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Old 04-03-10, 04:06 PM   #26
Oosbahnd&Weefay
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right, but the observed pressure INSIDE the tire is not the absolute pressure inside the tire, it's gauge, which is totally dependent on atmospheric pressure. take your bike underwater, and you'll notice the tires look horribly flat (ok, better example is inflate them at 10k ft and then descend). Temperature matters as well, and actually helps, since usually higher altitudes have lower temperatures, offsetting the problem. HOWEVER, there are cases where this might not hold (direct sun on black tarmac at high altitude) and it could, if you're having a very bad day, lead to problems. More likely that descending would be an issue with an under-inflated tire causing poor turn-in that leads to a crash.
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Old 04-03-10, 04:10 PM   #27
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Straight-forward discussion of altitude & tire pressure:

http://www.tirerack.com/tires/tirete...jsp?techid=167
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Old 04-03-10, 08:29 PM   #28
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The increase in tire pressure due to elevation changes while riding is insignificant.
However, outside temps (+ heat of the road surface) will have a greater effect in tire pressure change than any elevation gain here in the US.
Have bicycled in 117 degrees, and road temp had to be at least 140 degrees. Did let a few lbs of air out of my commuter's tires just to be on the safe side. BTW the humidity was a bare-bones 2%!
Just our observations . . .
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Old 08-07-17, 02:51 PM   #29
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Air pressure in leaky old windbags

Coming upon this old thread, I'm relieved that one poster gave up trying to be right. The question was asked and answered in two responses, but one poster took exception, over and over again, beating a number of adequate and well-referenced responses half to death.
Summary: Apparent/relative tire psi goes up by about 2.5 pounds from seal level to 5000ft, another 2 up to 10000ft.
In real conditions, temperature drops with altitude.
Tire pressure also drops with temperature drop, and in balance for typical climates, the lower air temps going up offset about one half of the apparent tire pressure increase. Death Valley to Mt. Whitney: probably less than 5 psi, 10 max.
Tires vary more day to day from temperature changes often than from even significant altitude changes. Check/adjust at the trailhead or the start of a ride, and be off.
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Old 08-12-17, 11:32 AM   #30
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Originally Posted by Wanderer View Post
"The absolute pressure remains the same,"

I rest my case....

Taking temp out of the equation, the pressure inside a sealed, container, under pressure - remains the same......

Who cares what the outside atmospheric is, as long as the container is sufficient to contain the internal pressure.

And, the density of the contained air is the same.

The gauge may read differently, because of the different atmospheric pressure; but, the pressure inside the tire won't change.
You "rest your case?"

"Who cares?" People who are concerned about tire failure, that's who!

The whole reason one is interested in tire pressure is indeed the differential between internal and external air pressure. And since external air pressure changes, it has a real-world effect on tire performance and safety. And it may be "minuscule" when considering bicycle tires, but not so with automobile tires.

Example, when I air up my van tires to 35 psi at sea level and then drive to Lake Tahoe, the tire pressure at my destination has changed to 39 psi (measured "cold," so it isn't a temperature-induced variation). Sure, there's the exact same amount of air in the tire, but that's a correct, but completely irrelevant fact! I'm now driving my tires at the wrong pressure! If I were driving at altitude for an extended period of time, I'd adjust them back to the recommended 35 psi cold pressure.

Your statement, although technically correct, is simply BAD advice. You're effectively saying that since there's no change in internal air, you shouldn't worry about it. This is wrong. The resultant internal air pressure after external pressure change (change in elevation) changes and thus is not the recommended pressure. This is what causes premature or uneven tire wear, and worse, tire failure (Think Ford Explorer tire failures).

But since most elevation changes people encounter don't affect their resultant tire pressures that much, it isn't an issue for MOST people. But those who are concerned and/or driving with a fully loaded vehicle, double-checking tire pressures and adjusting accordingly is a very good idea.

Last edited by LV2TNDM; 08-12-17 at 11:36 AM.
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Old 08-12-17, 04:52 PM   #31
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"Think about this ----- pressure inside a sealed container (the tire) won't change due to elevation........"

