Tire Pressure Puzzler

[TravelingMatt]

No, air has the same composition everywhere, with the only difference being how many molecules of it occupy a given space. The tire doesn't care where you put air into it. You're still just adding molecules into a fixed volume. It can come from a compressed air canister which has a pressure way more than 14.7 psi.

[LURE]

then the only way to get to a correct reading of softness, relative to the manufacturer's recommended 35 psi, would be to replace the air inside the tires with Breck air.

I don't see how this could be. The pressure in the tire will certainly change as the atmospheric conditions change, but to get the recommended pressure as your atmosphere changes, you just need to adjust the pressure accordingly, whether by adding air or losing some. Evacuating and refilling seems an unnecessary step.

Am I missing something? This seems to be getting overly complicated.

[Buckshot Jake]

Air pressure has more to do with heat than atmospheric pressure as some have alluded to. You aired down before you left, which I don't know why. Ideally, you would want to air UP before you left Houston so as to maximize your gas mileage. Anywhere from 38-42 for OEM Jeep tires. But anyway, as you were driving across the hot Texas desert at 80MPH your tires were naturally increasing in PSI, which explains the 35 when you landed in town. I track my Honda and it's the same concept, after every lap I need to air DOWN because the heat from the track has increased my tire pressure. Literally after every lap. Think about here in Colorado too, every fall tires out here begin to look defalted and we need to go get air. Naturally, the air temps are dropping.

Next time, air down at the trailhead, and as someone previously mentioned you are well safe going under 30 for offroad conditions. For OEM Jeep tires though I wouldn't go less than 25.

[LURE]

Air pressure has more to do with heat than atmospheric pressure as some have alluded to. You aired down before you left, which I don't know why. Ideally, you would want to air UP before you left Houston so as to maximize your gas mileage. Anywhere from 38-42 for OEM Jeep tires. But anyway, as you were driving across the hot Texas desert at 80MPH your tires were naturally increasing in PSI, which explains the 35 when you landed in town. I track my Honda and it's the same concept, after every lap I need to air DOWN because the heat from the track has increased my tire pressure. Literally after every lap. Think about here in Colorado too, every fall tires out here begin to look defalted and we need to go get air. Naturally, the air temps are dropping.

Next time, air down at the trailhead, and as someone previously mentioned you are well safe going under 30 for offroad conditions. For OEM Jeep tires though I wouldn't go less than 25.

I feel that a manufacturer suggested pressure would account for typical highway heat. I don't think you're average driver needs to be releasing air everytime they stop at a gas station on their road trip, unless you live in Canada and are runnin through Death Valley or other hotbeds of the US. Running laps at the local track is hardly comparable to daily driving, even offroading. For most of us average users I still feel this is overly complicated. Check your gages, release or fill up as necessary if your pushing far too low or far too high.

[Buckshot Jake]

I feel that a manufacturer suggested pressure would account for typical highway heat. I don't think you're average driver needs to be releasing air everytime they stop at a gas station on their road trip, unless you live in Canada and are runnin through Death Valley or other hotbeds of the US. Running laps at the local track is hardly comparable to daily driving, even offroading. For most of us average users I still feel this is overly complicated. Check your gages, release or fill up as necessary if your pushing far too low or far too high.

LOL, did you think I was under the impression they would just continue to naturally air up until they explode? It will level off eventually based on vehicle weight too. OEM Jeep tires are right around 35PSI. I know track conditions are different, but the concept is similar. I also have a 4x4 and air down/up quite frequently and log it. As I mentioned in my first post, air down at the TH, not before you leave for your road trip. That is really the point I want to emphasis and don't want to get lost in the arguments.

[Herbert]

TravelingMatt wrote:
No, air has the same composition everywhere, with the only difference being how many molecules of it occupy a given space. The tire doesn't care where you put air into it. You're still just adding molecules into a fixed volume. It can come from a compressed air canister which has a pressure way more than 14.7 psi.

So we know that there are the same number of molecules of air in the tire in Houston as in Breck. We also know that when we compressed those molecules to a reading of 32 psi in Houston we were 3 lbs "softer" than Jeep's recommended 35 psi. Those same molecules of air produce a reading of 35 psi in Breck. However, I think the tread remained 3 lbs softer than Jeep's recommended pressure. I think if I had aired those molecules down to a reading in Breck of 32 lbs, I would have been 6 lbs softer, not 3 lbs softer, than the 35 psi that Jeep recommends. If that is true, then I would need to replace the Houston air in the tires with Breck air. If i compressed 10.2 psi air in Breckenridge to 32 lbs in, that should be the equivalent of compressing 14.76 air in Houston to 32 pounds, i.e,, same number of molecules and the same relative "softness" in each case.

[LURE]

TravelingMatt wrote:
No, air has the same composition everywhere, with the only difference being how many molecules of it occupy a given space. The tire doesn't care where you put air into it. You're still just adding molecules into a fixed volume. It can come from a compressed air canister which has a pressure way more than 14.7 psi.

