Racing NASCAR At Texas Motor Speedway Is Just One Big Physics Problem

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Texas Motor Speedway was built during NASCAR’s westward expansion in the mid-to-late 1990’s as the sport sought to grow from its Southeastern niche into a national powerhouse. After scouting locations in Las Vegas, St Louis and the Dallas-Fort Worth metro area, Bruton Smith, CEO of Speedway Motorsports Incorporated, settled on a piece of land outside of Fort Worth owned by Ross Perot Jr.

[UPDATE: Since writing this piece, Aeden’s car, the #1 JR Motorsports/Carolina Carport Chevy of Sam Mayer, went on to win Texas!!! -MH]

Ground was broken on April 11, 1995 and by early 1998 a 150,000-seat, 1.5-mile (2.4 km) speedway was completed and ready for action. After securing a race date from North Wilkesboro Speedway, Texas Motor Speedway hosted its first weekend of NASCAR racing in April of 1997 with Mark Martin winning the Saturday Busch Series race and Jeff Burton winning the Sunday Winston Cup Series race. A second date was added in 2005 from Rockingham Speedway.

Texas Motor Speedway was once wildly popular and featured the third-largest purse on the schedule behind only the Daytona 500 and the Brickyard 400. Turns out that everything actually IS bigger in Texas. In 2005 the purses for these races were $14,322,448 for Daytona, $8,087,058 for Indianapolis, and $5,936,997 for the Texas fall race in a season in which the average purse was $4,902,352. In 2024 dollars, Texas Motor Speedway paid out a purse that was $1,694,587 higher than the average for that season. Of note, the 2024 purse is $330,901 lower than the inflation-adjusted purse from 2005.

The harsh Texas winters and oppressively hot summers quickly wore in the asphalt, combined with the high banking providing multiple racing grooves from the apron all the way up to the catch fence. For years, Texas Motor Speedway played host to fantastic racing. Side-by-side battles, photo finishes, fisticuffs, literally everything about this track was larger than life. Texas Motor Speedway even has the world’s largest jumbotron (22,692 sq ft / 2,133 sq m) on the backstretch. It had almost become an incarnation of Texas itself.

Things Got Weird

All of this was set to change ahead of the 2017 racing season. The track surface was in need of a touch-up, and persistent drainage problems in Turns 1 and 2 plagued the track whenever there was weather on a race weekend. Over the winter of 2016 into the spring of 2017 the track was not only repaved but reprofiled. To combat the drainage issues, Turns 1 and 2 were flattened from 24 degrees of banking down to 20 degrees. In an effort to reduce costs, the outer retaining wall was left in place and the track was simply widened to accommodate the reduced banking.

The result was a very awkward corner profile that caught many drivers by surprise when they showed up for practice ahead of the spring 2017 race. Opening practice saw Denny Hamlin and Kyle Busch go for a spin while Erik Jones and Chase Elliott were forced to bring out backup cars. Kasey Kahne got into the wall later in the weekend, but his team was able to repair the car.

Because of the abnormal shape of Turn 1, drivers are forced to take a much shallower entry than they would prefer, and it greatly reduces their mid-corner speeds. The first races on the reconfigured track turned into single file, strung out affairs as drivers fought for the bottom lane. There was no way to go side-by-side through Turns 1 and 2. While the greater banking in Turns 3 and 4 could have provided a second lane, driver’s entry speed was so low thanks to Turns 1 and 2 that they were barely out of the throttle at all in this corner, effectively negating any overtaking opportunity. The 2016 spring race on the old track surface featured 2,733 green flag passes and the 2017 spring race on the reconfiguration saw only 1,978. Clearly, there was an issue.

Then It Got Weirder

To improve the quality of racing, the track was treated with PJ1 traction compound in the upper lanes starting in 2019. This sort of worked, and I will emphasize sort of. Rather than being strung out around the bottom of the racetrack, the field would string out along the bottom edge of the treated region. The only added benefit was that if a driver got a run on the vehicle ahead of them, they could dip down to the untreated surface to complete the pass before sliding back up into the PJ1. The combination of PJ1 with the Cup series’ new 2019 “drafting” package saw the numbers briefly jump up to 3,489 passes but soon everyone had figured out the new package and track treatment combination. By 2021 the number of passes was down to a paltry 1,624.