The difference isn't significant, but in terms of physics, this is not true.

https://www.tirerack.com/tires/tiret...jsp?techid=167
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Old 08-13-17, 12:39 PM   #32
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More reading to enlighten you...... From ****** Physics

Since the cylinder is sealed, it is isolated from the changes in pressure of the outside environment so a change in pressure resulting from a change in altitude will have no effect. However, temperature also changes with altitude so if the sealed cylinder and its contents are in thermal equilibrium with the outside, the decrease in temperature with increasing altitude would cause the pressure inside the sealed cylinder to drop. So, with an assumed lapse rate of ~2 C per 1,000 feet increase in altitude, the temperature at an altitude of 5,000 feet will be 10 C lower than on the ground. If we assume a temperature of 20 C (or 293 K) at zero altitude, at an altitude of 5,000 feet with a temperature of 10 C (or 283 K), the gas pressure inside the cylinder will be ~3.4% lower

OR

The pressure only changes when you pump more gas into the cylinder or the cylinder changes volume. Since the amount of gas doesn't change and the volume of the steel cylinder doesn't change significantly, the change in pressure is negligible. The answer is no.
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Old 08-13-17, 01:33 PM   #33
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From that link to tire rack - maybe easier to understand. The pressure inside the tire remains the same, it's the gauge that changes.

" 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. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi. "

Soooo, using your own words, "your statement is simply BAD advice!"
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Old 08-13-17, 03:32 PM   #34
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Originally Posted by Wanderer View Post
From that link to tire rack - maybe easier to understand. The pressure inside the tire remains the same, it's the gauge that changes.
...

Soooo, using your own words, "your statement is simply BAD advice!"
Sure the 'absolute pressure' inside the tire is nearly constant (not quite since the tire volume will expand a bit if the external pressure decreases). But for all practical purposes (i.e. ride quality, resistance to pinch flats, possibility of blowing off the rim, etc.) what matters is the 'gauge pressure' and that does change with altitude.

When people just refer to 'tire pressure' it's implied that they mean 'gauge pressure' since that's the quantity that matters, that's what we measure with our pumps or pressure gauges, and that's what varies when we go up in altitude or load our bikes on airplanes
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Old 08-13-17, 03:39 PM   #35
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Originally Posted by prathmann View Post

When people just refer to 'tire pressure' it's implied that they mean 'gauge pressure' since that's the quantity that matters, that's what we measure with our pumps or pressure gauges, and that's what varies when we go up in altitude or load our bikes on airplanes
+1

Gauge pressure, or the difference between inside and outside pressure will change, and the change is equal to change in external pressure. While chemists are concerned with absolute pressure, the mechanical considerations will depend on gauge pressure, which is what we measure.

Referencing this chart we see a drop of about 2.5psi between sea level and 5,000' altitude. So the gauge pressure in a tire will increase by that amount, which is trivial in the scheme of things.

Even if we put the bike into an unpressurized aircraft and take it up to 40,000' we'd only get an increase of about 12psi, well within the working tolerance for tire pressure.
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Old 08-15-17, 01:39 AM   #36
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From that link to tire rack - maybe easier to understand. The pressure inside the tire remains the same, it's the gauge that changes.

" 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. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi. "

Soooo, using your own words, "your statement is simply BAD advice!"

You realize your quote actually confirms my post, right? This quote says exactly what I said. The gauge doesn't change, the effective tire pressure changes. Since there are the same number of "air" molecules in the tire after reaching altitude, there's no change in air quantity during an elevation gain or loss, I get it. But that same quantity of air is exerting more external pressure on the tire sidewall, tread, bead and rim because the surrounding atmospheric pressure is lower.

What happens if I were to continue climbing in elevation? Tire pressure would continue to increase (again, relative to external air pressure) until the tire eventually explodes before reaching the vacuum of space. Or do you rebut this assertion? Would you argue that this tire failure couldn't occur, but rather my gauge would explode because " The pressure inside the tire remains the same, it's the gauge that changes"?

Absurd. Beyond absurd.

Now perhaps we're talking past one another, but I'll stick to my original statement and say your cavalier attitude about the tire pressure NOT changing, but the gauge changing is indeed bad advice. Anyone driving a motor vehicle who wishes to drive at recommended tire pressures would not want to heed your advice, but rather check tire pressures after significant elevation changes and adjust accordingly.

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Old 08-15-17, 08:33 AM   #37
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What happens if I were to continue climbing in elevation? Tire pressure would continue to increase (again, relative to external air pressure) until the tire eventually explodes before reaching the vacuum of space. Or do you rebut this assertion?
If we're talking about a road bicycle tire then yes, I do rebut that assertion. Going from sea level up to the vacuum of space would only increase the gauge pressure by 14.7 lbs/sq.in. which is well within the normal safety margin for bike tires.
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Old 08-15-17, 08:52 AM   #38
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I'd bet that the temp change from 30F, to 80F, had more effect than the elevation change. It's usually pretty cold on top of Mt Evans.......
This is what I'm thinking too, if you ride from sea level to high altitude, the temp will typically drop as the air thins. Therefore, I expect the effect on your tires will be negligible.

