Does that make any sense? Basically, I measure the air pressure in my garage after the car has not been on the road--say for 24 hrs. Where I go to add air is approximately 3 miles from my house. How much should the pressure increase? How much is ambient temperature a factor, 95 vs 45 degree day? Thanks to you tire engineer types who know these kind of things. Nick

 

• n = number of moles
• R = universal gas constant = 8.3145 J/mol K
• N = number of molecules
• k = Boltzmann constant = 1.38066 x 10-23 J/K = 8.617385 x 10-5 eV/K
• k = R/NA
• NA = Avogadro's number = 6.0221 x 1023 /mol

This is what a physicist might tell you. The ideal gas law expresses the relationship between pressure, volume, and temperature of a gas.

Assuming that the volume of the tire is constant, your pressure after driving is related to the change in temperature, but of course you know that . .

So, what we need to do is calculate the pressure change that occurs when the temperature changes. Give me a few minutes and I'll have a number for you

 

unfortunately, the ideal gas law doesn't really apply when water moisture
is present in the mix. that's why those dreaded steam tables exist in the
back of thermodynamics texts for Mech Engr.

replacing the tire gas w/ nitrogen leads to more 'ideal gas' like behavior.

 

basically 1psi difference for 10 degrees change in ambient temp.

i recommend you just buy a air compressor ($25) and pump in your garage.

 

just add 2-3 more psi, come back home, let it sit overnight and bleed air next day.

 

Best idea for us non math majors!

regards,
Red

 

Was wondering if someone might point that out!

Plus, the ideal gas law won't work with a nitrogen inflated tire either, since the volume of a tire is related to the pressure because rubber tires are not rigid and the volume is affected by the weight of the car. . . .

Power outage due to the hurricane kept me from getting back to post some numbers, but as stated before, 1psi per 10'F is good enough of an estimate for most of us. . .

 

have a margin of error so just add accordingly for that error. For example mine differs by 5 so instead of leaving them at 35psi, I go to 40psi.

 

the dry nitrogen is so that as the tyre heats up due to rolling, a linear pressure increase occurs.
(tyres w/ 'wet' gas mixture heats up along a nonlinear curve and
will likely have a much higher pressure at the
same temperature than one filled w/ 'dry' nitrogen.)

btw, the old rule of thumb :
more than 1/4 mile of slow driving, the tyre ain't 'cold'

 

Nitrogen is the “only” way to go if your fanatical about tire pressures. Check-out the pit lane at any race track........

It is much more consistent (when heated) and your tire pressures will remain constant over a greater length of time.

I purchased my set up for the racing karts (running 12 psi) where +/- 1 psi is critical to grip and handling. I use nitrogen in all my vehicles and no longer have to “top-up-the-pressure" every couple of weeks. Best $300 spent.

And yes the Mini does handle like a kart....well sort of but the shifter kart is awhole lot more fun at 125 mph.

 

Check the air pressure when the tires are cold. Drive a goodly distance, until the tires are "hot." Check the air pressure again. Determine the difference between the two.

Next, decide what you want the cold air pressure to be, whether by reading the side of the tire, or some more involved method. Add the difference determined above to your ideal cold air pressure, and you will have the number you need for "hot" inflation at the air pump.

Or spend $20 or so at WalMart for a cheap air compressor, and fill your tires at home when they are cold. I got one when I ditched my run flats, and it comes in handy some times.

 

how do you fill up an inflated tire with nitrogen? Do you need one of those wheels with two valves or is there a special trick?

 

Never seen a wheel with two valves.

I just pop out the needle valve, release all that poo poo air, and fill 'er up with the good stuff.

 

ever heard of Kosei?

 

NAM is a great place. Great info. I got lost on the technical formulas--but hey, I still haven't figured out where the two trains will meet when 1 leaves Chicago at 1000AM going 80 MPH and the other leaves Denver--well you know the deal.

I think I am going to buy the compressor. Thanks again. Enjoy your journey. Nick

 

formulas...nitrogen...ow! ow! I've gone blind!

I got that cheap-o $15 compressor from WalMart. It takes more time to get the air in the tire than a fancy-schmancy compressor but I still get the right pressure.

 

yeh, air is free. free is good.

having an accurate (consistent) pressure guage is important too.
me? I use a pen type guage I bought years ago before stores
started to sell its crap versions.

 

lots of tyre places around this neck of the woods offer nitrogen fill.
(even the costco!!)