Tire Pressure and Altitude

[Villagerjjm]

Originally Posted by milkman55

Thanks for the link. I Googled atmosphere change and all I got was scientific garbage.

All my six tires are down 6 lbs from where they were in Texas at near 100f. It’s been down to 41f here in Gunnison and it’s also raining lightly. Since all my tires, even the Toad are all down the same relative amount, I think I will not add any more air.

That "scientific garbage" that you refer to is a small part of the science of physics. The study of physics has revealed a lot about how the world in which we live works. Not all of us are capable of understanding it.

I have always been curious about my tire inflation rates. The recommended cold pressure is supposed to be 35 psi. When I drive and the tires get warmed up, the psi will increase to 40 psi in each tire whether I am towing my trailer or not. Why is there no difference in psi when pulling a trailer as to when I am not? There is a difference in HEAT. The tires are hotter when pulling a trailer as compared to when I am not. That comes from the tire shop mechanics.

[RV Randy]

Originally Posted by Larry-NC

Let's see. The atomic weight of an Oxygen molecule (two atoms, its natural state) is 32 grams/mole. The atomic weight of a Nitrogen molecule (two atoms, also its natural state) is 28 grams/mole which is lighter, but the difference really isn't significant. If smaller molecules were to diffuse more rapidly, Nitrogen would leave the tire faster than Oxygen, not slower.

As Mike suggests, a more interesting test would be to see how fast the trace of helium (second smallest molecule in nature) diffuses from a tire. We all know how fast (overnight) it diffuses through a party balloon.

Larry

We use nitrogen in racing tires because it is dry and does not have any moisture in it compared to ambient air. This, plus nitrogen's properties, minimizes how much the pressure increases as the tires become hotter. I am not aware of any benefit from nitrogen regarding leaking out faster or slower than normal air.

[babock]

Originally Posted by RV Randy

This, plus nitrogen's properties, minimizes how much the pressure increases as the tires become hotter

Nitrogen behaves exactly like any other gas according to the ideal gas law.

[RV Randy]

Originally Posted by babock

Nitrogen behaves exactly like any other gas according to the ideal gas law.

Thanks professor, if I ever decide to get a Masters in physics I'm sure that will be useful information. In the mean time, I can confirm that all of the pro racing teams I have supported use nitrogen in the tires on the race cars.

[BandJCarm]

Originally Posted by TexasGolfBum

What do we that have nitrogen filled tires do?

Put 78% nitrogen in them when needed.

[babock]

Originally Posted by RV Randy

Thanks professor, if I ever decide to get a Masters in physics I'm sure that will be useful information. In the mean time, I can confirm that all of the pro racing teams I have supported use nitrogen in the tires on the race cars.

No need. High school chemistry is all that's needed. Racers use nitrogen because it's dry, convenient and relatively inert. Yes, my brother in law is a race car driver and I use to work with the Toyota racing team.

PV=nRT works with any gas.

[milkman55]

Well if we ever want to know about Physics, potato chip bags or race car tires, this will be the thread to consult. Unfortunately it was about the impact of altitude on tires.

[BriaBeck]

Great thread!


Lots of good info, explains the "how" and "why"... also appears to shred some faulty understandings of what's going on

[RV Randy]

Originally Posted by milkman55

Well if we ever want to know about Physics, potato chip bags or race car tires, this will be the thread to consult. Unfortunately it was about the impact of altitude on tires.

Well said! I was simply trying to answer someone's question regarding the value of using nitrogen.

I live in Colorado and as a former cop I interfaced with hundreds of longhaul truckers, RV driver's, etc. I don't remember seeing large numbers of them with blown tires because they drove from flatland to mountains and back without wringing their hands about tire pressure changes daily due to elevation, ambient temperature, time of day, direct sunlight versus shade and so forth. If you just use a large dose of common sense, set your psi based on the weight/loading of the vehicle you will be fine. The tires are designed to account for these minor fluctuations due to all these factors and still perform just fine

[dunk05]

Went from4800. In Albuquerque to South Dakota around 1400 saw slight change. Temp about same.

[Stetay]

We went to Banff Canada from so cal, mine went from 50 to 65 lbs

[larryandamy]

Originally Posted by milkman55

How much decrease in air pressure would you expect with an increase of 7,000 ft in altitude from where you filled them cold?

I had 61 lbs in the front and 63 lbs in the rear when we left So Texas in 90 degree heat. Now at 7700 ft and 50 degrees and pressure reads 55.

I now the temp has an impact, but altitude also?

It is the temperature change that impacts the tire pressure. The altitude is not a factor as it it compensated for by the guage.

