NASCAR's stock-car racing:

the introduction of Daytona International Speedway.

Daytona was unlike any race track

before it because of these:

banked turns.

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 races,

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.

Viewers will see stock cars rocket past

each other in the straightaways and think,

"Well, the faster car had more horsepower."

Not so true.

The speed that the driver uses to pass

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 me.

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 to make the ball curve.

Our stock cars don't 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 friction 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.

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 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.

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, say, a street car,

we'd constantly be turning right

just to 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, say,

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.