Mark Browne wrote:

I believe that what you are neatly trying to side-step in your consideration
is the presence of liquid water. If all we were talking about is water
vapor, even at 100% humidity, then I would completely agree that you are
right.

Unfortunately there *can* be liquid water trapped inside the tire. Some of
this comes from tire mounting compound, some from air compressors without
suitable dryers, some from water inside the tire. This trapped water inside
the tire can be standing on the surface, or inside the rubber.

Wasn't this discussion about using nitrogen in the tires? If the racers
are so particular why are they using wet air from a cheap air compressor
and still worry about the effects of moisture? Why are they using
practices and processes that are known to add worrisome quantities of
liquid water to a component that is so sensitive to moisture?

Which way do you guys want this? Perfectly predictable tire pressures
will be impossible to obtain when the mounting is done with shade tree
techniques as you describe.

If there is so much water in a tire despite the best efforts of "real
physicists and engineers on their staff (who)go to considerable effort
to control the presence of water inside the tire" then something is
missing in this equation.

In a Formula or NASCAR setting moisture can raise tire pressure
about 4 PSI in the corners.

I find this a bit hard to believe. If it is water vapor it will
respond exactly like the nitrogen or air and the pressure rise is due to
the temperature increase of the tire. That is one of the gas laws that
no one seems to want to follow. If there is liquid water in the tire a
whole new set of conditions exist that are still unlikely to produce the
effect you describe.

Look at it another way. If you can attribute that 4 psi increase to
moisture then why not control the amount of moisture in the tire ... dry
the tire and add a measured amount of water so that you can predict the
pressure increase and regulate accordingly? But if you can dry the tire
then why worry about moisture? If you cannot dry the tire then you can
measure the dew point of the filling gas and calculate the weight of the
water in the tire and predict accordingly. It sounds to me like you guys
are just using "rules of thumb" and are surprised when you get surprised.

What is the normal tire pressure on one of those cars? Thirty - forty
psig? Let's say the tire started out at 35 psi, for the water to boil
away and increase the pressure of the tire to 39 psi its temperature
would have to increase to somewhere around 285 degrees F. Do you run
your tires at those temperatures? For the tires to increase 4 psi in the
second or two of cornering due to boiling water the carcass temperature
of the tire would have to be damn near glowing since heat does not
transfer instantaneously to the water, nor does it cool instantaneously
as the car leaves the corner.

If the starting temperature and pressure of the tire in the pits was 35
psig at 80 degrees F and it heated up to 200 degrees in the corner its
pressure would raise to around 46 psig ... at that pressure any water
would still be water until it reached over 290 degrees ...

I have not even mentioned the fact that the heat to vaporize the water
comes from the tire and the other gases filling the tire. The transfer
of that heat actually cools the tire and the gas ... a minute amount,
yes, but the effect of water has to be an equally minute amount and I
have yet to be shown the mechanism whereby "normal" amounts of water in
a tire will produce the effect you describe in the conditions in which
tires operate.

If you can explain how race car tires somehow work differently than
other machines I would love to hear it. I am very open minded but this
smacks of voodoo engineering to me.

Rick