. The new asphalts also produced tons and tons of grip, but it simply never aged.

The Most Boring Race In NASCAR History

Because of this age-defying nature, Charlotte Motor Speedway became a single-lane racetrack after the repave with every driver wrapping their line right around the bottom, taking the shortest distance around the track. Even with multiple lanes of racetrack available, the bottom groove never lost any grip so it was pointless to travel a further distance running in the higher lanes.
As teams developed a better understanding of aerodynamics through the 2000s and 2010s the racing product at Charlotte Motor Speedway degraded and further approached unwatchable with every successive year. Perhaps the least exciting race in NASCAR history was the 2016 co*ke 600 where Martin Truex Jr led 392 of 400 laps on his way to victory 2.5 seconds clear of Kevin Harvick.

Prior to the 2017 co*ke 600, the track surface was treated with PJ1 traction compound on the middle and upper lanes in hopes of equalizing them with the bottom lane. Unfortunately, this led to drivers just running around the lowest part of the treated area in another single-file procession. Martin Truex Jr again led 233 of 400 laps before Austin Dillon stole the win by stretching his fuel mileage, leading only 2 laps but crossing the finish line first.

Subsequently, for the 2018 season, Charlotte Motor Speedway decided to take the fall race off of the oval and instead construct a road course known as the Roval in the infield. The Roval quickly became beloved by fans and the co*ke 600 was seen as a drag. In 2018 Kyle Busch led 377 of 400 laps and in 2021 Kyle Larson led 327 of 400 laps en route to their victories.

Thankfully, the Next Gen car has breathed new life into Charlotte Motor Speedway. The strength of this car on intermediate racetracks is unmatched in recent memory. Racing on intermediate tracks has been so good recently, and road courses so poor, that many fans are clamoring to abandon the fall Roval race and return to the traditional oval circuit. Despite being the longest race on the calendar, the last two renditions of the co*ke 600 have left everyone desperate for more.

The Xfinity Series cars were never as aero-sensitive as the Gen 6 Cup cars were. Running their spring Charlotte race in the heat of mid-afternoon has produced consistently fantastic action over the years with multiple lanes being viable options for racing. Watch below as Josh Berry and Justin Allgaier slug it out for the lead here in 2022.

If you’ve been following this series of articles, you’ll already be familiar with several aspects of intermediate racing, and all of these factors will be combined this weekend at Charlotte Motor Speedway. You’ll see them running on the fence like we talked about before Darlington Raceway. Aero racing will come into play like we talked about before Texas Motor Speedway. Drivers and teams will struggle with bumps in the racing surface like they did in Las Vegas. If you missed any of those, go back and check them out for a refresher. Instead of rehashing things we’ve already discussed, let’s take a couple of minutes to learn about something that separates Charlotte Motor Speedway from all the other intermediate tracks on the schedule.

What Makes Charlotte Unique

The first thing you’ll notice about Charlotte Motor Speedway is that its ageless asphalt surface is a lot darker than other intermediate tracks on the schedule. Below is Texas Motor Speedway whose surface is only seven years old.

AN INSTANT LEGEND!!! ????
In a week about “legendary moments” let’s look at last weekend, when Sam Mayer crossed the finish line .002 seconds ahead of Ryan Sieg, making it the 2nd closest Xfinity Series finish and the closest finish in Texas Motor Speedway history! #NASCARLegends pic.twitter.com/eejL5ZYnTo

— Texas Motor Speedway (@TXMotorSpeedway) April 18, 2024

Compare that to the eighteen-year-old surface at Charlotte Motor Speedway.

Is @J_Allgaier repeating this year? ????????

????️: https://t.co/l185pFISxN#BetMGM300 | @BetMGM pic.twitter.com/evRWEGwHVS

— Charlotte Motor Speedway (@CLTMotorSpdwy) May 16, 2024

As anyone who’s ever worn a black t-shirt in the summer knows, dark materials absorb a ton of heat. Because darker materials absorb more of the sun’s rays than lighter-colored ones, their temperature also increases at a faster rate. Charlotte Motor Speedway is the most temperature-sensitive track on the schedule for a multitude of reasons that we shall discuss below.

