MALE ANNOUNCER: If three tires have a lot of grip, does it matter what the fourth tire is doing?
MALE ANNOUNCER: The roar of the engines, the squeal of the tires, then the race to victory lane.
It all says, NASCAR. A race car is much more than steel, gas, rubber and speed.
A race car is a science experiment on wheels.
DIANDRA LESLIE-PELECKY: A turn at the race track looks like a smooth, continuous arc to you and me. But in
the minds of a driver and his crew chief, corners have distinct pieces. That's because the way the weight of
the car pushes each tire into the track is different in different parts of the turn.
And that affects how much grip each tire has.
BOB OSBORNE: The majority of the time you increase the load on two objects, they become harder to move,
relative to each other, between the tire and the surface you're applying the tire to, you increase the
load, the contact friction between the two increases, and you have improved grip.
DIANDRA LESLIE-PELECKY: A typical car and driver weigh around 3,600 pounds. The crew chief puts the car on
scales in the garage to measure how much of the car's weight pushes down on each tire.
STEVE LETARTE: The car sitting on the scales, we have the hypothetical percentages that we use as reference.
But really they don't mean anything. The driver's not in it, the hood's not closed, the car's not moving.
CHRIS ANDREWS: The driver steps on the accelerator, the weight of the car shifts to the rear.
The driver steps on the brakes, the weight of the car shifts to the front.
DIANDRA LESLIE-PELECKY: If the front tire of a bicycle hits a rock, the tire stops, but the bike rotates.
The same thing happens when you brake. The braking force creates a torque that shifts weight from the
rear wheels to the front wheels. This is called weight transfer, or more properly, load transfer.
NICK HUGHES: You're not creating load. You're just transferring it from one set of tires
to the other set of tires.
DIANDRA LESLIE-PELECKY: How much load shifts when you brake depends on three things.
How fast the car is speeding up or slowing down; how far apart the front and rear wheels are;
and how high the center of gravity is from the ground.
TOMMY WHEELER: The center of gravity is arithmetic average of the mean mass of the vehicle.
KENNY FRANCIS: If you could hook a crane to the car and hook it to the center of gravity, you could put
the car in any position and it wouldn't tend to move.
DIANDRA LESLIE-PELECKY: A higher center of gravity produces more weight shift.
CHRIS ANDREWS: The center of gravity is the height off the ground that the center of mass is of the vehicle.
Our cars, they're probably 16, 17 inches off the ground. And then as the car gets closer to the ground,
the center of the mass moves down.
TOMMY WHEELER: That is a constant battle for engineers and technicians in our sport. We're constantly trying
to get that center of gravity as low as possible to control load transfer.
DIANDRA LESLIE-PELECKY: Load transfer is why NASCAR turns have distinct pieces.
The drivers will refer to at least three parts of the corner. There'll be some that refer to six parts of
the corner, and some, nine. But it all kinda depends on the feel they have and what they're looking for.
DIANDRA LESLIE-PELECKY: The simplest way to break down a corner is: entry, middle, and exit. The driver
brakes and turns left when he enters a corner. Weight shifts from the rear wheels to the front wheels, and
from the left to the right. There's a lot of force pushing down on the right front tire, and not so much
on the left rear. In the middle of the corner, the car is roughly constant speed, but still turning.
KENNY FRANCIS: You'll see probably 70 to 80 percent of the load of the car on the right side tires, and
the other remaining portion on the left side tires.
DIANDRA LESLIE-PELECKY: Exiting the corner means accelerating and turning, which shifts weight from the
front to rear, and left to right. At a short track like Martinsville, or a flat track like New Hampshire,
sometimes the left front tire actually comes off the ground.
STEVE LETARTE: I could go out there and lay my hand on the ground and they could run it over with the left
front tire and it would barely hurt.
ANDRA LESLIE-PELECKY: If all the weight is on one tire, the other tires don't have as much grip, and you
can only go as fast as your slowest tire.
STEVE LETARTE: And that's what that whole track is about. If you can get the weight transferred over,
if you can keep the car flat and keep weight on the left front tire, that makes a car handle.
DIANDRA LESLIE-PELECKY: Teams use a special machine at the shop to study how the car's weight shifts on
different parts of the track.
JOHN PROBST: A seven-post in general is a way for us to simulate a car going around the track without
physically being at the track.
DIANDRA LESLIE-PELECKY: A seven-post rig uses four platforms on the wheels to push up, and three
actuators on the underbody that pull down and simulate the forces that push the tires into the track.
KENNY FRANCIS: Typically, like, let's say at Charlotte, for instance, you'll see about 3,000 pounds
of vertical load just on the right front tire. You'd probably see 2,700 pounds on the right rear tire,
700 to 1,000 on the left. And of course it depends on where you're at on the track and what part of the
corner you're in.
DIANDRA LESLIE-PELECKY: In the winter, I put a bag of sand in my pickup to give the rear wheels more grip.
You can think of a race car as having bags of sand over each wheel. But the sand bags' weight changes
when the car accelerates, brakes or turns. The car's grip is literally changing every moment of every lap.
So a turn ends up being a pretty complicated shape. But geometry is a lot more fun at 180 miles an hour.