Nitrogen in Tires
Elder User
Joined: Dec 2008
Posts: 745
Likes: 4
From: North Carolina
Nitrogen gas may be inert, but this has nothing to do with isentropic expansion. The expansion of nitrogen in your tires can be modeled the same as air using the ideal gas law, since at the temperatures and pressures in your tires, both air and straight nitrogen behave as an ideal gas.
PV=nRT
ALSO, just FYI, I just did a quick calculation and it seams that if you place a tire on a rim and then fill the tire with 100% N2 to 50 psig you will have about 95-96% N2 and 4-5% other gasses from the air that was in the tire before you added the nitrogen.
Elder User
Joined: Dec 2008
Posts: 745
Likes: 4
From: North Carolina
Close, but i think the most relevant law is the "third gas law" which states that the pressure is proportional to temperature.
Boyle's law states that where temperature is constant the volume of a gas is inversely proportional to the pressure.
Charles law states that volume is proportional to temperature.
All three of these laws are just derived from the ideal gas law.
Boyle's law states that where temperature is constant the volume of a gas is inversely proportional to the pressure.
Charles law states that volume is proportional to temperature.
All three of these laws are just derived from the ideal gas law.
Senior User
Joined: Sep 2005
Posts: 116
Likes: 2
From: Flagstaff
Close, but i think the most relevant law is the "third gas law" which states that the pressure is proportional to temperature.
Boyle's law states that where temperature is constant the volume of a gas is inversely proportional to the pressure.
Charles law states that volume is proportional to temperature.
All three of these laws are just derived from the ideal gas law.
Boyle's law states that where temperature is constant the volume of a gas is inversely proportional to the pressure.
Charles law states that volume is proportional to temperature.
All three of these laws are just derived from the ideal gas law.
You are correct, I should have posted "read up on Ideal Gas Law PV=nRT". My "trade" school (GaTech) education was a long time ago.
Senior User
Joined: Dec 2009
Posts: 128
Likes: 0
Not only that, but while you're topping off you can make your voice sound funny!
Postmaster
Joined: Jun 2008
Posts: 2,564
Likes: 11
From: Wayyy NoCal, USA
We used helium (like up to 200 psi) on some of our human powered vehicles during top speed runs.
Had to re-fill every run. Reason: Helium molecules are so small they pass through rubber.
That was the original reason for using nitrogen in tires, to reduce rate of leakage.
But it is 99% marketing. And some would say bad marketing, as it causes many not-so-educated folks to think they never have to check tire pressures. As use becomes more common, I bet we keep seeing more tire failures/blowouts/rollovers.
Had to re-fill every run. Reason: Helium molecules are so small they pass through rubber.
That was the original reason for using nitrogen in tires, to reduce rate of leakage.
But it is 99% marketing. And some would say bad marketing, as it causes many not-so-educated folks to think they never have to check tire pressures. As use becomes more common, I bet we keep seeing more tire failures/blowouts/rollovers.
Post Fiend
Joined: Jul 2005
Posts: 6,127
Likes: 6
Nitrogen gas may be inert,
ALSO, just FYI, I just did a quick calculation and it seams that if you place a tire on a rim and then fill the tire with 100% N2 to 50 psig you will have about 95-96% N2 and 4-5% other gasses from the air that was in the tire before you added the nitrogen.
ALSO, just FYI, I just did a quick calculation and it seams that if you place a tire on a rim and then fill the tire with 100% N2 to 50 psig you will have about 95-96% N2 and 4-5% other gasses from the air that was in the tire before you added the nitrogen.
Touche!
That is, unless you filled the tire / rim in a chamber that is purged with nitrogen!
The real benefit of filling with Nitrogen is that it is relatively inert.
Heat it up enough, and it is reactive, but much less so than Oxygen.
Furthermore, since N2 is normally purchased from a gas vendor, it is likely to contain very little moisture vs. normal compressed air, which is often filled to the saturation capacity of compressed air at its current pressure --- that is why all compressed air tanks have a water drain.
If you were to fill with dry air, much of the benefits of a Nitrogen fill are captured.
True, there is a small benefit in having a relatively more inert gas, but that is minor --- Oxygen will diffuse into the rubber from the outside, especially the tire damaging / rotting O3. (Ozone).
The bottom line --- if it came filled with nitrogen for free, sure!
But don't sweat too much or spend much trying to maintain it by only filling with Nitrogen.
