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Thread: Altitude effects on Tire Pressure

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    Altitude effects on Tire Pressure

    This is likely a discussion best held with a few libations. Definately NOT a "what tire pressure do you recommend" thread!

    Regardless of the tire pressure one regularly runs (from the Owner's Manual recommended pressures one-up and two-up w/luggage to the 38/42 we often see recommended for the RT) what does one do at altitude?

    Situation: We live at 6500 feet elevation in the Sierras. Most of our rides take us over 7400 feet through the passes to either 4500 feet in the Nevada deserts or to 300 feet in the California central valley in the course of a single day.

    Is there a general rule of thumb for setting air pressure to compensate for altitude changes? Does 40 psi inside a tire at 6500 feet (all other things being equal) mean 36 psi at sea level due to more ambient pressure at sea level?

    I've been giving the tires about 10% over what I regularly run to compensate for the lower altitude of my destinations w/o seriously over-inflating for altitude and getting great overall tire mileage and traction, but I still have that back-of-the-head question no one has ever really answered.

    Thanks for your thoughts, rants, or insights?

    Stan Moore
    South Lake Tahoe

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    Quote Originally Posted by 53133 View Post
    This is likely a discussion best held with a few libations. Definately NOT a "what tire pressure do you recommend" thread!

    Regardless of the tire pressure one regularly runs (from the Owner's Manual recommended pressures one-up and two-up w/luggage to the 38/42 we often see recommended for the RT) what does one do at altitude?

    Situation: We live at 6500 feet elevation in the Sierras. Most of our rides take us over 7400 feet through the passes to either 4500 feet in the Nevada deserts or to 300 feet in the California central valley in the course of a single day.

    Is there a general rule of thumb for setting air pressure to compensate for altitude changes? Does 40 psi inside a tire at 6500 feet (all other things being equal) mean 36 psi at sea level due to more ambient pressure at sea level?

    I've been giving the tires about 10% over what I regularly run to compensate for the lower altitude of my destinations w/o seriously over-inflating for altitude and getting great overall tire mileage and traction, but I still have that back-of-the-head question no one has ever really answered.

    Thanks for your thoughts, rants, or insights?

    Stan Moore
    South Lake Tahoe
    Since you're probably measuring gage pressures (actual-atmospheric), you're already using a pressure differential as criteria. Your concern and apparent choice would be correct, if you were measuring absolute pressures.

    Of course, I could be wrong.
    Cave contents: 99 R11RS, 2013 Toyota Tacoma, 03 Simplicity Legacy, 97 Stihl FS75, Dewalt DW625 & DW744

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    The pressure differential across the wall of the tire will increase by about 4.5 psi as you go from sea level to 10,000 ft, so if your tires are properly inflated at sea level, it would probably make sense to let a little air out before crossing high mountain passes, but, by the same token, it would make sense to put air back in as you descend, and this would all amount to a big hassle.

    I've gone from sea level to 10,000' and back to sea level in one day without touching the tires and survived, but if you are riding solo and start at 40/42 at sea level, you're going to be quite over-inflated at 10,000'.

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    Registered User 58058D's Avatar
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    Agree with Spokesman all the way. Guage pressure means you are measuring inside relative to outside, so as the outside pressure increases or decreases, the pressure inside changes in relation to that, given that you have a flexible tire, it will not likely be a straight calculation. I have the tire pressure monitors in my bike's wheels and I do note changes as I ride up and down, and from one temp range to another. Temp is another part of the equation (PV=nRT). So, when I left Sacramento at 45 feet above means sea level and a temperature of 95 degrees, my tires read 36/42 for 'temperature compensated pressure'. As I head to the coast, I cross passes around 4,000 feet and the temperature dropped to 53 by the time I got home. Temp compensated pressure changed a couple of times dropping down to 35/41 but then as I got into the twisties and hit it hard climbing the highest point, the pressure went back up on both, but at different rates. Overall, they never varied more than one pound per in^2 over the entire 190 miles. So, I don't worry.
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    Quote Originally Posted by SPOKESMAN View Post
    The pressure differential across the wall of the tire will increase by about 4.5 psi as you go from sea level to 10,000 ft
    If the day is not a "standard temperature" day, the variation can be different than that, especially if the temperature profiles at sea level and the altitude in question are different. Conceivably, if the temperatures vary in the worst possible direction, the changes could be more than 4.5 psi...hard to say how much.
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    535IS
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    Quote Originally Posted by 58058D View Post
    Temp is another part of the equation (PV=nRT).
    As someone who has taught it, I'm delighted that someone has brought up the Combined Gas Law. Now, can anyone tell me which variable in there has to do with the pressure on the opposite side of a (fixed-volume) membrane? Pressure measuring equipment might depend on relative pressures, but pressure does not. 35 psi is 35 psi whether it's here, on the moon, or at the bottom of the ocean.

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    gah..... physics!

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    Quote Originally Posted by 535is View Post
    As someone who has taught it, I'm delighted that someone has brought up the Combined Gas Law. Now, can anyone tell me which variable in there has to do with the pressure on the opposite side of a (fixed-volume) membrane? Pressure measuring equipment might depend on relative pressures, but pressure does not. 35 psi is 35 psi whether it's here, on the moon, or at the bottom of the ocean.
    I'm putting my money on "V".

