Tire Pressure Puzzler

[Wentzl]

I just got a quote for tires at Big-O and on the quote is a line, Nitrogen - - - -$32.00.

If the OP had nitrogen in his tires, would it change the discussion? More important, is it really worth $32.00 to use nitrogen instead of air?

[easyridertme]

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.

No, no, no. Just because it's "corded and rigid" doesn't mean it isn't going to flex--you'd have a damn uncomfortable ride if it didn't. Just have someone else drive your tire over a brick to see how rigid it is... As far as pressure is concerned as far as how soft or hard the tire is going to be, 35psig in Houston is going to be the same is 35 psig in Breck. Part of what TravelingMatt has been trying to say (if I can put words in his mouth) is that the relative pressure inside the tire increases because outside air pressure decreases going from Houston to Breck. Of course a tire is not going to visibly change to the extent a balloon or a bag of chips would, but it definitely responds to the increase in pressure relative to the atmosphere.

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.

I'm really struggling to come up with any reason it would make any sense whatsoever to do this. Why would a car company think one pressure is ideal at sea level, but think that a more rigid tire (due to the relative increase in pressure) is ideal at high elevations? It think it's reasonable to assume that most people see the sticker on their door that says 30psi, that they're going to inflate their tires to 30psi wherever the hell they are, not convert the pressure difference given the elevation to ensure that they're at the manufacturer's recommended psia...

[Herbert]

LURE wrote:
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.

The absolute pressure on the inside of the tire chamber did not increase. TravelingMatt made this point in his very first post when he said:

The tires didn't gain pressure.

The key here is that the number on a tire pressure gauge does not represent the absolute pressure inside the tire chamber; it represents instead the difference between that absolute pressure and the atmospheric pressure. The tire gauge gives you the result of a three part linear equation: Absolute Pressure - Atmospheric Pressure = PSIG. We can solve for absolute pressure because we know the atmospheric pressure and psig. In Houston, I set the tires to 32 psig, and at that reading the absolute pressure inside the tire chamber had to be 46.7 because 46.7 - 14.7 = 32 (the atmospheric pressure in Houston is 14.7 psi). When I measured the same tire in Breckenridge the gauge read 35, a reading which yields an absolute pressure inside the tire chamber of 45.2 because 45.2 - 10.2 = 35 (the atmospheric pressure in Breckenridge is about 10.2 psi). So actually, the absolute pressure inside the tire decreased somewhat from Houston to Breckenridge, notwithstanding the increase in gauge pressure, most probably due to temperature. I measured the tires in Houston at probably 90 degrees, and most probably around 70 degrees in Breckenridge. That difference in temperature would easily account for the 1.5 lbs drop in absolute pressure inside the tire chamber. According to Tire Rack there is a pressure change of about 1 psi for every 10 degrees Fahrenheit. By the way, I used the same gauge for all the measurements.

What this means is if I set my tires in Houston to 35 psig when the temperature is 70 degrees Fahrenheit, then the absolute pressure inside the tire chamber will be 49.7 because 49.7 - 14.7 = 35. If I set the same tire to 35 psig in Breckenridge when the temperature is 70 degrees Fahrenheit, the absolute pressure inside the tire chamber will be 45.2 because 45.2 - 10.2 = 35. Under those conditions the tire is going to be 4.5 pounds softer in Breckenridge than in Houston notwithstanding the fact that the gauge pressures are the same. I don't see how you can escape that math.

As for whether the tire chamber expands at higher altitudes, I don't think it does so to any appreciable degree. Yes, the tire is flexible and obviously gets distorted with a 4,500 lb body on top, hitting bumps and rotating however many times a second it rotates at 80 mph. But it is not like a balloon and I don't think its going to flex out at altitude when the vehicle is stationary. Tire Rack says: "Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level."

[LURE]

LURE wrote:
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.

I don't see how you can escape that math.

Maybe the absolute didn't change, maybe it did. Maybe your math is wrong (i'm not saying it is). I don't know. Maybe I'll think about it on Monday when I get back.

