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Old 10-20-2015, 10:25 AM   #1
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Tire Pressure


I know that you are supposed to check your tire pressure when cold (before being driven). However, that got me wondering if the ambient air temperature has any significant effect upon the pressure reading. Would I see much difference between checking the tires at 45ļ, as opposed to 80ļ?

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Old 10-20-2015, 10:42 AM   #2
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In short, yes. A tire with say 50 psi on an 80 degree day may read closer to 45 psi on a 45 degree day. Not something that needs to be done on a day by day basis, but always a good idea to check and adjust when average temperatures start to swing with the seasons. You may have noticed that a lot of dealerships, tire stores, etc. offer nitrogen, and one of the purported benefits is that nitrogen is less affected by temperature change, so tire pressure should remain more constant.

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Old 10-20-2015, 02:50 PM   #3
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He's right. Nitrogen has a 'bigger bubble than regular air so it doesn't 'bleed through tire' and heat doesn't effect it as much as it does with air. Adjust tire pressure to the vehicle pressure setting, not what's on the tire. That's max pressure. Tire pressure will rise 4 or 5 lbs when warm through normal driving but follow manufacture specs. Most if not all want tire pressure adjustment at ambient temp (outside temp).
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Last edited by Brainbucket; 10-20-2015 at 02:52 PM.
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Old 10-20-2015, 03:06 PM   #4
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In addition to temperature, altitude can also have a significant effect on tire pressure.

http://www.tirerack.com/tires/tirete...jsp?techid=167
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Old 10-20-2015, 03:24 PM   #5
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Quote:
Originally Posted by Brainbucket View Post
He's right. Nitrogen has a 'bigger bubble than regular air so it doesn't 'bleed through tire' and heat doesn't effect it as much as it does with air. Adjust tire pressure to the vehicle pressure setting, not what's on the tire. That's max pressure. Tire pressure will rise 4 or 5 lbs when warm through normal driving but follow manufacture specs. Most if not all want tire pressure adjustment at ambient temp (outside temp).
Bigger bubble? Please explain.
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Old 10-20-2015, 04:52 PM   #6
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Larger molecules, I believe.
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Old 10-20-2015, 06:19 PM   #7
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Oh dear, here we go again.

There is a bit of mis-info on Nitrogen in tires.

First off, regular air is 78% Nitrogen to begin with.

As for the size of the molecule? No, the O2 is not leaking out because it's smaller.

The main advantage of N in tires is NO moisture. The expansion coefficient of Nitrogen vs air is negligible. Remember, air is 78% N. The real difference is standard shop air, even though it goes through a dryer still has moisture in it. As any who has worked on tires, it's not unusual to unmount a tire and find water in it.

About the ONLY real advantage of N over regular air is long term use. If you have a car that keeps it's tires for 5 or more years, the O in the air will eventually react with the rubber to create an oxide and cause the tire to potentially have a tread separation. Think Ford Explore/Firestone Tire debacle.

So...over time, the O2 in your tire reacts with the rubber.

And for those who claim the big pressure swings....look at your style of tire. If you are running one of those low profile tires, pressure changes are going to be much more obvious due to the smaller volume.
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Old 10-20-2015, 08:56 PM   #8
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Didn't mean to change the topic to nitrogen, and only tossed it out because if the OP does any further research on pressure changes they will most likely run across the topic at some time or another. And I did say "purported benefit" because, although it is promoted that way, or at least that seems to be the main thing they sell it on around here, I'm not so sure that it makes much difference either. Will add though that while I don't know how much change there is above a certain ambient temperature, so may or may not be much of a factor in AL or SOCAL, yes, here in the upper midwest, it does have a measurable affect. Temps are back up a bit this week, but, in the preceding weeks, we went from daytime highs in the low 80's to overnight lows in the high 20's, and I had to add air to all of our vehicle tires, including a 3/4 ton Ram and a Corvette, so it's not limited to any specific type of tire. The 'vette was down the most, about 6-8 psi, but the truck, three trailers, and the wife's car, were all down around 4-5 psi each. On the other hand, I have noticed that our fleet of about 20 vehicles at work are not as fussy, but makes sense because the temperature in that building is more regulated, so, by the time they set overnight, the temperature is more constant when they are checked each morning. Anyway, not something that I worry about, unless of course one is down appreciably more than the others, but, as I mentioned earlier, just something that I watch as the seasons change. Just routine maintenance. Anything close to typical this winter means that I will probably add air to everything but the 'vette again around Thanksgiving or so, and then go the other way mid to late March.
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Old 10-20-2015, 11:09 PM   #9
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I'm not buying the argument that nitrogen is less sensitive to temperature. The ideal gas law applies here, PV=nRT. Molecular weight (or volume) doesn't enter in to this.

Volume is essentially fixed, so pressure is proportional to temperature (Gay Lussac's law, as my inner physics teacher would say). Remember to add 1 atm (14.7 psi) to the gauge pressure that you measure and measure temperature in either Kelvin or Rankine.
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Old 10-20-2015, 11:51 PM   #10
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Quote:
Originally Posted by LanterDan View Post
I'm not buying the argument that nitrogen is less sensitive to temperature. The ideal gas law applies here, PV=nRT. Molecular weight (or volume) doesn't enter in to this.

