# Load Transfer – Science of Speed

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
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.