Hmmm. It's been a long time since I studied the theory, but as I recall
the ideal gas law refers to a dry, ideal gas, which air is not. If you
wanted to really calculate this change (and I don't) you would need to
compute partial pressures for each gas and add them all together. Then,
since air always has a substantial amount of water vapor in it you'd still
be fairly far off since water vapor acts differently than gas.
It is kind of entertaining to think about this after all these years of
ignoring physics (and chemistry)
-----Original Message-----
From: BillDentin@aol.com [mailto:BillDentin@aol.com]
Sent: Tuesday, July 23, 2002 6:14 AM
To: cartravel@pobox.com
Cc: fot@autox.team.net
Subject: Re: Tyre Pressure/Temperature
In a message dated 07/23/2002 7:43:54 AM Central Daylight Time,
cartravel@pobox.com writes:
> Subj:Re: Tyre Pressure/Temperature
> Date:07/23/2002 7:43:54 AM Central Daylight Time
> From:<A HREF="mailto:cartravel@pobox.com";>cartravel@pobox.com</A>
> CC:<A HREF="mailto:fot@autox.team.net";>fot@autox.team.net</A>
> Sent from the Internet
>
>
>
> Ok, I tried to be quiet, but my chemical engineering background won't
> let
> me.
> Air in your tires (or nitrogen, argon, hydrogen, hellium, whatever)
obeys
> the
> ideal gas law, PV=nRT. The tire volume is fixed, so the absolute
pressure
> increases in direct proportion to the absolute temperature. Let's say
you
> have 20 psi in your tires at 70 degrees F. The absolute temperature is
70
> +
> 460 = 530 degrees R and at sea level the absolute pressure is 20 +
> 14.7 = 34.7. If the temperature drops to 50 deg F (510 deg R), the
> pressure will drop to 34.7(510/530) - 14.7 = 18.7 psi. If the
> temperature increases to 130 deg F (590 deg R) during a race, the
> pressure goes to 34.7(590/530) - 14.7 =
> 23.9 psi.
>
Amici:
This stuff is great. I am memorizing it verbatim, and plan to use it a
cocktail parties. While I am totally unqualified, I love to talk Hi Tech.
Can anyone tell me how VIAGRA works. Someone told me it was 2% nitrogen,
.5%
argon, 3% hydrogen, 6% helium and 88.5% Fix-a-Flat.
Seriously, I appreciate this thread. We've been playing with tire
pressures
for years, and while we look really cool with our clip boards, pyrometers,
and tire gauges.....truth be told..... we're still just 'playing with tire
pressures'.
Years ago the big muckity muck in BFG'S Competition Department (I forget
his
name), lectured me on tire pressures for about twenty minutes, and then
said,
"In the final analysis it is VERY PERSONAL. You have to run what feels
good
for you."
I think the latter is especially true at our level of competition.
FORMULA
ONE we ain't.
Bill Dentinger
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