Dr. Diandra: Turning at Talladega: 100 mph on ice

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The secret — and the challenge — of winning NASCAR races is in the turns. While we normally focus on drafting and pack racing when NASCAR visits Talladega Superspeedway, Talladega was built for turning fast.

Even if you covered the turns in ice, a car could still take them at about 100 miles per hour.

To understand how that’s possible, we have to first understand how cars turn under normal circumstances.

How race cars turn

Talladega is just plain huge. The infield is 247 acres. That’s big enough to fit the Disneyland theme park (160 acres, which includes only the theme park and not the whole resort) and still have enough room left over to almost get the Mall of America in there too — which, come to think of it, is not a bad way to describe the tamer parts of the Talladega infield.

Large tracks like Talladega give drivers more time to build up speed down long straightaways. Wide, sweeping turns don’t force cars to slow down as much as tracks like Bristol or Martinsville.

At Bristol, about 60% of each lap is turning. At Talladega, it’s more like 10-15%. But even at only 10% of the race, drivers can’t win Talladega unless they can master its corners. The turns are where speed is gained or lost. How a car exits a corner determines its ultimate straightaway speed, and how the car enters a turn plays a large part in how it exits.

The force that makes a car turn is called the centripetal force. Even if you’re not familiar with the word, you already know all about centripetal force. You experience it every time you use a cloverleaf highway interchange or ride a merry-go-round or a loop-the-loop roller coaster.

The word ‘centripetal’ means center-seeking. Since cars turn left and right, you may wonder how the turning force always points toward the center of the turn.

Let’s do a thought experiment. Imagine tying a ball to a string. Now whirl it over your head in a horizontal circle.

The string forces the ball to travel in a circle. At every point on the path, the force on the ball points toward the center of the circle, as I’ve shown below.

A graphic showing a ball whirling around in a horizontal circle on a string, and the forces keeping it in a circle
The ball moves in a circle because the string forces it to do so via centripetal force. Centripetal comes from Greek and means ‘center-seeking’. The centripetal force always points toward the center of a turn.

The exact same physics apply to a race car — except the numbers are bigger.

The minimum weight of a Cup Series car (with driver) is about 3,675 pounds. Let’s turn the car in a circle with a radius of 1,100 feet, which is about the turn radius at Talladega. A driver taking the turn at 190 miles per hour requires a little more than 8,000 pounds of force.

That’s four tons.

If the force is any less than that, the car breaks traction and hits the wall. Then we’re talking about some very different physics.

Of course, race cars don’t have strings. Those four modest-sized patches of tire rubber in contact with the track must create those four tons of turning force.

A graphic showing how a race car turns and the directions of the forces making it turn.

In the video below (from the FS1 broadcast of the Martinsville Cup race earlier this month), I’ve shown how the centripetal force points toward the red-and-white curbing throughout the turn. You can also see how exiting the turn prepares the driver to take the best line down the straightaway.

But tracks like Talladega give the tires a little more help turning the car.

Taking it to the bank(ing)

Martinsville is a half-mile track with only 12 degrees of banking. Pole speeds tend to be in the mid-90 mph range. Compare that with Bristol, a similarly small track, but with pole speeds around 130 miles per hour. Banking makes all the difference.

In addition to being the longest track, Talladega has the highest banking of any track NASCAR runs this year: 33 degrees. For reference, most modern staircases have angles between 30 and 35 degrees.

Below, I’ve drawn the banking of a couple NASCAR tracks to scale.

Comparing the baking at Martinsville, Auto Club of California Speedway and Talladega

That banking is magic. To understand why, let’s look first at how a car turns on a flat track.

In the diagram below, the car is turning left and you’re looking at it from behind. The centripetal (a.k.a. turning) force points left at the moment we snapped these pictures.

The left track is flat. I’ve indicated the force the track exerts on the car with red arrows and labelled it ‘track force’. Gravity (which I didn’t show so as not to clutter the diagram) points straight down and exactly offsets the track force. That’s where friction originates.

But gravity and the track contribute nothing in the left or right directions that might help (or hinder) the car turning.

A graphic describing the forces acting on a car on a flat and on a banked track.

Now compare that with the same car turning on a banked track, which I show on the right side of the above picture.

Gravity still acts down. (Gravity always acts straight down.) The track force is still perpendicular to the track. But when the track is banked, some of the track force points in the direction of the centripetal (turning) force.

The banking actually helps the car turn.

Of course, there is a trade-off. A banked track provides slightly less frictional force. But the net effect is that a banked track allows a car to turn faster because the banking contributes turning force. The force from the banking adds to the force from the tires. The higher the banking, the more turning force help the track provides.

Talladega on ice

In fact, a banked track can provide so much help turning that it can compensate for a loss of friction.

Imagine a sudden, concentrated storm that covers Talladega’s Turn 4 with a sheet of solid ice. The tires don’t touch the track at all in that turn. There is no friction.

Because of the 33-degree banking, a car could still travel through turn four at Talladega at a maximum speed of just about 103 mph.

That’s not to say the car would be well behaved through the turn: It would be sliding instead of rolling and you wouldn’t be able steer – but you would make the corner.

At Martinsville, with only 12 degrees of banking and about a 200-foot turn radius, the maximum speed at zero friction would be only about 37 mph.

If a car travels too slowly around a high-banked track, gravity will pull the car down the banking. We will likely see that this week at Talladega, and next week at Dover, as well. Dover’s reputation as a ‘self-cleaning track’ originates from physics.

On a banked track, you have to move fast to beat your competitors – and to beat gravity, too.