How Racing Tires Work

If you’ve followed motor racing for any amount of time, you’ll have heard mentioned numerous times that higher track temperatures mean reduced grip, and this is true for any type of racecar. There are two reasons for this phenomenon. First, as asphalt heats up, the oils in its petroleum binder will start to rise up to the surface of the material and reduce its coefficient of friction slightly.

The second part is a bit more complicated. A racing tire has a specific temperature range that it produces the most grip in. This is why you see teams in series that allow it, like Formula 1, for example, use heated blankets to pre-warm the tires so that they are closer to their optimum temperature range when they are installed on the car. Tires will absorb some heat from outside components, like the brakes, but a majority of the heat is created by surface friction as the tire accelerates the car laterally and longitudinally.

“Track temperatures can drastically affect tyres. When you’re driving on a track that’s nice and cool, the tyres cool down pretty much every time you race down the straight. But on the other hand, when the track is scorching hot, you are basically driving on top of a saucepan, heating up the tyres, so they don’t afford you as much grip.”
– George Russell, Mercedes AMG Petronas F1 driver

He raises an interesting point with this quote, but let’s look at it in a different way by thinking about tire heat being the product of work done by the tire. The temperature at which a tire behaves optimally is a fixed point, and the difference in temperature between its starting and ending point is a measure of the amount of work a tire can do. Tires will reach a lower, steady state temperature on the straightaways and then rapidly heat up as load is applied in a corner. If the track surface is keeping heat in the tire, then the tire can do less work before it reaches its optimal temperature. This is the primary reason why you see lap times quicken whenever there is a drop in temperature. You can see this roughly illustrated below where the thick vertical lines represent how much work a tire can do before being heated to its optimal point.

During multiple races in the 2013 season, Formula 1 installed thermal imaging cameras onto several cars for use during the broadcast. This clip of Sebastian Vettel making a lap around Suzuka provides an excellent visualization of how tires heat up and cool down during a lap.

It’s worth noting that in oval racing, the straightaways make up a much smaller percentage of the lap than at road courses like Suzuka. This makes any bit of temperature reduction that can be achieved while the car is traveling in a steady state significantly more important.

In addition to changing the overall grip level, track temperature will also change the balance of the racecar. One thing to note about NASCAR tires is that they are designated by side of the car, not front or rear. In open wheel racing, and some tin-top road racing series, the front and rear tires are different sizes and the compound can be tuned for the corner of the car that it will be installed on. Take a look at Kyle Larson’s Mclaren Indycar ride for this weekend.

It’s quite apparent that the front and rear tires are different sizes and could never be exchanged for one another. So, how do NASCAR tires differ, and how are they put into sets of four?

For the Xfinity Series this weekend, teams will be allocated 6 sets of tires. The teams will be given 12 left side tires and 12 right side tires that their tire specialist will sort into sets. Cup series teams will receive 14 sets of tires for their race, which is twice the distance of the Xfinity Series race. The two predominant factors in tire sorting are size and spring rate.

When looking at a batch of tires, the difference in circumference from smallest to largest tire can be about 3/16” (4.5mm), and the spring rate of a tire can vary by as much as 25lb/in. In oval racing, we talk a lot about stagger, which is simply the difference in circumference of the left side tires versus the right side. If you’ve ever knocked a solo cup over on a table you may have noticed that it rolled in a circular motion. This is because the open end up the cup is bigger than the base of the cup. That is the basic concept of stagger. During a normal race it is crucially important that the front and rear staggers are kept consistent from one set of tires to the next because increasing the stagger at one end of the car will make that end want to turn more, decreasing will do the opposite.

