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Old 11-27-2007, 11:36 AM   #1
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Higher Altitude Tire Pressure

I was just thinking the other day... And this really doesn't apply to me, as I'm very close to sea level. But for people driving at higher altitudes....

When we measure tire pressure, we're measuring gauge pressure - the difference between local atmospheric and inside the tire. So, given a higher altitude, tire pressure will appear higher than the same amount of air inside the tire at sea level.

So gauge pressure changes, but the amount of force your tire must hold up doesn't (lets say constant gravity ).

That would mean, under inflation of tires is likely more common at higher altitudes EVEN THOUGH gauge pressure is within the normal pressure range...

Okay, we're not talking about much here...

Sealevel: ~ 101kPa
5,000 ft: ~ 84 kPa (difference of 2.9 psi over sea level)
10,000 ft ~ 69 kPa (difference of 4.6 psi over sea level) <--- seriously, that's one high city

Just something to chew on because if every bit didn't count - I'd be driving an SUV
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Old 11-27-2007, 01:56 PM   #2
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If it were a piston or other solid direct acting mechanism I agree but keep in mind the pressure holding the car up is the same that pushes the sides of the tire out and the top up. I would bet that 5 psi at sea level would hold up a full size SUV in a vacuum just fine.
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Old 11-27-2007, 02:20 PM   #3
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Quote:
Originally Posted by kamesama980 View Post
If it were a piston or other solid direct acting mechanism I agree but keep in mind the pressure holding the car up is the same that pushes the sides of the tire out and the top up.
If we were measuring in terms of absolute pressure - yes, it's the same mechanism as far as physics are concerned. But we're not - we're taking relative measurements. Even in a vacuum - you don't magically get more force from the ground because the gauge pressure changes. Draw a quick FBD and it will quickly become apparent
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Old 11-27-2007, 03:15 PM   #4
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OH NO! i read FBD and instantly knew it ment Free Body Diagram! dang u physics!!!LOL but anyways, what trebuchet said is correct.
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Old 11-27-2007, 03:31 PM   #5
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This assumes that your gauge is measuring the pressure relative to outside air pressure. Digital gauges probably measure with a pressure transducer so there would be no counteracting outside pressure as with a Bernoulli tube type gauge. Also to correct you on the higher altitude, lower outside pressure would result in less pressure collapsing the tire so it would be over inflated at high altitude if you didn't let some air out. As an example go the other way and put the 32psi tire into a hyperbaric chamber at 32 psi and now the tire can not support any weight because it has no pressure in it relative to the outside of the tire then increase outside pressure more and the tire would actually collapse without any vehicle weight on it. Remember air pushes in all directions including up from the ground via the space between the treads.
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Old 11-27-2007, 04:23 PM   #6
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The thing is if you put the pressure you want in the tire at the altitude you are staying at...it will be correct. If you change altitudes then the pressures will either rise or drop a bit.
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Old 11-27-2007, 09:19 PM   #7
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I maintain my position but lack the degree or linguistics to explain to my or your satisfaction but I'll still try...actually, I mostly maintain my position but agree with your result though not for the wording you gave. or something, how bout I try to explain my thought process and see what others think. (grrrr... when can we get telepathy and leave the silly limitations of language to pass on ideas behind)

upon reflection I agree that tires will be underinflated more often, but by a percent. I'll try to make up for my lack of quantitative accuracy with qualitative reasoning for I went for a real-world/hands-on degree in school, not physics, engineering, look at how water spirals 12 year program (no offense).

<p>at sea level (at given temp etc) it takes X mass of air to create 35 psi in the tires. at altitude it takes less than X mass (lets say 30&#37; of X at 10k feet using your numbers above) thus, losing Y amount of air from your tires at 10k feet will drop significantly more PSI than at sea level. Basically the curve for mass of air in the tire vs PSI will be compressed at altitude<p>
<p>HOWEVER What holds the car up though is the relative pressure which is 35 psi higher than whatever atmospheric is whether it's
5 psi atmo or 50 psi atmo (hate to live on that planet...how many Gs I wonder?) concurrently the force on the tire is caused by the relative pressure SO if you have a leak in the tire it'll take the same amount of time to deflate to atmo pressure. since, as you say, the SUV weighs the same at 10k ft as 0 ft, the force is the same, the pressure difference is the same, and unless you're inflating your tires with an air mass meter, you're fine.</p>
I dunno if I made any sense but I entertained myself for 5 minutes
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Old 11-27-2007, 09:56 PM   #8
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I was thinking about this on my ride home earlier (rather than pay attention to the 10pm traffic ) as I felt I was missing something after my post (which is bothersome for me ). But you're totally right, my mistake I was totally stuck on abs pressure and missed a critical point

Quote:
OH NO! i read FBD and instantly knew it ment Free Body Diagram! dang u physics!!!
Even though I was wrong, that doesn't change that you're a physics nerd too (even though FBDs are useful outside of physics too)
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Old 11-28-2007, 05:37 AM   #9
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Tre you may have a point about the air pressure gauge though . . . different pressure gauges may measure pressure differently and their accuracy MAY be affected at different altitudes. Should they measure absolute pressure or relative pressure would depend upon how they are designed.
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