What I think to be true here is that relativity matters. 35 psi is 35 psi and your tires will be inflated properly no matter what happens for the result to be 35 psi. This is why tires are measured, recommended, and tested with gauge pressures.

[easyridertme]

I don't see how you can escape that math.

There's no need to escape the math, because you're paying attention to the wrong thing. My main point was this: the only thing that matters, in terms of how "soft" or "hard" a ride is going to be, is the differential pressure. Not absolute pressure. And the gauge pressure is the measure of that differential.

But it is not like a balloon and I don't think its going to flex out at altitude when the vehicle is stationary. Tire Rack says: "Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level."

It's more a like a balloon than it is a solid wooden wagon wheel--the fact that pneumatic tires are flexible and the air in them compresses is the whole reason we no longer use solid tires. And Tire Rack isn't quite correct there--the number of air molecules is the same, but that doesn't guarantee that the volume won't change... How about instead we compare it to a mylar balloon, which is all floppy without much inflation, but that won't change its shape--much--after pressure reaches a certain point. With tires, the cords (as well as the thickness and strength of the rubber) act to mitigate, but not entirely eliminate, increasing volume with increasing pressure. Mylar balloons seem to jive pretty well with this photo, where, as you continue inflating, it's harder to notice the difference each step up makes in terms of shape and size.



So, what happens to mylar balloons (or regular balloons, or bicycle tires, or basketballs or anything else you pump up with air) as they get closer to their breaking strength while you inflate them? Well first, the pressure differential between atmospheric pressure (whatever it is where you are) and the absolute pressure inside the balloon/ball/tire increases, as you can see on any gauge. What is more, as you continue inflating--that is, increasing the pressure differential--the balloon/ball/tire gets harder and more rigid. Can we agree on that?

So imagine a world where we didn't inflate balloons/balls/tires by forcing additional air molecules inside, but instead decreased the amount of air molecules on the outside by putting it in a vacuum chamber? Can we agree that the balloon/ball/tire would still become more rigid as we sucked air molecules out of the chamber? This is because your changing the pressure differential, you're not changing the absolute pressure inside the object. It doesn't matter where you take the tire--into space where the atmospheric pressure is zero or 100 meters below the sea where the "atmospheric" pressure is now around 147 psi. https://www.pmel.noaa.gov/eoi/nemo1998/ ... ssure.html That tire is going to have the same rigidity as long as your gauge reads 35psi, meaning that the absolute pressure would be 35psi in space and 182psi under the sea.

[spiderman]

You went a little bit too far in the final example of doing the experiment under the sea. Your conclusion would only be valid if bulk modulus >> Young's modulus. This is true for rubber balloons but false for car tires and mylar balloons.

I think that the entire 14er community would be in agreement if you simply stated that mylar balloons should not be used as a substitute for vehicle tires.

[easyridertme]

You went a little bit too far in the final example of doing the experiment under the sea. Your conclusion would only be valid if bulk modulus >> Young's modulus. This is true for rubber balloons but false for car tires and mylar balloons.

I'm not sure I follow where you think I went wrong? My conclusion, if illustrated by imperfect examples, is that the stress and strain on a hollow object would stay the same as long as the difference in internal and external pressure remain constant, regardless of how low or high the baseline (atmospheric) pressure is. Why would there be any difference whether the external force was from water or air? Simply stated, my point was that a tire that is inflated to 35psi in Badwater Basin is going to be as deformed and as rigid as a tire inflated to 35psi in the Mt. Evans parking lot.

I get that tires act differently than balloons, the only point I was making with the comparison (and, frankly, a mostly irrelevant one), was that changes in tire pressure are going to cause the tire to deform--isn't that the whole point of airing down in the first place?

I think that the entire 14er community would be in agreement if you simply stated that mylar balloons should not be used as a substitute for vehicle tires.

I dunno, I think America's automakers have just been too conservative to experiment with it... Could be the next great revolution.