So we know that there are the same number of molecules of air in the tire in Houston as in Breck. We also know that when we compressed those molecules to a reading of 32 psi in Houston we were 3 lbs "softer" than Jeep's recommended 35 psi. Those same molecules of air produce a reading of 35 psi in Breck. However, I think the tread remained 3 lbs softer than Jeep's recommended pressure. I think if I had aired those molecules down to a reading in Breck of 32 lbs, I would have been 6 lbs softer, not 3 lbs softer, than the 35 psi that Jeep recommends. If that is true, then I would need to replace the Houston air in the tires with Breck air. If i compressed 10.2 psi air in Breckenridge to 32 lbs in, that should be the equivalent of compressing 14.76 air in Houston to 32 pounds, i.e,, same number of molecules and the same relative "softness" in each case.

32 is 32

35 is 35

[XPLSV]

The assumption that volume is constant inside the tire is not theoretically correct. As the atmospheric pressure decreases, there is potential for thee internal pressure to force expansion of the tire. This is likely something we would never observe with a typical tire gauge when measuring between sea level and Breckinridge, but there is likely a small expansion. An well known example is weather balloons: they have a specific number of helium molecules when released in a rather deflated state. As the balloon rises higher in the atmosphere, the pressure differential between the internal helium and external atmosphere changes, with the balloon expanding, thus causing an internal increase in volume and decrease in pressure, assuming a constant temperature and the constant miles of gas. Same type of situation kills a rockfish when you haul it up from a 200 foot depth to the surface and it’s swim bladder expands rapidly.

Yes, temperature will play a part, too.

For our purposes, we are fine with assuming a constant volume in the tire for our measurements (negligible contribution).

[TravelingMatt]

So we know that there are the same number of molecules of air in the tire in Houston as in Breck.

Whew, at least we're getting somewhere.

We also know that when we compressed those molecules to a reading of 32 psi in Houston we were 3 lbs "softer" than Jeep's recommended 35 psi.

You didn't "compress molecules" like they do at Fermilab or CERN or wherever. And you didn't even "compress" the air; you released air in reducing the gauge pressure from 35 to 32 psig. The actual pressure in the tire in Houston is now 32 + 14.7 = 46.7 psia. (We use "psig" and "psia" for gauge pressure and actual pressure, respectively. Yay, I'm finally using freshman year engineering classes.)

As long as the temperature of the tire is maintained, the pressure inside the tire will be 46.7 psia. Yes, tires do change in volume, but at the range of pressures used for normal driving this is negligible.

Those same molecules of air produce a reading of 35 psi in Breck.

Yes, 35 psig. That's because the atmospheric pressure is lower by 3 psia in Breck compared to Houston. It's still 46.7 psia inside the tire.

However, I think the tread remained 3 lbs softer than Jeep's recommended pressure. I think if I had aired those molecules down to a reading in Breck of 32 lbs, I would have been 6 lbs softer, not 3 lbs softer, than the 35 psi that Jeep recommends.

"Aired those molecules down"... shudder.

I can't speak for how tire manufacturers (it's them, not Jeep) determine their recommended pressures, but strongly presume that because everyone uses gauge pressure to measure tire inflation, and because people drive at a wide range of altitudes that are not automatically corrected for -- people don't get out and check a barometer and air out their tires every time they go up a mountain - - the manufacturers base their recommended rating on gauge pressure. Thirty-five psig is likely in the middle of an ideal range from say 33-37 psi.

IF the manufacturer says tires should be kept at 35 psig regardless of altitude, it would be wise to leave the tires at 35 psig in Houston and air them out slightly when you get to altitude when they would likely register 38-39 psig (cool) due to the greater difference between the pressure of the atmosphere and the 49.7 psia inside the tire.

It MAY be that the manufacturer figures a recommended psia -- actual -- that should be maintained regardless of elevation, and then issues a recommended pressure that is just a conversion to psig near sea level, because that's where most people drive. If this is the case, then you shouldn't adjust your tires at all, because they always contain the same cool psia.

If that is true, then I would need to replace the Houston air in the tires with Breck air. If i compressed 10.2 psi air in Breckenridge to 32 lbs in, that should be the equivalent of compressing 14.76 air in Houston to 32 pounds, i.e,, same number of molecules and the same relative "softness" in each case.

No. To maintain the same psig in Breck as in Houston, you REMOVE air in Breck, just enough to get it down to 35 or whatever psig.

The air in Houston contains 78% nitrogen, 21% oxygen and 1% other stuff.

The air in Breck contains 78% nitrogen, 21% oxygen and 1% other stuff. There's just less of it than there is in Houston.

The air in your tires contains 78% nitrogen, 21% oxygen and 1% other stuff. There's just more of it proportionally in the tire, volume for volume, than there is outside in Houston or Breck, because it registers at a positive psig.

When you inflate or deflate a tire, you're just adding molecules to or removing molecules from the chamber inside the tire. Air doesn't have a specific pressure at which it comes in or goes out. Air pumps, whether hand-operated or mechanical, simply take air from the atmosphere, compress it and shove it into the tire, basketball or whatever thing the pump is connected to. If you want you can bottle up some Houston air, take it to Breck, rig up some contraption to pump that into your tire and it would be the same as adding the same number of molecules of Breck air straight from the atmosphere.

