Tires and Pressure – Science of Speed

Tires and Pressure – Science of Speed


MALE VO: The anticipation at a race track grows as laps are
completed. But anticipation isn’t the only thing building. 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: Talk about a high-pressure job. The only
thing keeping a car going 200 miles an hour down the front stretch and out of the wall, are its tires. How the rubber meets the road
determines how fast the car goes. And that’s why tire pressure is one of the most versatile tools in a crew chief’s arsenal. STEVE LETARTE: I was a tire man for a lot of years before I became a
crew chief. And I learned a long time ago that a tire low on air- low, not flat- low on air, is a happy tire. DIANDRA LESLIE-PELECKY: Tires heat up. The tires on your car may
reach 160 degrees Fahrenheit. NASCAR drivers make their tires work a little harder, and the tires get hotter. LISA SMOKSTAD: We measure tire temperatures with a pyrometer. It
actually has a probe, and we stick it in the tire in three different locations. STEVE LETARTE: Your tire, over the course of a long run, at a place
like Martinsville, with braking, in can be 220, 230 degrees on the surface. A tire, while it’s running on the track can be well over
300 degrees. DIANDRA LESLIE-PELECKY: The increase in temperature comes from
friction between the tire and the track. Depending on the track surface and the speed, tires can get from 250 to 325 degrees
Fahrenheit or more. Your car owner’s manual tells you to measure the tire pressure when
your tires are cold. That’s because hotter tires have higher pressures. Air molecules are constantly moving. In fact, the average
air molecule is moving over 1,000 miles an hour. Tire pressure comes from air molecules moving around inside the tire. When an air
molecule hits the inside of the tire, it pushes outward. While the gas molecules inside the tire are pushing out, the air molecules
outside the tire are pushing in. LISA SMOKSTAD: The lowest pressures that we use at any NASCAR
track, up till this point, would be a Martinsville, which is the smallest track that we attend. And we can get as low as eight pounds
on the left side. DIANDRA LESLIE-PELECKY: People in NASCAR talk about tire
pressures in terms of pounds. What they mean are PSI, or pounds per square inch. Pressure is force, which we measure
in pounds, divided by the area the force is acting on, which we measure in square inches. LISA SMOKSTAD: After a pit stop and the tires come over the wall,
the first thing I do is take the caps off and check the pressure. DIANDRA LESLIE-PELECKY: A tire pressure rating of 23 PSI means
there’s actually about 38 PSI in the tire. 23 PSI plus about 15 PSI that compensates for the atmospheric pressure. NICK HUGHES: Pressure is proportional to change in volume and
change in temperature. We run radial tires, which are very stiff, so the volume doesn’t change very much. So as a result, as the
temperature increases, so does the pressure. DIANDRA LESLIE-PELECKY: Higher temperatures mean the molecules
are moving faster. The faster they move, the more often they hit the sides of the tire, and the more force they exert. More force
means more pressure. This increase in pressure is called the build. JOSH BROWNE: You’ll see at most race tracks on the left side tires,
you might see 10 PSI buildup from cold to hot. And you might see 20 to 30 PSI buildup on the right side tires. LISA SMOKSTAD: At Homestead, the right front pressure will build up
to like 23 pounds. The right rear will build around 15. Left rear will be about 8 with this new car, and the left front will be around
15 pounds. DIANDRA LESLIE-PELECKY: A right front tire at Homestead, starting at
the minimum inflation pressure of 47 PSI, would have a tire pressure of 70 PSI at the end of a long run. Passenger car tires are usually
filled with air- but not NASCAR tires. LISA SMOKSTAD: We use nitrogen in the tire instead of air because
it’s a dryer gas, and it hopefully will control the buildup of the air pressure during a run. JOHN PROBST: We typically use nitrogen because it is the predominant
gas in our atmosphere. If we have a single gas, you know, I think that it more readily allows us to calculate builds, and effects from
temperature, and things like that. DIANDRA LESLIE-PELECKY: Even having a single gas in the tires
doesn’t make the crew chief’s job any simpler. He knows the tire pressure’s going to change, but he also has to figure out when. BRANDON THOMAS: So you’re trying to time, when does the tire hit
that optimal air pressure. And also you’re experimenting throughout the course of the weekend of, do we get to that air pressure early?
Do we sustain and get to that air pressure late? Do we wanna up that air pressure in the cold state to get it hotter sooner, and be a
higher performance vehicle earlier in the run or later in the run? DIANDRA LESLIE-PELECKY: One reason drivers swerve during
cautions is that they’re trying to get a little extra friction and get a little more heat in the tire. The extra heat increases the
tire pressure, and hopefully gives them a little more grip. So if you’ve ever wondered who’s under more pressure at the end of a
green flag run, the driver or his crew chief- the answer is, it’s neither. It’s the tires.

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  1. I found this very interesting, I have a converted ambulance campervan (European style only 4 wheels, no doubles on the rear) It weighs 2980kg. I run my Continental Fourseasons2 tyres at near max pressure, 69PSI. Am I running the risk of being to "hard" and risking punctures? They are all season tyres so have soft blocks to the touch.

  2. The reason you swerve or turn left to right aggressively is to clean your tires off. Under caution your tire will cool and pick up debris. To have a good restart you need as much of your tire in contact with the track.

  3. Ahhh Americans and their stupid lazy boring sports. They couldn't race against Europeans around corners, so they invented a stupid oval track to go around like a toy train!

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