One of those phrases you tend to pick up as a NASCAR fan without thinking is “cookie cutter track”. That’s the accusation commonly directed at the one-and-a-half mile tracks (like Texas Motor Speedway, which we’re visiting this week). The complaint is that these tracks are so identical that it’s almost not worth bothering to watch. But are they really identical?

My disclosure, before we start: I am a fan of short tracks. I like being able to see the whole race and I like watching drivers try to pass each other. So I started out with a bit of a bias toward the intermediate tracks — Atlanta, Charlotte, Chicagoland, Homestead, Kansas, Kentucky, Las Vegas and Texas — myself. But there is nothing a scientist likes more than sitting down with a pile of data and trying to make sense of it, so (armed with Google Maps, a load of data from racing-reference.info, and the Excel file in which I’ve collected track parameters), I dug in to see for myself how similar these tracks are. After a couple false starts, I decided to try to develop a taxonomy. Taxonomy is a Greek word meaning a ‘method of arrangement’. My 1.5-mile track taxonomy is shown below.

The Differences

We start at the top with 1.5-mile tracks and a missing hyphen. The first distinction is the track shape because not all ovals are equal. Homestead actually is oval shaped. The other tracks have one of two shapes: the D-shaped oval and the quad-oval. Atlanta, Charlotte and Texas fall in the latter category, with Chicagoland, Kansas, Kentucky and Las Vegas in the D-shaped oval Camp. The difference is evident in the figure below. The photo in the upper left is Kansas (our representative of the D-shaped oval) and in the lower right is Atlanta. The difference is subtle: the D-shaped oval is more of a triangle while the quad-oval has a double dogleg. I’ve driven at Texas and you can see two distinct angles in the wall as you approach them. The D-shaped oval looks like someone grabbed an oval in the middle of one of its long sides and pulled on it. The D-shaped oval is not specific to intermediate tracks. California, Michigan and Richmond are D-shaped ovals, too.

Within the D-Shaped oval category, no two tracks have the same corner banking. Kentucky is 14°, Kansas is 15°, and Chicagoland is 18°. Las Vegas is in a class of its own, as it has progressive banking that runs up to 20°. (Homestead also has progressive banking.) The quad-ovals all have the same corner banking (24°), so we can’t differentiate that class any further in this level. These three tracks really are very similar. To make any distinction, we have to look at things like the backstretch length. Charlotte and Texas have approximately the same backstretch length (~1350 ft), while Atlanta has an 1800 ft. backstretch. Although Charlotte and Texas have similar frontstretch lengths, they do differ by 300 feet, so if you were really looking for an excuse to put them in separate categories, that’s about the most obvious division.

The tracks definitely race differently. The pole speeds on these track vary from an average of 174.8 mph to 193.0 mph (values given are averages over the last four races run). Out of curiosity, I plotted the pole speeds for the tracks as a function of different variables and finally found the following relationship. Note that there is no data for Kentucky since the first Cup race was run just last year and starting order was determined by owner points. The pole speed definitely depends on corner banking, which makes perfect sense. Banking helps the cars turn by providing some of the required centripetal force. More banking means more speed given that the track length is constant.

There is still, however, a 4.2 mph difference on the three quad-oval tracks, which suggests that there are other factors to be considered beyond shape.

The track surface makes a huge difference in speed. Asphalt is a composite of aggregate (stones) and binder (bitumen). A host of variables such as the size distribution of the aggregate, the chemical makeup of the asphalt and the conditions under which the asphalt is laid down have a huge impact on the track’s grip and how it wears. The track is changed constantly by the weathering and no two tracks experience the same combination of factors. The diagram at left shows how the aggregate (grey) and asphalt (black) wear over time. More of the aggregate is exposed with time and sharp edges get worn down. The track also changes in response to temperature and, again, different tracks will change in different ways. Atlanta, for example, is known as a tire-eating track because its rough surface is very hard on rubber.

The same issue arises over the course of a single race. When you hear a driver or crew chief talk about “chasing the race track”, it means that the setup they had that worked so well at the start of the race didn’t work as well during the race. A track changes significantly over the course of a race: it heats up due to friction between tires and the track, plus it may heat or cool due to the way the Sun hits the track (or portions of the track) or even just because a race goes into evening and the overall temperature changes. Different weather means different racing.

In addition to the small-scale roughness discussed above, some tracks have unique, larger perturbations in the track surface. Texas has a major bump between Turns 1 and 2 that was caused by the track settling over the entrance to the infield. In 2007, they drilled a bunch of holes in the area and injected a structural urethane to try to fix the giant distraction. They made it better, but you have still heard drivers all week talking about “the bump”. This isn’t unique to Texas: Charlotte has a big bump entering Turn 1. Those bumps pose major challenges for setting up the suspension. The ideal position for the splitter is as close to the track as possible – but if there’s a big bump, you have to make sure that the splitter doesn’t hit the bump. There are also issues like seams and patches, where the texture or type of asphalt changes, that challenge drivers.

The Similarities

This is not to say that these tracks don’t share some similarities. They are all fairly wide (50-60 feet) compared to the smaller tracks. The most important similarity is less a function of the track and more a function of the car. The current version of the NASCAR stockcar is highly aerodependent on one-and-a-half-mile tracks. Aerodynamic forces go like the speed squared, so these high-speed tracks have three-to-four times more emphasis on aero than short tracks.

A car depends on air rushing over it to push its tires into the track. Turbulent air – like you find in the wake of a high-speed car – doesn’t provide as much downforce as laminar (straight-flowing) air. This is why drivers value “clean air”. If you’re the first car in line, you don’t have turbulent air from the car in front of you because there is no car in front of you. Another feature of 1.5-mile tracks is that, because it is larger, you don’t run up on lap traffic as much as you do at a short track, and there’s plenty of room for a lapped car to get out of the way. At these tracks, being out front gives you have a huge advantage. That leads to a car that can easily put quite a distance between itself and the rest of the field.

The ‘aeropush’ effect happens when you get too close to the car in front of you. The air coming off its rear end is turbulent and doesn’t give you as much downforce as laminar flow would provide. It’s like running over ice: the only thing you can do is slow down. The aero-push makes it really hard to pass because you have to get close to the car in front of you in order to pass it. If the cars weren’t so dependent on aerodynamic downforce, then losing a fraction of that downforce wouldn’t affect them a significantly.

The Conclusion

I’d say there are actually only three ‘cookie cutter’ tracks: Texas and Charlotte are identical twins that get their hair cut differently and refuse to wear identical clothing. Atlanta is a fraternal twin to Texas and Charlotte. Lumping the D-Shaped Ovals in with these tracks, however, is unfair. The issues that many race fans have with racing at these tracks requires changing the car rather than changing the track.

Atlanta is known as a really rough track that eats tires.