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    Small road corner junkie pffog's Avatar
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    Quote Originally Posted by 535is View Post
    As someone who has taught it, I'm delighted that someone has brought up the Combined Gas Law. Now, can anyone tell me which variable in there has to do with the pressure on the opposite side of a (fixed-volume) membrane? Pressure measuring equipment might depend on relative pressures, but pressure does not. 35 psi is 35 psi whether it's here, on the moon, or at the bottom of the ocean.

    But with tires, the differential or gauge pressure does determines profile, fill a tire with 35 psi at sea level and take it down to the bottom of the ocean, and I will guarantee it will NOT appear inflated, it will look more like it was melted to the rim.

    That said the BIGGEST force on the tire is not from the minor pressure differential at high altitudes, but from holding up the weight of the bike and the butts sitting on them. And the force of gravity is pretty much a constant.

    So yes, atmospheric pressure will effect what you measure, and as mentioned buy others heat will change things more than altitude.

    Bottom line, just go and ride, and stop being such worry warts, check for severe under or over inflation, but stop worrying about a pound or two.

    That said, I will give those that want something else to worry about something new, TREAD TEMPERATURES!! Temps have the most effect on traction and wear, and are effected by tire pressure, load, speed, ambient temps, moisture, road surface, tire construction, tire compounds, rim design and probably a few more. .......................I think I just heard some peoples heads explode................

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    Registered User breyfogle's Avatar
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    Quote Originally Posted by 535is View Post
    Now, can anyone tell me which variable in there has to do with the pressure on the opposite side of a (fixed-volume) membrane? Pressure measuring equipment might depend on relative pressures, but pressure does not. 35 psi is 35 psi whether it's here, on the moon, or at the bottom of the ocean.
    Tyre "inflation" is determined by delta P across the tyre body (i.e. outside pressure - internal pressure). Putting 35 psi gauge pressure into a tyre at sea level (about 15 psi ambient) really means you have 50 psi internal ( 35 psi gauge(delta P) = 50 psi internal - 15 psi external ). Take that same tyre to the moon and you will still have 50 psi internal but your external pressure is now 0 so your gauge reads "50 psi" and your tyre will be rather stiff.
    '89 K75S Original Owner
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    Registered User amiles's Avatar
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    I'd be leery of trying to set tire pressures while the tire is hot from riding as this introduces what I would believe to be an unpredictable variable. Stopping for an hour or two (or more) so as to have a "cold" tire pressure would seem impractical while on a trip.

    While riding, if an altitude change would large enough to drop tire pressure, increased friction would likely cause the tire pressure to rise back up to a higher level. This might even reach a point of equilibrium (the decrease being compensated by the increase).

    I suspect that if you run a proper cold tire pressure, any foreseeable change in altitude on paved roads in the U.S. would not cause a significant problem.

    Note the numerous wrecked bikes with ruined tires at the tops and bottoms of steep grades, Pikes Peak for example and the prominent mention of this problem in Safety classes and popular writings on the subject.

    This seems akin to adjusting carb jetting for altitude. Generally if just passing through it usually isn't worth the effort in exchange for tolerating a bit of driveability annoyance. If the different altitude visit will be for a longer time, then the change may be worth the effort.

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    I live in Reno and face the same problem. I usually run 34, 38 at home and 35, 40 when heading down the hill. Most of the year, it is warmer at lower altitudes so the pressures don't change enough to worry about. If I am staying at low altitudes for a few days, like the annual Death Valley ride, I readjust in the morning.

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    Small road corner junkie pffog's Avatar
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    Still think everyone is missing the point of the air molecules in the tire , they are there to HOLD THE BIKE AND RIDER OFF THE PAVEMENT!!! That is why you add pressure with weight!!

    Not sure what the contact patch area actually is, and it varies with the tire, wear etc, but if we say it equates to an approximate 2.5" circle on each tire, that is about 10 square inches of surface area, put bike and rider weighing 800 lbs on it and you have 80 psi deforming the tire. When atmospheric pressure is the major external force we don't care, as the tire is NOT doing anything but hanging out in space.

    My bet is if you checked with a motorcycle or auto racing team, they will tell you they run HIGHER gauge pressure at altitude, in effect maintaining the tires sea level pressure.
    Last edited by pffog; 07-02-2010 at 01:28 PM.

  14. #14
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    Quote Originally Posted by pffog View Post

    Bottom line, just go and ride, and stop being such worry warts, check for severe under or over inflation, but stop worrying about a pound or two.
    Quite possibly the sagest advice I've seen on tire pressure in a long, long time.

    Use your butt, there will be a point (probably some where between 30 and 42 PSI, lower if you are off-road on your GS) that will make you happy. It's easy to over think some things.
    Steve
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    It's really quite simple. Going up 10,000 feet without bleeding off air IS adding 4.5 psig. Descending 10,000 feet without adding air IS subtracting 4.5 psig. Note that I did not say "is like." (We are talking cold tire pressures, as always, and assuming constant barometer. Barometric effects are pretty insignificant, unless you're in a hurricane or tornado.)

    Is 4.5 psig significant? I would say that this is the individual's call. If I started at the extreme low end of the permissable inflation range and subtracted 4.5 psig, I'd call that significant, but the rim would not be on the pavement, unless I happened to hit a big pothole.

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