Volume is essentially fixed, so pressure is proportional to temperature (Gay Lussac's law, as my inner physics teacher would say). Remember to add 1 atm (14.7 psi) to the gauge pressure that you measure and measure temperature in either Kelvin or Rankine.
Correct.....the difference between N and O is so small, it's not a factor. And when you factor in the ratio of N to O in our air, 3:1....even if there was a significant difference, it would have to be a lot to overcome the ratio difference.

The simple fact is....normal compressed air has moisture in it. That is what is causing the large changes in pressure.
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Old 10-21-2015, 09:01 AM   #11
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I was giving a GENERAL reason. NOW I know I have to give the scientific explanation next time. Didn't know this was a science class. So hear we go. Copied so...

Air is 78 percent nitrogen, just under 21 percent oxygen, and the rest is water vapor, CO2 and small concentrations of noble gases such as neon and argon. We can ignore the other gases.There are several compelling reasons to use pure nitrogen in tires.
First is that nitrogen is less likely to migrate through tire rubber than is oxygen, which means that your tire pressures will remain more stable over the long term. Racers figured out pretty quickly that tires filled with nitrogen rather than air also exhibit less pressure change with temperature swings. That means more consistent inflation pressures during a race as the tires heat up. And when you're tweaking a race car's handling with half-psi changes, that's important.

Passenger cars can also benefit from the more stable pressures. But there's more: Humidity (water) is a Bad Thing to have inside a tire. Water, present as a vapor or even as a liquid in a tire, causes more of a pressure change with temperature swings than dry air does. It also promotes corrosion of the steel or aluminum rim.
If I ever need to top off a tire when I'm out on the road, I'll always briefly depress the tire chuck's valve with my thumbnail and vent some air. If my thumb gets wet, there's water in the line. Some gas stations don't do a very good job of keeping the humidity out of their air system. I don't even like to use a water-based tire-mounting lubricant unless I can let the tire bake in the sun for a couple of hours before I air it up and seat the bead. I've dismounted tires (not mine) that had several quarts of water inside—probably from a compressed-air hose that collected water and was never purged properly.

How is water relevant to a nitrogen discussion? Any system that delivers pure nitrogen is also going to deliver dry nitrogen. Filling tires with nitrogen involves filling and purging several times in succession, serially diluting the concentration of oxygen in the tire. This will also remove any water.
It's certainly simple, although time-consuming, for a tire technician to fill and bleed tires. But most shops use a machine that not only generates almost pure nitrogen by straining the oxygen out of shop-compressed air, but will also automatically go through several purge cycles unattended. Some shops have been charging as much as $30 per tire for this service. I think that's too much. If you're buying a new tire, it should be far less. Still, the nitrogen generator, filling system and technician's time aren't free—the dealer is entitled to some return for that.
So, to answer your specific questions: With nitrogen, your tire pressures will remain more constant, saving you a small amount in fuel and tire-maintenance costs. There will be less moisture inside your tires, meaning less corrosion on your wheels. You will not be able to feel any difference in the ride or handling or braking, unless your tire pressures were seriously out of spec and changing to nitrogen brought them back to the proper numbers. This was copied. I already knew this but this fella did a better job explaining it. I'm a 38 years ASE re-certified master auto tech. This is all I have done since I was 9 yrs old. I'm not a cop pretending to be a auto tech, not a carpenter, (the only nail I can hit is my thump nail) not an English teacher as my grammar is awful, not a plumber even though I do some plumbing on vehicles. What I'm trying to say is this is all that I do and I do it well. So next time I come on here I will try do a better job trying to explain it. Sorry for the general explanation.
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Last edited by Brainbucket; 10-21-2015 at 09:10 AM.
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Old 10-21-2015, 10:41 AM   #12
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Here is a very good right up on the N2 vs O2 molecule.

http://www.getnitrogen.org/pdf/graham.pdf

Quote:
O2 "permeates" approximately 3-4 times faster than does N2 through a typical rubber, as is used in tires, primarily
because O2 has a slightly smaller effective molecular size than does N2.
Hence, this would explain the long term loss of pressure when using air.

But....it has nothing to do with pressure stability relative to temp. That is caused by moisture in the air.
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Old 10-21-2015, 12:03 PM   #13
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This is an interesting thread, not so much for the topic, but for the wildly different opinions and pseudo science used to "document" them. The previous comment in this thread extolling the virtues of nitrogen in tires was taken from a Popular Mechanics blog. For an opposite opinion that basically says nitrogen is a waste of money, see this article from Edmunds http://www.edmunds.com/car-care/shou...-nitrogen.html

So who are you going to believe, and what is your basis? Two relatively well respected publications reaching opposite opinions. Hard to know who to trust.
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Old 10-21-2015, 03:44 PM   #14
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Personally I'm just too lazy. I just do the regular air. I check air pressure every oil change or major temp swing or when my tire light comes on.
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Old 10-22-2015, 09:08 AM   #15
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Originally Posted by Brainbucket View Post
Personally I'm just too lazy. I just do the regular air. I check air pressure every oil change or major temp swing or when my tire light comes on.
Kind of like cleaning gutters!

I'm lucky I can just push a button on the dash and it tells me what the tire pressure is, they have to be about as accurate as the regular gauge that gets thrown around in the tool box.

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