This all got my attention as I have a goal of riding Mauna Kea - from sea level to 13,500 ft of elevation gain.
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Old 08-15-17, 09:48 AM   #39
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Wanderer:
"I'd bet that the temp change from 30F, to 80F, had more effect than the elevation change. It's usually pretty cold on top of Mt Evans......."

This is what I'm thinking too, if you ride from sea level to high altitude, the temp will typically drop as the air thins. Therefore, I expect the effect on your tires will be negligible.

This all got my attention as I have a goal of riding Mauna Kea - from sea level to 13,500 ft of elevation gain.
No, I'd take Wanderer up on his bet. Ascending Mauna Kea will increase tire pressure (i.e. 'gauge pressure' for those still confused) by almost 6 psi based on the altitude change. A drop in temperature from 80F to 30F (which would be pretty extreme) would only reduce the pressure by about 1.5 psi so there'd still be a net pressure increase. But not enough of a change to create any need for concern assuming normal road bike tires.
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Old 08-15-17, 02:15 PM   #40
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29.66 is the barometric pressure on Mt Evans, right now - Denver is 29.67.....

Fly specks in pepper -------
This is a common misconception.

The ambient pressure in Denver will rarely be greater than 25"Hg and Mt. Evans ambient pressure is going to be a good 8-9"Hg lower than Denver.

This is because barometric pressure is _not_ local ambient pressure. It is ambient pressure numerically adjusted to a presumed sea level value.
This allows meteorologists to compare barometric pressure readings without needing to consider altitude. The correction is roughly +1"Hg per 1000' altitude above sea level, though it is non-linear and read from a table.

If this were not true, aircraft altimeters wouldn't function. Note that such altimeters have a barometric pressure adjustment. When the altimeter is adjusted to local altitude, the sea level-compensated barometric pressure is displayed in the Kollsman window.

Another misconception floating around in this thread: The atmospheric temperature lapse rate applies as you rise through the air above the ground...NOT when you follow sloping ground to higher altitude. This is because most of the heat in the atmosphere is transferred to the air from the ground, so the air temperature near the ground is not a strong function of altitude. Due to less atmosphere above, sunlight is a bit stronger at altitude, so the ground-level air temperature at high altitude locations can get quite high...100F + days are regularly recorded at Albuquerque Sunport elv. 5,355'MSL.

Last edited by kevbo; 08-15-17 at 02:36 PM.
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Old 08-16-17, 06:35 AM   #41
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No, I'd take Wanderer up on his bet. Ascending Mauna Kea will increase tire pressure (i.e. 'gauge pressure' for those still confused) by almost 6 psi based on the altitude change. A drop in temperature from 80F to 30F (which would be pretty extreme) would only reduce the pressure by about 1.5 psi so there'd still be a net pressure increase. But not enough of a change to create any need for concern assuming normal road bike tires.
I don't disagree with you on a standard 25mm tire; however, I'm a fan of big tires (I ride a fatbike for gravel centuries). Because Mauna Kea includes a significant section of steep gravel roads, the bike I expect to use will be a Breezer Radar Pro with 45mm tires. The low-pressure, large-volume tire will see a more signification change in tire pressure.

This chart is for fatbikes, so not strickly relevant here, but you get the idea that pressure in low-pressure, large-volume tires can change by a significant percent with the change in temp:


Using 80F to 30F at 12.5 psi to 10 psi; that's a 20% reduction in the tires pressure. That 2.5 psi wouldn't be signification in a 25 mm tire are 100 psi; but for a large-volume, low-pressure tire it is a noticeable change.

I still believe that the change in air pressure and temp with balance out ... at least to the point where the rider will not notice the change.

BTW - I can't be the only one to buy a bag of chips before driving into the mountains and have it burst from pressure change, right? We live in Minneapolis (950 ft) and we frequently drive to Summit County (up to 11,000 ft at the Eisenhower Tunnel). We've had chip bags explode (loudly) in the car while drive up I-70. Chip bags are not bike tires, but it's obvious that the change in pressure has an effect on airtight vessel.
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