 

The PJ1 track treatment also caused significant problems for the NTT Indycar Series racers. PJ1 is a heat-activated compound and requires drivers to “work in” the lanes that it has been applied to. With the smaller fields and lighter vehicles, the Indycar Series could never quite get their cars to work in the treated areas of the track, turning their races into single-file processions around the narrow band of untreated track. Teams also noted that because of the darker color of the treated asphalt, track temperatures were significantly higher in this area, further reducing grip.

Ahead of the March 2022 Indycar race series and track officials tried several methods to remove the PJ1 from the track surface. The track was scrubbed with a chemical wash and steel mesh to try and peel up what was left from the fall 2021 NASCAR weekend. An additional practice was even proposed to allow drivers to work more Firestone rubber into the remaining compound. No compound was applied to the track surface in 2023, but Texas Motor Speedway was still dropped from the Indycar schedule for 2024.

What we’re left with is an imperfect track with an equally imperfect racing surface, but still the race must go on. So, what does it take to get around Texas Motor Speedway? Let’s talk about it

How To Make It Work

The difference in banking angle creates two very different corners with Turns 3 and 4 acting like a slingshot sending drivers rocketing down the front stretch, topping out at just over 190mph (306 kmh). Drivers will brake fairly heavily for an intermediate track on entry to Turn 1 and decelerate down to around 155 mph (250 kmh).

They must take a shallow entry to keep the back of the vehicle settled under braking and it is tempting for them to rush back to the throttle in the middle of Turns 1 and 2 as, visually at least, the track is incredibly wide. However, the banking falls off in an awkward manner on the exit of the corner causing drivers to lose grip and struggle with oversteer if they get too high up the racetrack.

In reality, there’s just several parking lots worth of unusable asphalt in the upper areas of Turns 1 and 2. Drivers will then arrive at Turn 3 at just over 180 mph (290 kmh) and only have to decelerate down to around 170 mph (273 kmh) mid-corner. They will take a much bigger arc into Turn 3 and get back into the throttle as soon as possible to keep the vehicle loaded up on the right rear throughout the turn. In the bass ackwardness of Texas Motor Speedway, drivers will enter the slower, less stable corner much faster than they will enter the corner where their vehicles are gripped up and ready to rip wide open. Both ends of the track drive completely different.

Let’s take a ride onboard with Noah Gragson during the 2022 spring Xfinity race. You can clearly hear how long he is out of the throttle for in Turns 1 and 2 versus how he barely cracks the throttle on entry to Turns 3 and 4 before immediately jumping back in the gas.

What does this mean from a setup perspective? Looking at a free-body diagram of a car on a banked corner you will see that the normal force (FN), or the force acting vertically with respect to the car, is a product of the banking and the car’s lateral acceleration.

Track Diagram

At Texas Motor Speedway, the vertical loads in the faster, steeper banked Turns 3 and 4 are about 20% greater than they are in the slower, flatter Turns 1 and 2. This causes the suspension to compress, aka travel, significantly further at one end of the track than the other. If the aero platform is set correctly in Turns 3 and 4, the vehicle will be too high in Turns 1 and 2 to make optimum downforce. On the contrary, if the aero platform is set correctly for Turns 1 and 2 where there is less banking and drivers are struggling for grip, the vehicle will bottom out in Turns 3 and 4 and be undriveable. The catch-22 of Texas Motor Speedway is that in the corner where you need to generate the most grip the car will be the least optimized for generating it.

So, what exactly does it mean when someone talks about “aero platform”? Put simply, every vehicle of any kind has an optimum pitch (combination of front and rear ride heights) and yaw at which it will make the most efficient downforce. By efficient we mean most downforce to least drag or the optimized point of that ratio.

Xfinity cars and Cup cars make downforce in very different ways, but the same principles still apply. An Xfinity car produces downforce in the traditional sense where it primarily utilizes the top surface of the vehicle with a smaller percentage coming from the underbody. Cup cars are the exact opposite.

