Dr. Diandra: Explaining Ross Chastain’s Martinsville move

Ross Chastain provided a superb physics lesson at Martinsville. Here’s how he managed to pass five cars in the last half-lap of the race.

Turning requires turning force

Imagine swinging a tennis ball tied to a string above your head. The ball moves in a circle because of the string.

That string provides a force that makes the ball turn. This turning force always points toward the center of the turn. Physicists call it ‘centripetal force’, but I think ‘turning force’ is more descriptive.

Turning at a race track utilizes the exact same physical principle — except without the string.

And these 3,675-pound cars require a lot more turning force than a tennis ball does.

The amount of force necessary to turn is proportional to the mass that’s turning, times the speed, times the speed again, all divided by the turn radius.

Physics tells us that:

• Faster turns require more turning force.
• Making tighter turns (like Martinsville) requires more turning force.
• Heavier cars need more turning force.

Let’s figure out how much force you normally need at Martinsville. The pole speed was 96.078 mph, but each car’s speed varied throughout a lap.

• During practice, cars reached 114-119 mph on the straightaways.
• Drivers entered the turns from 75-85 mph, depending on the driver and the age of their tires.
• Most drivers slowed to around 60 mph before speeding up again to exit the turn.

Let’s say a driver takes Martinsville’s 202-foot-radius turns doing an average of 80 mph. That requires a turning force of 7,775 pounds.

The four tires must generate all of that almost four tons of turning force.

Sir Isaac Newton discovered that force is equal to mass times acceleration. Chastain is 5’9”. I’ll guestimate his weight at around 160 pounds. That makes the total weight of the car and driver 3,675 pounds.

Dividing the force by the mass, the acceleration of a car turning at 80 mph at Martinsville is normally about 2.1 Gs, where G is the acceleration due to Earth’s gravity.

Your head, which weighs around 10 pounds, would feel like it weighed 20 pounds when subjected to a 2G acceleration.

Compare that to astronauts on the Space Shuttle, who experienced about 3Gs during takeoff.

Ross Chastain’s turning model

On the final lap, Chastain needed to pass two cars. But none of the cars he needed to pass were close enough for him to get to them.

Chastain floored it going into Turn 3. Instead of relying on just the tires for turning force, Chastain used the wall to help him turn. That gave him enough turning force to turn a lot faster.

Chastain’s lap time on Lap 499 was 20.758 seconds. His lap time on lap 500 was 18.845 seconds. As my colleague Dustin Long noted, that’s the fastest lap ever by a stock car at Martinsville Speedway.

Chastain did the first half of the lap normally. It would take him about half the time of lap 499: 10.379 seconds.

That means he completed the last half of the lap in 8.466 seconds. He had to run an average of 112 mph from the midpoint of the backstretch to the start/finish line.

He didn’t run 112 mph the whole way. Let’s assume he entered the turn at 122 mph, which would be 37-47 mph faster than anyone else did. We’re talking about a turning force of 18,079 pounds, and an acceleration of almost 5Gs.

Isn’t 5Gs dangerous?

A human being can tolerate 5Gs for a short time. A 10-pound head would feel like it was 50 pounds under a 5 G acceleration. But that’s not the primary problem.

The human body is optimized for the 1 G acceleration the Earth’s mass provides. When your body accelerates faster, it has to work harder to circulate blood. Without sufficient blood flow, organs don’t get enough oxygen.

A warning sign of excessive G’s is losing peripheral vision and the ability to see colors. When the brain senses a lack of oxygen, it shuts down the least important functions first.

But if the high acceleration persists, the person who is accelerating eventually blacks out. Fighter pilots wear pressurized suits to ensure their circulation remains normal.

Much of what we know about how the human body tolerates high accelerations is thanks to Air Force Colonel John Stapp. He experimented on himself in the 1950s, surviving 25Gs for a little over a second and a peak force of 46.2Gs.

Unfortunately, his experiments permanently damaged his vision. However he lived another 45 years, dying at age 89 in 1999.

Ross Chastain not only made it into the Championship Four, he provided a great answer for every student who asks their math and science teacher: When am I ever going to use this?