When tires are manufactured, Goodyear will inflate each tire to a specified pressure and apply a vertical load to it. The difference between the static and loaded radius is used to calculate the spring rate of the tire. Teams have differing theories about where they want the soft tires in each set but the most important thing is that front to front-to-rear and side-to-side spring rate split is the same from one set to the next.

The main point of all of this is that the tire compound is the same whether or not the tire ends up the front or the rear of the car. This is significant because while the tires are the same compound, they will operate in vastly different conditions depending on which axle they are installed. Typically, in oval track racing the tires are punished as follows in terms of most load to least: right front, right rear, left rear, and then left front.

The front tires on a stock car also operate at a higher temperature than the rear tires due to their proximity to the engine. With minimal airflow in the engine compartment, heat from the engine soaks into the front tires which are only a few inches away. Combining all of these factors you will end up with a right front tire that operates around 50* F hotter than the right rear tire. The left front, despite experiencing less load, will still run slightly hotter than the left rear due to engine proximity.

Now, let’s see how this can affect the car’s balance when temperatures begin to shift. In the graph below we have a bell curve-shaped graph that roughly demonstrates grip versus temperature. The 0 point at the center of the graph represents the optimal operating temperature for this generic tire compound. The initial state (Front 1 and Rear 1) starts with a balanced car where the front tire is 25 degrees above the optimal point and the rear tire is 25 degrees below it, both generating equal grip. If we shift the curve and cool both tires by 25 degrees you can see in case 2 (Front 2 and Rear 2) that the front tire is at the optimal temperature and the rear tire is 50 degrees below optimal.

By cooling the tires, we have added front grip and reduced rear grip which will shift the balance towards oversteer. The opposite would be true if the track temperature was increased.

As stated earlier, Charlotte Motor Speedway is easily the most temperature-sensitive racetrack on the schedule. Due to the darker track surface of Charlotte Motor Speedway as compared to other intermediate racetracks, the track surface temperature can vary wildly over the course of a day. While the 2023 race weekend was marred by weather, we can take a look back at the track temperature versus air temperature for the 2022 co*ke 600 which ran from 6pm to 11pm. You can see how, despite relatively stable air temperatures, the track surface temperature dropped by 25 degrees over the course of the event.

Even something as simple as clouds covering the sun for a couple of minutes can change the track surface temperature by 10 degrees. Teams must make sure to account for all of these weather conditions when discussing the balance of their car with the driver.

Additionally, due to the higher percentage of petroleum binder in the asphalt mixture, the surface coefficient of friction also varies more with temperature than at traditional racetracks.

Cup Series teams must account for this carefully when building their setup for the co*ke 600 as the race will start in the daytime and end in the evening, but all series racing this weekend must account for it to an extent. The Craftsman Truck Series teams will practice in the heat of the day at 1:35pm but their race will go green at 8:45pm. The Xfinity Series teams are practicing at 3:30pm on Friday when the forecast is for it to be cloudy, but racing on Saturday at 1:00pm when it’s forecast to be sunny.

Remember earlier, I added the caveat of “a normal race” to the discussion of stagger? Well, the co*ke 600 is not a normal race as it starts in the sunshine and ends under the lights. One tool that is often utilized by Cup Series teams is gradually reducing the stagger of their tires from the set of tires they plan to start the race on to the set they plan to finish on. Reducing the stagger gradually from one set to the next will add understeer to the car which compensates for the oversteer caused by cooling track temperatures. By building this adjustment into their tire sets, teams can eliminate the need to make a chassis adjustment during pit stops, which saves them valuable time on pit road.

Charlotte Motor Speedway is one of the easiest tracks for teams to miss their balance due to the changing conditions from session to session and over the course of the race. Even something as simple as cloud cover building can make the racecar handle very differently and teams must be careful to mind their surroundings and forecasted conditions when making setup decisions.

Thanks for coming along for the ride. Get comfy and enjoy a weekend slam full of racing. Happy Memorial Day, everybody!

All photos author or Daylon Barr unless otherwise noted