Tire Pressure Puzzler

[Herbert]

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.

[TravelingMatt]

The tires didn't gain pressure. Gauge pressure measures the difference between the pressure in the tires and the atmospheric pressure. Whether you're in Breck or Houston, the same number of molecules are inside the tire of the same volume.

Atmospheric pressure at sea level is 14.7 psi. At 9500' atmospheric pressure is closer to 10.2 psi. Within the margins of error of typical tire pressure measuring equipment, 14.7 + 32 is equivalent to 10.2 + 35. If the temperature was higher in Houston than in Breck, say by 10C or 18F, it could account for most of this difference. Ten degrees C is about a 3.7% difference in absolute terms (10/273 ~= .037) and (1.037)(45.2) = 46.8, basically the same as 14.7 + 32 = 46.7.

As to your other question, I'd say no, because again it's the same number of molecules inside the same volume regardless of where you are. The molecules inside the tire don't know what's going on outside of it.

[oldschool]

The tires didn't gain pressure. Gauge pressure measures the difference between the pressure in the tires and the atmospheric pressure. Whether you're in Breck or Houston, the same number of molecules are inside the tire of the same volume.

Atmospheric pressure at sea level is 14.7 psi. At 9500' atmospheric pressure is closer to 10.2 psi. Within the margins of error of typical tire pressure measuring equipment, 14.7 + 32 is equivalent to 10.2 + 35. If the temperature was higher in Houston than in Breck, say by 10C or 18F, it could account for most of this difference. Ten degrees C is about a 3.7% difference in absolute terms (10/273 ~= .037) and (1.037)(45.2) = 46.8, basically the same as 14.7 + 32 = 46.7.

As to your other question, I'd say no, because again it's the same number of molecules inside the same volume regardless of where you are. The molecules inside the tire don't know what's going on outside of it.

Now THAT'S an answer! Well done sir!!

[Mtnman200]

Matt gave a good explanation. For more info than you ever wanted to know, google the "ideal gas law."

[Herbert]

I can see this is going to be a short thread because I think your answer is correct on both counts. This article from Tire Rack is also useful and contains a chart of atmospheric pressures at different altitudes.

https://www.tirerack.com/tires/tiretech ... techid=167

As for the relative softness of the tread, I think the only way to get the tires correctly calibrated in Breckenridge would be to evacuate all of the Houston air from all of the tires and replace it with Breck air. Then the air inside the tires and the air outside would be apples to apples.

I was surprised by the magnitude of the difference in the atmospheric pressures in Houston and Breckenridge. It made me appreciate the effect on my body a little more. I won't be so hard on my body in the future when it is trying to adjust to the Breck altitude.

[Voshkm]

Nobody said there would be math

[Mtnman200]

Nobody said there would be math

A train leave Chicago heading west at 55 mph...

[justiner]

As for the relative softness of the tread, I think the only way to get the tires correctly calibrated in Breckenridge would be to evacuate all of the Houston air from all of the tires and replace it with Breck air. Then the air inside the tires and the air outside would be apples to apples.

I'm usually completely boneheaded with such matters, but I wanna say the only thing different would be the air pressure. What makes up the air would be inside the tire would be the same, no matter in Houston or Breck.

Air pressure inside a closed system like a tire makes sense; same principal that explodes bags of chips on an airplane.

[ezabielski]

Jeep asks for 35 lbs per tire (front and rear) on my vehicle. I let them down to 32 lbs each.

Not really what you're asking for but airing down to 32 basically isn't going to make any difference for your ride comfort and traction on 4WD roads. 3 PSI is within in the margin of error for two different tire gauges, in my experience. You'd want to go down to at least 20. A lot of people run in the teens by default for 4WDing, and single digits for snow, deep sand, or getting unstuck. You definitely would want to air up before driving on the highway.

[TravelingMatt]

As for the relative softness of the tread, I think the only way to get the tires correctly calibrated in Breckenridge would be to evacuate all of the Houston air from all of the tires and replace it with Breck air. Then the air inside the tires and the air outside would be apples to apples.

For the purpose of inflating tires air is the same everywhere. It's 78% nitrogen, 21% oxygen and 1% other stuff. That 1% other stuff includes differences in water vapor (relative humidity), carbon dioxide and various pollutants. But these differences don't significantly make air heavier or lighter at a given pressure or temperature. Pollutants, for example, are typically measured in parts per million, which is the sixth significant figure, if not parts per billion, the ninth significant figure. Tire pressure matters at most to two significant figures.

[peter303]

For the purpose of inflating tires air is the same everywhere. It's 78% nitrogen, 21% oxygen and 1% other stuff. That 1% other stuff includes differences in water vapor (relative humidity), carbon dioxide and various pollutants. But these differences don't significantly make air heavier or lighter at a given pressure or temperature. Pollutants, for example, are typically measured in parts per million, which is the sixth significant figure, if not parts per billion, the ninth significant figure. Tire pressure matters at most to two significant figures.

I have seen a number of claims for improved tire lifetime with mostly nitrogen air, but not scientific studies. Some of the just-so reasoning here:
https://cars.usnews.com/cars-trucks/bes ... ou-need-it

[Herbert]

Justiner wrote:
I'm usually completely boneheaded with such matters, but I wanna say the only thing different would be the air pressure. What makes up the air would be inside the tire would be the same, no matter in Houston or Breck.

I think there is difference, and this is somewhat the crux of matter. The tires had 14.73 psi air in them, which is the only air my Houston compressor could grab. That did not change -- the air was trapped in a closed cylinder. The only thing that changed was the outside atmospheric pressure. I think the tread therefor remained 3 lbs "softer" than the manufacturer's recommended 35 lbs. I think if I had aired down in Breckenridge to 32 lbs, I would then have been 6 lbs below the recommended 35 lbs. If all this is true, 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.