In 1959, something happened that revolutionized NASCAR’s stock-car racing: the introduction of Daytona International Speedway.
Daytona was unlike any race track before it because of these: banked turns. The turns had towering walls that sloped downwards to the center. Walls that NASCAR’s stock cars would drive onto. Daytona’s banks were a whopping 31 degrees, significantly steeper than the relatively flat 12-degree banks at Martinsville or Occoneechee Speedways.
In the first year of Daytona, stock-car drivers qualified at speeds of more than 140 mph. And today, at the same track, that speed is more like 200 mph — in large part because of the steep banks. Which raises the question: How do banked walls help cars go faster?
Detractors of NASCAR joke that, to finish a race, all you have to do is turn left. To NASCAR fans’ chagrin, it’s somewhat true. For the majority of NASCAR tracks, most of the lap is completed while turning, or cornering. What critics misunderstand is that it’s the turns where good drivers earn their keep. Oftentimes, viewers will see stock cars rocket past each other in the straightaways and think that the faster car had more horsepower. The speed that driver uses to pass, however, comes largely from the momentum they collect in the curve they just left.
The winningest NASCAR drivers, then, are the ones that understand the corners the best, change direction the fastest, pick the best lines, and apply power at the right times to navigate the corners better than their competitors. It’s the corners where the races are won. Going straight is easy. Newton’s law of inertia tells us that an object going straight will keep going straight until something makes it change direction.
So driving a stock car on a straightaway, even at 180 mph, would be fairly easy for you or I. It’s turning that presents some challenges. To turn, a force needs to push the car sideways. That force is centripetal force. Imagine a ball attached to a string. When I twirl the ball in a horizontal circle, the tension in the string provides the centripetal force needed to make the weight curve.
Our stock cars do not have strings attached to them. The centripetal force needed to move the car left is caused, instead, by friction at the tires. But at high speed, the force of traction at the tires alone is not enough to pull the car to the left.
Let me explain by example. Think about turning sharp circles in a flat parking lot. The faster you go, the more unsteady the car will be. With enough speed, the car will slide out. For cars traveling above 180 mph, friction at the tires alone is not enough to get the cars moving to the left. For example, taking the first turn at Bristol Motor Speedway at 130 mph requires an immense 16,000 pounds of force to move the car to the left. That’s where high banks come in handy. When an object presses onto a surface, the object feels an equal force in the opposite direction. So for a stock car on a flat track, the track will push up with a force equivalent to the weight of the car.
On a banked track, however, only part of the force from the track goes straight up. The angle of the track directs the rest of the force towards the center. And that’s exactly the direction the driver is trying to turn. The extra force from the banked track, combined with the friction from the tires, is enough to turn the car safely. So the steep, banked turns let drivers maintain greater speeds into and through the turns.
While the banked track isn’t the only thing helping the car corner — aerodynamic downforce too helps the car generate lateral force — it is one of the most important factors keeping stock cars cornering at speed. NASCAR’s banks are for cars going at race speeds. At lower speeds, the 33 degree bank at Talladega Superspeedway would be enough to slide a car down to the bottom of the track. In fact, if you or I wanted to take a lap around Talladega in a street car, we’d constantly be turning right to just stay up on the wall.
But you don’t need to be a stock-car driver to test a banked turn for yourself. Banked turns exist on our roads, too, on freeway on-ramps and interchanges. For heavy vehicles like trucks and buses, friction alone may not provide enough force to turn safely, especially if the driver doesn’t slow down enough. A slightly banked turn, with a gentle grade of 15 degrees or less, can help push the vehicle into the turn.
So, for NASCAR, banked turns simultaneously create lateral force that, in addition to friction force at the tires, create enough centripetal force in total to get stock cars moving to the left but also enable them to travel at higher speeds without sliding or flying off the track.