On Wed, 05 Nov 2003 02:20:32 GMT, Rick wrote:

Steven Shelikoff wrote:


None of that changes the fact that aircraft braking requirements and
capabilites and tire heating have nothing to do with race car or boat
trailer braking or tire heating.

Never said it did, I just made the statement that tires are more likely
to be heated than cooled by the wheels and brakes and used aircraft
tires as a spectacular example.

And I'm just saying that while in racing, the brake rotors themselves
can get extremely hot during braking, if there is so little heat taken
away that the rotors alone are causing the wheels to be heated up to
over 250 degrees then something's wrong with the setup. The same
statement might not be true for an airplane, where the brakes are
applied hard for only a short time and then they have hours to cool
down.

While it may be true for aircraft
braking that the tire is more likely to be heated by the brakes then by
the heat from tire friction, that's not true for most types of racing
and especially NASCAR restrictor plate racing, when the brakes aren't
even used but the tires still get very hot and might benefit from
cooling through the wheel.

I have absolutely no idea what "restrictor plate racing" is, what do you
do, run with them? 8-)

Uh, yeah. You run with them. They limit the horsepower available on
the superspeedways to around half of what it normally available. That
way, the cars never get going fast enough to have to use the brakes.

If the brakes are never used then the brakes won't add heat. Unless the
area of the wheel exposed to the filling gas is a fair proportion of the
area of the sidewalls then I can't see much heat going out the wheels
regardless of the gas used. Are you sure there is a large area of wheel
surface exposed anyway?

If there's not an inner liner, then yes, a large area is exposed. An
inner liner is used for some races and not for others.

As a quick and dirty example, Nascar wheels are 15" dia x 9.5" wide.
The tires are 27.5" dia with a width of not more then 13.2". To make
things easier, assume flat sidewalls, which will make the area
calculation below come out on the low side. The sidewall area is around
2*(27.5-15)*pi = 78 sq in. Also ssume the wheel is a cylinder, which
will also make the area calculation come out on the low side so it sorta
cancels out. Also, assume that the bead takes up around 1/2" of the
wheel width on each side even though it's a little less, so the area
calculation of the wheel area will be a bit low. So the surface area of
the metal inside the tire is around 15*8.5*pi = 400 sq in, or about 5
times the sidewall area.

I haven't seen a racing tire up close and personal but if they are like
most other tires the bead/s run pretty close from side to side and it
doesn't appear that there is much metal not covered by rubber in most
wheels.

If you're using passenger cars as your example, you need to look at
today's larger and wider wheels mounted with very low profile tires.
They're closer to most racing wheel profiles. The area of the wheel
inside the tire is significant.

Anyway, I don't buy the "runs cooler" argument for nitrogen any more
than anyone should buy the "nitrogen expands less" nonsense.

I don't buy the "runs cooler" argument either. But I do buy the
argument that you can control the amount of moisture in the gas easier
if you're filling it with nitrogen then when plain compressed air.
There's no reason I can see that extremely dry compressed air shouldn't
work as good as nitrogen. But it may be cheaper and easier for the
teams to buy a tank of compressed nitrogen then to dry compressed air to
the same level of water content.

Steve