# Tandem Cycling - Tire pressure vs elevation

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View Full Version : Tire pressure vs elevation

Thigh Master
04-03-10, 08:04 AM
How much does tire pressure increase with an elevation increase of 5,000'?

PMK
04-03-10, 08:23 AM

PK

specbill
04-03-10, 10:04 AM
On the practical side....we drive up and down between 5000 ft and near sea level on a regular basis to ride and never think of or notice any impact on tire pressures...

Thigh Master
04-03-10, 12:09 PM

PK

Thanks so much! How does one figure that out?

Wanderer
04-03-10, 12:13 PM

Oosbahnd&Weefay
04-03-10, 12:44 PM

That's not true, or not at least in the sense the OP means.
The absolute pressure remains the same, but NOBODY measures absolute pressure, you measure gauge, or relative, pressure. That's the pressure as referenced to the Pa (atmospheric pressure). If you're terribly concerned, there are tables known as "standard atmospheric tables" which will show average pressure and density and temperature at altitudes. The difference in pressure at these altitudes is the difference that you'll observe.

-an Aeronautical and Astronautical engineer

Wanderer
04-03-10, 01:08 PM
"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.

merlin55
04-03-10, 03:03 PM
"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.

A bicycle tire is sealed, but not a rigid container, hence the 2.5 psia decrease on the outside of the tire, results in the air inside the tire expanding the tire a bit....just the same amount as if you stayed at the same place where the tire was first inflated, and then the tire pressure was increased by 2.5 psia. The thus the volume of the tire increase at altitude, hence the density as in lb/ft^3 decreases slightly.

Think about weather ballons that take instuments up to over 50,000 feet. When they are launched they are only partially filled, because at the very high alititudes and low pressures, the ballon is now completed filled due the low pressures on the outside of the ballon.

Dean V
04-03-10, 03:10 PM
It is the gauge pressure that counts in this situation, not absolute. If you were to pump your tyre up to 100psi (gauge) and the put the wheel in a pressure vessel set at 100 psi, what was a "hard" tyre will become flat and have zero load carrying capacity.

Wanderer
04-03-10, 03:12 PM
I would hazard a guess, that the volumes we are talking about, and the minute atmospheric changes, have very little, if any, effect on the tire. And, that dealing in these pressures (internal and external) the tire is plenty rigid enough to withstand the changes, without expanding.

I would also hazard an opinion, that normal atmospheric (barometric) changes at any same level, have just about, if not more, than the same effect.

I think we are trying to pick the proverbial fly specks out of the pepper ----- it just ain't worth it, and you probably won't even taste it.........

The barometer in Albuquerque, about 5200 ft, is 29.93 Hg, right now, while the barometer in North Aurora, about 600 feet, is 29.81, with almost the same weather conditions. The barometer in Frisco Colorado, at about 9,000 ft, is 29.79, although it is quite a bit colder there.... None of the above will have any significant difference in tire pressure (discounting any temp differences.)

The temperatures, at much different altitudes, however, will have a much bigger effect, as the trapped air expands and contracts with big changes. If I remember correctly, it will be about a pound for every 10 degrees.....

Wanderer
04-03-10, 03:34 PM
It is the gauge pressure that counts in this situation, not absolute. If you were to pump your tyre up to 100psi (gauge) and the put the wheel in a pressure vessel set at 100 psi, what was a "hard" tyre will become flat and have zero load carrying capacity.

While I agree with you in theory - I know of nowhere on earth that has an atmospheric pressure anywhere close to 100 PSI........ LOL

We are talking the difference in pressure of 14.4#, and 14.65# here.... You can blow thru a straw at higher pressure differences here.... LOL

Oosbahnd&Weefay
04-03-10, 03:39 PM
"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.

The pressure ratings on tires are in gauge, and the gauge pressure determines: Traction patch size, side load deflection, pressure on the 'hooked' portion of the rim, as well as the bead, total available traction, rolling resistance, etc. Everything you care about in tire inflation (all those arguments people have over proper tire pressure) is dictated by gauge and gauge alone. You never notice absolute pressure.