[Herbert]

TravelingMatt wrote:

When you inflate or deflate a tire, you're just adding molecules to or removing molecules from the chamber inside the tire. Air doesn't have a specific pressure at which it comes in or goes out. Air pumps, whether hand-operated or mechanical, simply take air from the atmosphere, compress it and shove it into the tire, basketball or whatever thing the pump is connected to. If you want you can bottle up some Houston air, take it to Breck, rig up some contraption to pump that into your tire and it would be the same as adding the same number of molecules of Breck air straight from the atmosphere.

Thank you for sticking with this TravelingMatt, and I concede that it does not make any difference whether I fill up the tires with "Breck air"or "Houston air". Having said that, the fact remains that the tires were 3 lbs softer than Jeep's recommended pressure of 35 psig in Houston where the absolute pressure inside the tire was 46.7 lbs (14.7 + 32 = 46.7). That absolute pressure inside the tire remained the same in Breckenridge, unless the tire flexed out somewhat in Breck's lower atmospheric pressure which I don't believe would have happened to any material degree. The tire chamber is corded and rigid. So I think that although the psig was 35 lbs in Breck, the tire remained 3 lbs softer than Jeep's benchmark if Jeep's benchmark is made with reference to sea level. I have not found anything yet that says that Jeep's recommended tire pressure is made with reference to any particular altitude.

[LURE]

TravelingMatt wrote:

When you inflate or deflate a tire, you're just adding molecules to or removing molecules from the chamber inside the tire. Air doesn't have a specific pressure at which it comes in or goes out. Air pumps, whether hand-operated or mechanical, simply take air from the atmosphere, compress it and shove it into the tire, basketball or whatever thing the pump is connected to. If you want you can bottle up some Houston air, take it to Breck, rig up some contraption to pump that into your tire and it would be the same as adding the same number of molecules of Breck air straight from the atmosphere.

Thank you for sticking with this TravelingMatt, and I concede that it does not make any difference whether I fill up the tires with "Breck air"or "Houston air". Having said that, the fact remains that the tires were 3 lbs softer than Jeep's recommended pressure of 35 psig in Houston where the absolute pressure inside the tire was 46.7 lbs (14.7 + 32 = 46.7). That absolute pressure inside the tire remained the same in Breckenridge, unless the tire flexed out somewhat in Breck's lower atmospheric pressure which I don't believe would have happened to any material degree. The tire chamber is corded and rigid. So I think that although the psig was 35 lbs in Breck, the tire remained 3 lbs softer than Jeep's benchmark if Jeep's benchmark is made with reference to sea level. I have not found anything yet that says that Jeep's recommended tire pressure is made with reference to any particular altitude.

This is a serious question cause I'm wondering in my mind if it's true: Did the absolute pressure remain the same though? The tire wouldn't have to flex or expand for the pressure to change. Absolute pressure is just a measure of the pressure in the container, relative to zero. The force exerted on the inside of the container, the absolute pressure, must have increased due to the two things 1) heat and 2) change in atmosphere. In this case we will just assume it was the change in atmosphere, i.e. lower pressures in breck made the tire want to expand, but couldn't expand and because the tire resisted the expansion, the force on the inside of the container increased, which is why the gauge pressure increased. What's the correlation on force on the walls of a container with a 1 unit change in absolute pressure versus a 1 unit change in gauge pressure? Is it not one to one?

I admit, I'm not an engineer, but took plenty of physics in college, which still wouldn't make me an expert, but i'm just trying to rectify this in my mind. I now see what you're saying though.

I feel that gauge pressure is still properly indicating what the force on the walls of the container - i.e. the tire - is.

[EZsummits]

I just returned to Houston from a week in Breckenridge. I drove the trip this time in my stock 2015 Jeep Wrangler. Because I had planned to drive to a couple of 4WD trailheads, I aired my tires down just a bit before leaving Houston. Jeep asks for 35 lbs per tire (front and rear) on my vehicle. I let them down to 32 lbs each. That seemed to me like a reasonable compromise between the highway driving and the trailhead driving. I thought they would heat up a little bit on the highway, and would probably deflate a little in the colder Breckenridge climate. After a couple of days in Breckenridge driving to a couple of 4WD trailheads, I checked them to see how much air they had lost being beat up on the rocks and in the somewhat colder environs. To my surprise, they had not lost any air. To the contrary, each tire checked in at 35 lbs, 3 lbs more than when I left Houston. So the puzzler is: How did the tires gain pressure? And if you answer that question correctly, the next question is: Was the tread of the tire as relatively "soft" in Breckenridge at 35 lbs as it was in Houston at 32 lbs? Anyone want to take a crack at that?

I ran a couple of keyword searches in the forum to see if this issue had been discussed before, and I couldn't find anything precisely on point.

What time of day did you air down your tires before leaving for your trip from Houston and what was the temperature at the time? When you got back to Houston how long did the car sit before you checked the pressure and what was the temperature at the time? I'm betting you aired down in the morning when it was cooler before you left and that both your tires and the temperature were hot when you measured the pressure after returning.