Some fast women librarians produced a copy in record time of "The
Physics of NASCAR," 2008, by Diandra Leslie-Pelecky, your typical
female pickup-truck-driving professor of physics at U. of Texas at
Dallas (she researches bio-medical nanomaterials during the
off-season).
While getting NASCAR team mechanics and engineers to talk to her and
taking a few spins around the track herself, Dr. L-P got some details
about tire pressure and nitrogen, what I should do to avoid disastrous
goathead blow-outs, and how to tow cars into crash-test barriers at
150 mph.
None of it applies to bicycle tires.
***
What kind of tire pressures do they run in NASCAR?
Anywhere from 10 to 45 psi, with each tire different at different
tracks:
"At Atlanta, Goodyear's minimum recommended tire pressures were 22 psi
(left front), 20 psi (left rear), 48 psi (right front) and 45 psi
(right rear). At Martinsville, which is a much shorter track with less
banking and lower speeds, the minimum pressures would be 10 psi (left
front and left rear), 23 psi (right front) and 22 psi (right rear).
The right-side tires carry much more of the weight during cornering,
which is why they are inflated to higher pressures."
--p.156
***
How hot do NASCAR tires get and how much does the pressure rise?
Up to 300F and at least 7 to 20 psi:
" . . .tires reach temperatures of about 160°F on the expressway, but
the tires on the No. 19 [car] can reach temperatures of up to 300°F
during a race. After two laps here at Atlanta, Swifty [the team's tire
specialist] said, the right-side tire pressures will increase by about
11 to 12 psi and the left-side pressures by 7 to 8 psi. After a long
run, the left-side pressures can increase by 10 psi and the right-side
pressures by 20 psi. One of the most important things Swifty will do
during the upcoming practice is measure tire pressures after the tires
are removed from the car to find out how much of a 'build' they
experience. Starting at a lower tire pressure means that the pressure
will be just right after the tire heats up."
--p. 157
***
How fussy are NASCR teams about pressure?
Awfully fussy:
"'Swifty, I want a half-pound out of the right rear,' Josh [tream
director] said, then repeated, 'Half-pound. Right rear.'"
--p. 248
***
Now it's time for the nitrogen.
Here's what Dr. L-P gathered while watching tire prep:
"I've been saying 'tire pressure' instead of 'air pressure' because
after recording the tire information, Swifty removes the valve stem
from each tire and lets out all the air the Goodyear technicians put
into them. He then refills each tire with ultra-high-purity dry
nitrogen from a gas cylinder."
"Air is 78 percent nitrogen, 21 percent oxygen, and 0.9 percent argon.
The remaining 0.1 percent is a mixture of other gases. Air is mostly
nitrogen already, so why bother with the 22 percent of air that's not
nitrogen? The problem isn't the oxygen--it's water vapor that is often
mixed in with the air. The pressure exerted by water molecules in the
tire can change significantly near water's boiling temperature of
212°F (lOO°C). Small temperature changes in that region can produce
large pressure changes. Races are sometimes won and lost by
thousandths of seconds, so a team can't overlook any possible
advantage."
"All the above arguments tell us, however, is that whatever gas you
use in your tires should be as free of water vapor as possible. So why
not just use dry air? Dry air usually is produced by passing regular
air through a compressor and a dryer, which removes some--but not
all--of the moisture. You could purchase dry air; however, you can get
dry nitrogen with a third less water than dry air for the same or
slightly less cost. NASCAR doesn't mandate what gas to use in the
tires, but you'd be hard pressed to find anyone using air on pit
road."
"That doesn't mean that everyone uses nitrogen, although the
alternatives are slim. Only ten of the naturally occurring ninety-two
elements in the periodic table are gases at room temperature. We can
eliminate gases that are flammable (hydrogen), poisonous (chlorine),
or radioactive (radon). The most promising of the remaining gases is
argon. You can buy argon with about one-quarter of the water vapor in
nitrogen, but it's about twice the price. Swifty told me that some
teams use argon or an argon/nitrogen mix, but that most don't find
enough of a performance difference to bother with the extra tanks."
--p. 157-158
***
What protects NASCAR racers from catastrophic blowouts when they run
over goatheads at high speed?
Two air chambers:
"Regardless of the type of gas in the tire, a blowout can be a
race-ending or potentially more serious event. It's bad enough to lose
a tire doing 65 mph on the highway. Imagine what it's like to have a
flat at 175 mph with other cars just a few inches away."
". . . . The best compromise is a pneumatic tire that goes flat in a
way that allows the driver to retain some control over the car.
Watching Swifty make last-minute adjustments as we got closer to
practice time, I noticed that he had to unscrew two caps for each
tire. Each tire is actually two independent tires: an outer tire,
and an inner liner that is pressurized from 12 to 25 psi higher than
the outer tire. If the outer tire loses pressure due to a puncture,
the inner liner remains pressurized, almost always allowing
the driver to get the car to the pits safely. While these tires still
go 'flat,' they make a flat tire a manageable event instead of a
race-ending catastrophe."
"NASCAR requires the inner liner at all tracks longer than a mile, as
well as on the right-side tires at Bristol, due to the high banking
that places a lot of stress on the right-side tires."
--p. 159
***
Completely unrelated to tire pressure and nitrogen . . .
How do you crash-test economically at 150 mph? That is, how do you get
cars up to that speed and hit a barrier at a 25 degree angle?
Turbo-charged or nitrous-oxide tow-trucks, cables, and pulleys:
"'I actually didn't realize how interesting making a car go fast could
become,' Dean smiled. 'Because of the expense of IRL [Indy Racing
League] cars, they made us prove that we could make a car go fast, so
we were testing with a Ford Festiva.' The Festiva was the designated
stand-in for an Indy car only because it was about the same weight."
"'Have you ever seen a Ford Festiva go 110 miles per hour?' Dean
laughed. 'Their suspensions are really not built to handle that type
of speed.' Dean laughed even more as he recalled their first test with
a real IRL car, which took place in front of the IRL brass. They used
a heavy-duty truck to get the car up to speed via an arrangement of
pulleys and a steel tow rope."
"'We get the car up to 90 miles per hour and there was so much drag in
the Festiva that when the tow truck would shift gears, the Festiva
would keep the tow cable taut.' He used his hands to represent the tow
truck and the race car. 'A race car rolls better, so it gets to
ninety, our tow truck shifts, and the race car catches up some with
the tow cable and the tow cable drops off. We're at the point of no
return, we can't brake the car, and we're coasting. It rolled into the
wall at about eighty-five.'
"MwRSF [Midwest Roadside Safety Facility] now uses two tow trucks, one
turbocharged and one with nitrous oxide, and a much different tow
ratio--and they don't shift anymore. The ultimate goal had been 175
mph, but Dean says, 'We towed an Indy car into a barrier at 150 miles
per hour and 25 degrees. IRL said that was enough.'"
--p. 205
Cheers,
Carl Fogel