Properly inflated tires is your best deal.
Post Fiend
Joined: Jul 2005
Posts: 6,127
Likes: 6
A regular service station fill.... according to wise guy calculation above.. leaves 4-5% O2 in the tire unless the tire / rim were filled in a Nitrogen gas purged enclosure.
So a nitrogen fill goes from 78% nitrogen with regular air to maybe 95%.
Now, if that benefit is for free... .sure!
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Post Fiend
Joined: Jul 2005
Posts: 6,127
Likes: 6
Do an estimate of the weight of the helium gas compressed (remember you have to overfill it a bit because of the rapid leak down)....
vs.
The weight of ordinary air.
Then see if there is any difference in displacement.
Ask yourself if you can make as much of a difference visiting the John before going off on your ride....
Postmaster
Joined: Oct 2009
Posts: 3,432
Likes: 7
Actually, when a tire heats up under severe stresses, the oxygen in the air inside the tire can more readily react with the rubber, causing it to become harder and more brittle over time. Under normal wear and tear, this can weaken the overall structure of the tire. This may seem insignificant, but how many blowouts occur in theis country, both from underinflated tires (air leaks out, nitrogen is less prone to this) and due to the rubber breaking down and failing? I think its a hard case to settle, but before arguing that regular air is fine, I think we should consider that there are problems that occur with normal air. I'm not sure how much longer a tire can hold up with nitrogen filled tire, but I do know that for vehicles that are driven infrequently, so that the tires get old but do not see a lot of use, nitrogen has some very strong advantages. It can keep the rubber supple longer and decrease the chances that the tire will be flat when you go to use the vehicle. For everyday use, harder call.
Cargo Master
Joined: Sep 2007
Posts: 2,624
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From: OR and UT
All this talk about Oxygen vs. Nitrogen and not one of you has mentioned how often you need to change the air (or nitrogen) in tires!
I know some say every 3,000 miles is "cheap insurance" while others say the manufacturers recommended interval of 7,500 miles is fine considering the tighter manufacturing tolerances of modern tires and wheels. Then of course you have the die-hard fans of synthetic air who say they only have to change the air in their tires every 15,000 miles (talk about drinking the Kool-aid).
Don't even get me started on whether an oiled cotton (K&N style) air filter is better than a paper air filter when it comes to actually inflating the tires!!!
I know some say every 3,000 miles is "cheap insurance" while others say the manufacturers recommended interval of 7,500 miles is fine considering the tighter manufacturing tolerances of modern tires and wheels. Then of course you have the die-hard fans of synthetic air who say they only have to change the air in their tires every 15,000 miles (talk about drinking the Kool-aid).
Don't even get me started on whether an oiled cotton (K&N style) air filter is better than a paper air filter when it comes to actually inflating the tires!!!
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Cargo Master
Joined: Mar 2006
Posts: 2,502
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From: Bend Oregon
All this talk about Oxygen vs. Nitrogen and not one of you has mentioned how often you need to change the air (or nitrogen) in tires!
I know some say every 3,000 miles is "cheap insurance" while others say the manufacturers recommended interval of 7,500 miles is fine considering the tighter manufacturing tolerances of modern tires and wheels. Then of course you have the die-hard fans of synthetic air who say they only have to change the air in their tires every 15,000 miles (talk about drinking the Kool-aid).
Don't even get me started on whether an oiled cotton (K&N style) air filter is better than a paper air filter when it comes to actually inflating the tires!!!
I know some say every 3,000 miles is "cheap insurance" while others say the manufacturers recommended interval of 7,500 miles is fine considering the tighter manufacturing tolerances of modern tires and wheels. Then of course you have the die-hard fans of synthetic air who say they only have to change the air in their tires every 15,000 miles (talk about drinking the Kool-aid).
Don't even get me started on whether an oiled cotton (K&N style) air filter is better than a paper air filter when it comes to actually inflating the tires!!!
Post Fiend
Joined: Jul 2005
Posts: 6,127
Likes: 6
Did you hear about putting an air filter to clean the air inside each tire continuously?
Get all that metal flakes, dirt, grit, and sand left inside the tire cleaned so it does not damage the tire!
Then there is the CCV mod, where CCV gases are directed away from the tires by a long hose extending 5 ft behind the tailgate.
Oh, don't forget, there is the high voltage mod for the TPS system to raise their transmitter output so you never miss a low tire pressure warning.