Driving Bristol Motor Speedway has been described as ‘flying jet fighters in a gymnasium’. Up until last March, Bristol laid claim to the highest banking of any NASCAR track at 36°, and that was around just 0.533 miles of track. Immediately after the spring race, they tore up the old concrete and laid an entirely new track. The new track joins Las Vegas and Homestead in having progressive banking.

Why bank a track at all? Newton’s Law of Inertia (NASCAR version) states that a car coming straight down the backstretch at Bristol at 120 mph will keep going straight at 120 mph unless a force acts to make it change either speed or direction. A change in at least direction is pretty important because the backstretch is only 650 ft long and then there’s a turn.

The force that makes the car turn is called the centripetal force. Centripetal means ‘center seeking’, which is appropriate because the force that makes the car turn always points toward the center of the turn. The magnitude of the turning force is the mass times the speed squared divided by the radius of the turn. That means the faster you’re going or the smaller the turn radius, the more force you need to turn to the car.

Kevin Triplett, the Vice President of Public Affairs at Bristol Motor Speedway, was kind enough to share with me that Bristol is slightly egg shaped (“mainly noticeable only from an aerial shot nearly directly over the speedway and the drivers and crew chiefs driving the cars and working to set up the cars”, he said). So there are two turn radii: turns 3 and 4 have about 256 feet and turns 1 and 2 have about 242.45 feet.

Let’s assume Jeff Gordon (150 lbs) in the Car of Tomorrow (3450 lbs) for a total of 3600 lbs for the car and driver. Going around turns 1 and 2 at a speed of 110 mph requires a turning force of about 12,000 pounds (six tons).

Where does all that turning force come from? On a flat track, it all comes from friction between the tires and the track. For the example above (turns 1 and 2), each wheel has to generate 3000 pounds of turning force. To first approximation, the turning force is proportional to 1 to 1.2 times the force pushing the car downward. Let’s assume (again, keeping things simple) that each wheel supports the same amount of weight, which would be 900 lbs. Where do the other 2100 pounds of turning force come from? Yes, there's downforce, but that's an awful lot of downforce to generate.

This is where banking becomes important. The forces that point toward the center of the track (to the left on my diagram because the cars are all driving into the screen) are the ones that contribute to the turning force. On a flat track, the friction between the tires on the track provide all of the force. All the other forces act straight up or down.

On a banked track, the force the track exerts on the car is partially up, but partially toward the center of the track. A banked track helps the car turn by providing some of the turning force. The steeper the banking, the larger the portion of the force from the track that points toward the center of the turn. Not all of the frictional force between the track and the tires is pointing toward the center on the banked track; however, the amount of force we lose there is more than made up by the force we gain from the track banking.

Bristol was a one-half mile asphalt track with 22° banking when it opened in 1961. About 18,000 people attended the first NASCAR race there. In fall 1969, the track was renovated and re-measured, creating a track of 0.533 miles in length and an amazing 36 degrees of banking. You can tell the date of the change from the qualifying speeds. The pole for the spring race was 88.669 mph, while the pole for the fall race was 103.432 mph.

Bristol had been an asphalt track, but by 1991, the top qualifying speeds had risen to 117-188 mph because the cars were better engineered. This was great for race fans, but really hard on the track. The asphalt was having a tough time standing up to the high speeds and the high temperatures generated by the tires on the highly banked surface.

In 1992, the decision was made to change the track surface to concrete. Concrete also uses aggregate, but uses cement instead of asphalt as a binder. (Concrete is the combination of aggregate and cement.) Concrete tends not to be as sensitive to temperature changes and it stands up much better to the cars. There wasn’t much change in the qualifying speeds due to the change in surface.

The concrete had lasted 15 years, but the cumulative toll of freezing and thawing, weather and the racecars finally required re-building. Kevin tells me that the turns had developed a ‘crown’, just like sometimes happens on football fields. That made for some very large bumps on the track. The turns had gotten convex. In March 2007, less than twelve hours after the very first Car of Tomorrow race, workers started removing the concrete for another re-configuration.

When they re-did the track, they introduced progressive banking, which means that the degree of banking changes as you move along the track. The new track changes from a banking of 24° at the lower end to 30° at the top. They also made the track about three feet wider, as I’ve drawn below. The track surface now is actually concave – if you ran a string from the top of the track to the bottom, there would be a gap between the string and the track because the track is now more bowl-like. I’ve shown in light blue what the track would look like with constant banking of 24°. The larger triangle is a constant banking of 30°.

The higher the banking, the more turning force you get, which means you can go faster; however, when you run at the top of the track, you have to drive a longer distance than if you run near the bottom. Most of the drivers were testing different lines along the track to figure out which one their car worked better on.

So the big question this weekend’s races raised in my mind was: why were the Busch race and the Cup race so different? The Busch race looked like the old Bristol, but the Cup race had a lot more racing and a lot less crashing. (If David Ragan had been able to keep the bungee cord around his shifter, he might not have been responsible for half the cautions.) The possibilities are: a) the Cup guys are just better drivers; b) The Cup guys were being extra careful because there were only three races left before the chase; and c) the Car of Tomorrow. I’m sort of leaning against option ‘a’ because there was a pretty big overlap in the drivers both nights. I’m not sure anyone is going to admit to ‘b’, so it may remain a mystery. At least until the Spring race, at which time I hope to get to see the progressive surface for myself.

Many thanks to Kevin Triplett, the Vice President of Public Affairs at Bristol Motor Speedway for sharing the story of the track and the dimensions with me!