City-Data Forum I Give Up! RE: Tire Pressure
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Yesterday, 08:09 AM
 10,823 posts, read 7,892,645 times Reputation: 19258

I follow directions. Makes life easier for me.

Yesterday, 09:00 AM
 4,216 posts, read 1,812,117 times Reputation: 11744
Yeah, but all that science stuff is just your opinion. You can choose to believe that if you want, I'm going to go with what some guy told me down at the local bar.

(sarcasm, sarcasm)

Yesterday, 01:08 PM
 Location: Davidson, NC 110 posts, read 53,137 times Reputation: 269
Quote:
 Originally Posted by Gungnir Neither really. Gas constant of nitrogen is 9 J/Kg K higher than air. However the molar mass of Nitrogen gas is 28g, and air is 28.9g. pV =nRT p = Pressure V = Volume n = number of moles R = gas constant T = Temperature in Kelvins Presuming both tires are the same volume, and at stp. Pressure is proportional to the number of moles (particles) of the substance and its gas constant. Thus the gain of 9J/Kg K is offset by the increased numbers of particles needed for a given volume and temperature. In fact the 9 J/Kg K nitrogen gains is against 287 J/Kg K so nitrogen only gains approx 4% versus air in heat absorption, but air loses 4% on quantity needed. Next to moisture, what happens to water vapor on air compression? Ever wonder why the drain plug on a compressor squirts water? The amount of water vapor in compressed air is negligible, certainly less worrisome than a TPMS sensor sitting on a shelf at your tire store. That's a red herring (although you need some insight to realize it). So yes it's a scam, though I'm not even sure the suppliers know it's a scam.
My insight is 20 years in professional motorsports and seeing first hand the difference from air out of a compressor (like most home garages or fuel stations have and I’m assuming most people are using to top up their tires. Are people really going to tire stores or mechanics for air? That makes me sad if so.) to dry nitrogen out of a bottle. Now grant it this an extreme situation where 0.5psi differences do actually effect the handling of the car given it spends most of its time on the travel limiters for the suspension so, the tire is basically functioning as the spring.

The amount of moisture in compressed air may not be much but, the amount that can accumulate in a compressor tank that is cycling through compressing, cooling, decompressing can be significant. It's the unknown of how much moisture are you introducing to the tire that's problematic for pressure gain.

https://www.popularmechanics.com/car...ogen-in-tires/

Again not necessary for typical street use but, if you are a car person (and presumably you are if you are in the car section) and there is a better and more consistent way to do something, why not do it? Of course I also drive pretty spiritedly and don't view my car as an appliance to get me from point A to B so my views might be weighted a bit differently than your typical driver.

I will agree the pricing on a nitrogen can be a scam. At \$35 for nitrogen purge fill, ok. \$200, yeah that is a scam.

Last edited by hotrod2448; Yesterday at 01:35 PM..

Yesterday, 01:53 PM
 Location: Itinerant 5,652 posts, read 3,932,050 times Reputation: 4341
Quote:
 Originally Posted by hotrod2448 My insight is 20 years in professional motorsports and seeing first hand the difference from air out of a compressor (like most home garages or fuel stations have and I’m assuming most people are using to top up their tires) to dry nitrogen out of a bottle. Now grant it this an extreme situation where 0.5psi differences do actually effect the handling of the car given it spends most of its time on the travel limiters for the suspension so, the tire is basically functioning as the spring. The amount of moisture in compressed air may not be much but, the amount that can accumulate in a compressor tank that is cycling through compressing, cooling, decompressing can be significant. It's the unknown of how much moisture are you introducing to the tire that's problematic for pressure gain. https://www.popularmechanics.com/car...ogen-in-tires/ Again not necessary but, if there is a better and more consistent way to do something, why not do it?
However we're not discussing track, just general motoring, how many times has your general motoring car had 212F centre tread temps? You're also familiar that a racing tire has very little in common with a sport radial you put on a road car, yes? I could get maybe 100-120 miles from a set of slicks, I'm disappointed to get less than 10k from a set of road tires (unless I've been knowingly burning them).