[mrfikser]

Ok folks here is how it works...a steel cylinder filled with 50psi at sea level will still have 50 psi in outer space...the 50psi is the amount of pressure the substance in the container exerts against its container, a tire is not a steel cylinder so it will expand slightly when going higher and the outside air pressure pushing against outside of the tire is reduced, how much the tire expands is relative to the way it was built, the more it expands the more the internal air pressure will drop. Tires are pretty rigid these days and I would imagine not expand more than a percent or 2, so air pressure drop inside the tire will be negligible....as far as using nitrogen in tires, its use has almost no change over plain old air...air is slready about 78% nitrogen so you are only adding about 10% more of it when fill with pure N (it is actually a fair amount less N because the tires are already full of air before you add the N, i.e., you are not adding it to a vacuum)...some folks say that the pure N is better because 1. it is a larger molecule and thus has a harder time squeezing out between the rubber molecules. 2. the lack of oxygen helps stop the breaking down of the rubber 3. that N is dryer then air, helping to prevent rusting of the rims...all three have been debunked by many separate testing companies...while there is a very slight validity to all three reasons, even all together it does not make enough of a difference to change the life expectancy of the tires or rims..

[WolfWhistle]

Did you know a 12 hour clock that doesn't run is absolutely perfect twice a day? And, its unlikely a running clock is every perfect?? Sorry, I digress.

Another thing... who has an air pressure gauge that is calibrated and traceable back to The NIST? I have a half dozen gauges and they all read differently... even the TST-507 can't agree. Oh, as a side note... that TPMS is the greatest thing created even considering sliced bread. I traveled 12,000 miles and when I got back home my tires were 1 lb under inflated, actual and not temperature compensated. I didn't add any air the entire trip. As I watched pressure start low then go up, I figured with such a wide swing in pressure it didn't matter. Tires look good, momma's happy... I'm happy.

WW

[de_runner]

Originally Posted by Joe_GA

Altitude does not change the pressure of a gas (air) in a closed container. According to Boyle's Law, pressure in a closed container changes if:

1.) temperature changes
2.) number of molecules increases or decreases
3.) volume changes

In other words, temperature and pressure of a gas in a closed container are directly proportional. If the temperature decreases, the pressure decreases and vice versa. Yes, it is true that atmospherical air pressure decreases as altitude increases, but that is air that is not contained in a closed container.

Yup that is correct. A tire if sealed properly is a closed container. A change in temperature will result in a change in pressure. This would explain the change in pressure if any. The volume of the container, i.e., the tire might change some since it is comprised of an elastic structure. However, I would expect the delta vol to be comparatively small. While the tire stucture is rubber it's elasticity is small compared to a balloon.

[mrfikser]

1 other thing that I feel is worth mentioning is that the Min pressure stamped on the tire is not the maximum pressure that the tire can be run at and that if you are going to be traveling faster than (I believe) 66mph it is recommended that you add 10 more psi to the printed psi on the tire, but still don't exceed the speed rating for the tire..

[Eagleslink]

Nitrogen is an inert gas and should not react to small temperature and pressure changes. Primarily used in aircraft tires.

[JackRoth]

We live at sea level and just went over 10,000 foot mountain passes. The difference in tire pressure from altitude change is less than the inaccuracy of what is being used to measure your tire pressure.

Keep it simple, fill the tires to the recommended pressure when the tires are cold. Tire pressure ratings are measured at 70F, there is about a 1PSI change for every 5F change so at 50F you may wish to underfill by 4PSI. Pressure increases as temp increases.

As far as attitude, air pressure is about 15PSI at sea level, at 10,000 ft it's about 10PSI, the difference is reduced by tire strength. Tire temperature has a much greater effect than air pressure.

[Larry-NC]

Originally Posted by Eagleslink

Nitrogen is an inert gas and should not react to small temperature and pressure changes. Primarily used in aircraft tires.

Umm,no, nitrogen is not an inert gas.

Stop reading here if you don't believe in science.

An inert element is one which has not been observed to form compounds with other elements. The elements generally considered to be inert gases are those in Group 18 (last column) of the Periodic Table, namely

• Helium
• Neon
• Argon
• Krypton
• Xenon
• Radon
• Oganesson

All of these were originally thought to be "inert", sometimes called the "noble gases" because they wouldn't associate with other elements. Since the 1960s it has been found that some of these elements would combine under unusual conditions, often with fluorine, a very active element, in conjunction with pressure, temperature, and/or sunlight. Of these elements, only Helium and Neon have not been found to form compounds.

Nitrogen, on the other hand, combines easily to form stable compounds. It's not inert at all.

How about the absorption refrigerator in your trailer? It's simply gaseous ammonia, NH3, nitrogen and hydrogen.

Ever wash your trailer windows with an ammonia-based glass cleaner? That's NH3 in H2O (water).

Nitrous oxide (laughing gas) used to eliminate pain during dental treatment?

TNT, the explosive? It's Trinitrotoluene, C6H2(NO2)3CH3.

Definitely not inert.

Larry

[Eidolon]

Nitrogen gas is actually pretty much non-reactive in most settings, such as say - a tire. No, it is not a noble gas, but it is far less reactive than oxygen.


In the context of this forum, it is basically inert.