An Xfinity car has a splitter at the front and a spoiler at the rear that generate downforce, as well as side skirts on the sides (obviously) just like the pre-Next Gen era Cup cars. Let’s talk about how these devices work.

A splitter generates downforce in two ways. The first is by creating a high-pressure region on the top, exposed edge of its surface where it meets with the nose of the car. Incoming air meets a sharp angle between the top of the splitter and the nose of the car and acts to push down on the top surface of the splitter. You can see the difference in pressure and flow between a car with and without a splitter in Figure 15 from the 2019 SAE study “Optimization of Race Car Front Splitter Placement Using CFD.”

The second way it creates downforce is by accelerating airflow under the body of the vehicle. According to Bernoulli’s equations, in a fluid flow as velocity increases pressure decreases. This is the principle on which airplane wings generate lift and downforce is simply lift in the opposite direction. So, when we talk about downforce, try to imagine the car acting like an airplane wing flipped upside down.

Here’s a handy, albeit crude, sketch of how this approximately works.

Longer Path Equal Tranist

The underbody of an Xfinity car is smoothed out with several features that help to maintain this high-velocity flow channeled under the car by the splitter. Behind the splitter is what is known as a radiator pan, which is a flat piece of metal that essentially extends the splitter from the nose of the vehicle back to the bottom of the engine. The dimensional rules for this pan can be seen here denoted in the red hatched area. The angle of this pan can be used to shift the center of low-pressure fore and aft underneath the car.

How A Splitter Works
Source: NASCAR RULE BOOK

Teams will take great care underneath the car to make suspension components and body panels as smooth as possible to help maintain the flow generated by the splitter. Anything that slows down this stream of air will reduce downforce and in extreme cases, turbulence can even create lift from under the vehicle.

Former Stewart Haas Racing fabricator Brian Murphy notes in this 2021 X post that in the Gen 6 days, teams would mold the nose of the car to raise the center of the splitter. This effectively channeled air underneath the body of the car and “fed” the pan, creating more downforce.

NASCAR attempted to crack down on this by enforcing straight edge checks of teams’ splitters in pre-race inspection but it’s hard to put the genie back in the bottle.

On Cup Series cars, which generate downforce primarily from the underbody, you will see that the splitter is purposely stepped up in the middle to help feed air into the floor and diffuser.

One thing to note about splitters is that they are very sensitive to ride height and perform optimally at a specific height relative to the ground. The downforce produced by a splitter increases proportionally with decreased ride height up to a certain point. If the splitter becomes too close to the ground it will start to stall as there won’t be enough room between the bottom side of the splitter and the racing surface for air to be channeled underneath.
Xfinity cars also rely heavily on something known as side force.

When a car is in yaw while cornering, one side of the car is exposed to the air while the other side is hidden. Air hits the exposed side of the car at an angle and the reaction force pushes against that side of the car. Take a look at this photo of Noah Gragson and Ty Gibbs from 2022 at Charlotte Motor Speedway. Using the white and blue lines on the inside of the track you can see how much the rear of the car is shifted towards the outside of the corner. This is called skew and it’s what creates an aerodynamic force pushing the car towards the center of the corner.

Twocarsontrack
Photo: GM

Commodore’s Garage on iRacing provides a great example of how the net force acts on the car.

How Air Works On Car
Source: Commodore’s Garage/iRacing

If we take a step back in time to the Gen 4 Cup Series cars with their “twisted sister” bodies, you can see how the car was built to maximize this side force principle.

Looking at the car from above you can see how the body clearly resembles that side profile of an airplane wing.

Skew Nascar
Source: Dodge

Just like with front downforce created by the splitter, side force is also very dependent on ride height.

Xfinity cars and CRAFTSMAN trucks utilize what are called “side skirts” to seal up the sides of the car to the race track. The door panel on the car stops at the bottom of the frame rail and then small flat pieces of material are added that hang down below the frame rail. These pieces are trimmed to lengths determined by the teams so that they will be flush with the surface of the racetrack while the car is in a corner. There are two reasons why these need to be as flush with the track surface as possible. First, if the material was cut too short and didn’t quite reach the ground then the team would be giving up surface area on which the air can act to create side force. Second, if there is a gap between the skirt and the ground then airflow from the side of the vehicle will get underneath it and interfere with the nice, clean flow created by the splitter.