Now I will give you Nitrogen has a marginally larger van der Waals radius, 155pm to oxygen 153pm (picometers). Air does leak at a marginally higher rate, two tires with 30psi lost 3.5 psi with air to 2.2 psi with N2 over 12 months, whoopdy doo. However, if the issue is oxygen, what you lose is more O2 than N2, and you're refilling with 78% N2.

To explain, if you have a tire filled with air, 78% N2, 20% O2 (because its reacted with the rim forming Al2O3 or the tire), 2% other gases. Ok you lose 20% pressure say, let's assume it's all the O2, ok you now have a tire filled with 78% N2, 2% other gases, you pump more air in so that 20% (only) is exactly the same air comp, but only 20% O2, so now the tire is 5% O2, 2% other gases, and 93% N2. Unless you purge that tire it's going to progressively approach 98% N2, 2% other gases. Now conversely unless you purge the tire, you gain nothing from inflating with nitrogen, because you're pushing in maybe 6 cubic inches of pure N2 (at stp) into 5 gallons of air (at stp).

Water is almost nothing, at worst you're looking at 0.14% of the total gas volume (sea level water ratio of atmosphere), presuming you don't use a tank compressor, nor any scrubber on the output. Best case is dry air, so no difference in H2O than tank N2.

The cheapest consistent way to do something is check your tires regularly, use a tank compressor that's regularly drained and dried, with a regulator and dessicant cartridge. How hard is that? If you're not even bothering to check your tire pressures weekly, you don't care enough to know what the very marginal benefits (if they're real in road cars) are with N2.
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Yesterday, 09:17 PM
 Location: Myrtle Creek, Oregon 11,551 posts, read 11,908,621 times Reputation: 18014
Quote:
 Originally Posted by cvetters63 Upping the pressure a bit improves handling by decreasing sidewall flex and the resulting tread lift on the inside of the tire under cornering, making the tires more responsive and lets the tread do more and better work. But there is a point at which you are overinflating the tires for the load and will ride mostly on the center of the tread wearing out your tires faster in the center. And of course it rides harsher as there is little sidewall flex to cushion the ride. The max pressure on the sidewall of the tire is for the max load that tire will be carrying, which tends to be about 1300-1600lbs per tire for typical passenger car tires, or 5000+ lbs per 4 tires. If your car and it's passengers/cargo don't weigh 5000+lbs then don't inflate to max. Light truck tires run higher pressures, especially when you get to load range E tires like I have on my tow rig, which are designed to be run at 60+psi to carry the loads that the truck is designed to carry.
Most people do not own an air compressor, and wouldn't think of adjusting tire pressure to load if they did. For groceries, 32 psi is just fine. If you are loading six 150 lb. teenagers into a mini-van, 32 psi is a recipe for disaster. It's safer if they put a few extra pounds in the tire and accept the rougher ride.

Today, 05:53 AM
 Location: Cleaning CAT VOMIT out of radiators 1,990 posts, read 716,591 times Reputation: 1600
Quote:
 Originally Posted by Larry Caldwell Most people do not own an air compressor, and wouldn't think of adjusting tire pressure to load if they did. For groceries, 32 psi is just fine. If you are loading six 150 lb. teenagers into a mini-van, 32 psi is a recipe for disaster. It's safer if they put a few extra pounds in the tire and accept the rougher ride.
Acc. to tirepressure.com, for a typical late model Grand Caravan, 225/65-R17 OEM, the specified F/R cold pressure = 36psi. And a suspension engineered and tuned accordingly for a smooth ride at that pressure. For a recent Sienna, 35psi cold.

So I think the vehicle mfgs are doing their homework, and won't repeat the same mistakes as Ford, compensating for a high C/G(center of grav.) plus ancient suspension with 1960s tire pressures, as they did on the inaugural Explorer.
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