All three vehicle types in NASCAR’s national touring series feature a spoiler on the rear decklid. A spoiler generates downforce by creating a high-pressure region over the trunk of the car or the bed of the truck as can be seen in the handy illustration below. Xfinity cars and CRAFTSMAN trucks, which produce downforce primarily from the top side of the vehicle, will often appear pitched down on their nose with the splitter just above the racing surface and the spoiler high in the air.

On the Next Gen Cup Series cars, downforce is generated primarily by the underbody and rear diffuser. This is why you will see cars running with the nose up and the rear of the car down, opposite to an Xfinity Series car or CRAFTSMAN truck, as they are trying to force feed as much air to the diffuser as possible. The rear diffuser generates more downforce at lower rear ride heights, so teams will try to get the back of the car as low as possible. The Next Gen cars have travel limiters inside of the shock body and teams will try to get the car to ride just above its engagement point. When the travel limiter is engaged, the suspension acts as a solid rod and all of the forces must be absorbed by the sidewall of the tire, often leading to a rapid failure. The 2022 Cup Series race at Texas Motor Speedway was plagued with cautions as numerous cars bottomed out the travel limiters in Turns 3 and 4 and sent their drivers spinning out of control. Most notably was Alex Bowman who suffered a concussion and was sidelined for multiple races. Other drivers afflicted included Kyle Busch, Christopher Bell, Chris Buescher, Kevin Harvick and Martin Truex Jr.

Because of the compromised aero platform and one-lane nature of the racetrack, Texas Motor Speedway is a track where you will see “aero racing” come into play more than at any other track. In NASCAR when we refer to aero racing, what we mean is how drivers manipulate the air around other cars to pass or defend. Let’s discuss how this works:

The most common bit of aero racing that you’ll hear about is an overtaking technique called packing air. Air flow around a vehicle can be visualized in much the same way as the wake of a boat with air being directed around either side of the nose.

Howwakeworks
Source: ACE BOATER

To make a pass by packing air, the overtaking driver will attempt to turn under the defending driver in the middle to exit of the corner and place their right front nose alongside the defending driver’s left rear quarter panel. The wake off of the overtaking driver’s nose will push against the left rear of the defending driver’s car which counteracts their side force and causes the car to oversteer.

When done properly the defending driver will have to lift out of the throttle and counter-steer slightly to recover their vehicle, breaking their momentum and allowing the overtaking driver to pull ahead of them on the straightaway. If the overtaking driver gets a bit too aggressive, or if the defending driver is already struggling with oversteer, packing air can cause a crash even without contact between the vehicles. Watch below as Austin Cindric attempts to pack air on Noah Gragson to overtake him for 3rd place in the 2021 My Bariatric Solutions 300 and inadvertently sends him careening into the fence off Turn 2. Upon seeing the replay, it is clear that there was no contact between the cars and it was simply the aero wake off the 22 car that caused the accident.

 

You will sometimes hear the commentators refer to one driver as having “put it on their door” in reference to another driver and this is an important defensive technique at aero racetracks. In this scenario the overtaking driver has positioned their vehicle inside of the defending driver and attempts to go through the corner side by side with them. The defending driver will try to get as close as possible to the inside car which is where the term putting it on their door comes from. In this scenario, the wake flowing off the right front of the inside vehicle and the left front of the outside vehicle is squeezed into the small area between them. Sticking with Bernoulli’s Principles from earlier, as a continuous fluid flow is squeezed into a smaller area, its velocity will increase, and its pressure will decrease.

Physics
Source: GSU Hyper Physics Lab

Therefore, the airflow will create a low-pressure region on the left side of the defending vehicle and on the right side of the overtaking vehicle. More importantly, it will pull the defending car towards the center of the corner creating slight understeer and more stability. For the overtaking car, it will negate most of their side force and want to pull the car towards the outside of the corner, creating oversteer. Drivers who crash from this position often describe the outside car as having sucked them around. Watch below as Kyle Larson crashes while trying to overtake Bubba Wallace for the lead of the 2023 Cup Series race at Texas Motor Speedway. Ironically, he follows right in the skid marks left by another car that had crashed in the same spot earlier in the race.

You will also hear a lot of talk, and complaints from drivers, about aero blocking. Aero blocking is essentially mirror driving but instead of blocking the overtaking driver with your physical vehicle, they are being blocked with air. Referring back to the boat image from above, the “wash” region is basically the turbulent bubble that exists behind a vehicle as the over and under-body air streams re-converge. You can see this recirculation region in some of the Next Gen CFD images published by NASCAR while the car was being developed.

Carwash Nascar
source:NASCAR

The fact that underbody downforce produces a smaller turbulent region behind a vehicle was the these behind not only NASCAR’s Next Gen car but the current Formula 1 regulations as well. Regardless of how a vehicle produces downforce though, this region exists behind any vehicle moving through air.

For a trailing vehicle, the result is a net loss in downforce when operating in the turbulent region behind the leading vehicle. This loss in downforce increases exponentially as the distance between the vehicles closes, which is why you will often hear spotters telling the driver to “take their line away.” What they want the driver to do is adjust their racing line to put turbulent air onto the line that the trailing driver has been driving in. The trailing driver will then have to either adjust their line to compensate or deal with the reduced downforce while trying to set up an overtake.

Welcome to Texas Motor Speedway, it’s tricky and imperfect but it’s the same for challenge for every driver and team. There’s only one option, and that’s to cowboy/cowgirl up and get it done.

Photo: Dylan Barr/Big Machine Racing

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13 thoughts on “Racing NASCAR At Texas Motor Speedway Is Just One Big Physics Problem”

    1. “Equal Transit Theory” actually doesn’t have merit 🙂 In fact, the flow over the upper surface of a wing is faster than you could explain solely by the longer path… two air particles that reach the leading edge at the same time (one going over, one going under) will not necessarily meet at the trailing edge at the same time.

      The higher speed over the upper surface (to first order) occurs because the Kutta Condition must be satisfied. The Kutta Condition says that flow must leave a sharp trailing edge smoothly… this forces higher speed over one surface (upper if you’re trying to produce lift, lower if you’re trying to produce downforce, which is the same as lift but in the other direction). We can mathematically describe the Kutta Condition using a bound vortex… and in fact we see this bound vortex in the wing tip vortices that are produced when a wing generates lift.

      So Kutta says flow departs the trailing edge smoothly, which means higher speed flow over the upper surface. Then Bernoulli says that the total energy in an inviscid flow must remain constant… so higher speed (kinetic energy) results in lower pressure (internal energy). So Bernoulli is correct, but it’s only half the story.

      “So smarty pants,” someone might ask. “How come airplanes can fly upside down?” You need to define the upper and lower surface… the short version (I don’t know how to include a picture) is the upper/lower “split” happens at the location where an air particle goes either over the top or under the bottom of the wing (at the stagnation point). Aerobatic airplanes are usually designed with a symmetric airfoil (so you can fly upside down as easily as right side up). Regular airplanes are usually designed with cambered airfoils so they can operate more efficiently right side up. But they can still fly inverted.

      Note: if the wing is stalled then the Kutta condition is no longer satisfied…

    1. Ok then I guess 15 below with a 40 mph wind is “cozy”.
      Seriously though, the hail you described sounds more like summer than winter.

    1. I agree. High level of research, development and engineering goes into the building, preparation and racing of these highly developed and sophisticated machines. And yet it is presented to consumers as something for Bubba and Cooter to argue over Chevy vs. Ford vs. Toyota, as if those nameplates have anything at all to do with these race cars.

      1. In a way, Nascar being so “simple” (ie, just turn left), means that the niche technical areas become much more important than in other forms of racing. So we can read a really deep dive into them, and see the effects. In a different racing series subtle aero effects are obscured by other things so they’re harder to visulise.

  5. These Friday insights into the physics & strategy of NASCAR are so dense I’ve taken to saving them till Saturday when I have the time to savor them. Keep this up and I might actually watch one of their races!

    1. I actually almost went to Richmond Raceway because of these NASCAR pieces. We hadn’t planned for it because I hadn’t realized the race was right then. We didn’t have the hotel/clothes/etc for another night. I’m really digging them.

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