Standard sea-level atmosphere is 15 PSI. Take your road bike into space, and assuming you had inflated your tires to 120 PSI with your floor pump, you'd notice them at 135 PSI. That would be significant. a tire will become noticeably 'more inflated' at higher altitudes. The difference is more important on mtn bikes where pressures are in the 60 PSI range, and a 5 PSI change is significant.

The altitude pressure change is all independent of temperature. It gets important when you: inflate your tires to the recommended max at low alt., get on the bike, and ride in the hills (adding heat to the tires via braking) sure the altitude is only 2-5 PSI in increase, but the combined effects could (not terribly likely) cause a bead to slip. More likely, you'd just have the tube fail (still not all that likely).

Wanderer
04-03-10, 03:50 PM
"1 atm = 101.325 kPa is the Normal Atmospheric Pressure at Sea Level.
1 atm = 760 mm mercury = 760 torr = 101 325 pascals = 29.9246899 inches of mercury = 14.6959488 pounds per square inch."

See what miniscule variations we are talking about here? With "here and now" even more slight variations, between 600 - 9000 feet.

It's not worth even considering......................

Who, among us, has a 0-100# gauge calibrated within even 1#, let alone 1/4 pound????

p.s. Absolute pressure is what happens to be holding us upright..... when on two wheels - guage pressure is just a way of measuring it.

Oosbahnd&Weefay
04-03-10, 04:06 PM
but atmospheric pressure resists it. To give an example, a tire with a hole in it is still at ~15 psi, but it won't hold you up. it's the difference between internal and external, i.e. gauge. Regardless, I don't want to sound hostile, so I'm not going to keep harping on it.

Wanderer
04-03-10, 04:16 PM
And, I'm not taking it as hostile - but, with no hole, it is the absolute pressure within the tire, that is resisting the miniscule 14# of atmospheric pressure, that is holding us up.

Anything could be used as a gauge, even a pile of bricks - just remove one to determine the difference in flex of the contact patch.

I guess my whole point is - that we are talking about a difference that is so small, it's not worth considering. Especially, considering that, even at drastic elevation changes, the atmospheric pressure can be the same, or even "upside down."

Temperature, during a single day, has more of an effect, than elevation changes, and we don't worry about that.

I'd be more concerned that my tire had a small shard in the rubber, working it's way inward...... LOL

Check your tires, with a guage, every day, and squeeze them every so often, (when stopped,) to verify that they continue to hold air, and, are still hard.

Shimagnolo
04-03-10, 04:23 PM
So what happens when you seal empty water bottles, then descend 7000'?

http://www.dim.com/~ryoder/MtEvans/IMGP0086.jpg

Oosbahnd&Weefay
04-03-10, 04:26 PM
lol. awesome photo. Btw, since maybe it isn't clear, whenI say gauge pressure, I don't mean the pressure a gauge reads. Gauge pressure is a definition Pg=P-Pa

Wanderer
04-03-10, 04:28 PM
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.......

Oosbahnd&Weefay
04-03-10, 04:31 PM
going from cold to warm would inflate, not deflate them. Ironically this shows that teh altitude greatly outplayed the temp in that case.

Wanderer
04-03-10, 04:31 PM
lol. awesome photo. Btw, since maybe it isn't clear, whenI say gauge pressure, I don't mean the pressure a gauge reads. Gauge pressure is a definition Pg=P-Pa

I understand completely! I come from a background in the Natural Gas Industry - back when I had to work for a living.... many people don't realize that they have 14# of pressure in their tire, even when it's flat.............

Wanderer
04-03-10, 04:36 PM
29.66 is the barometric pressure on Mt Evans, right now - Denver is 29.67.....

Fly specks in pepper -------

Shimagnolo
04-03-10, 04:43 PM
The temperature was ~50F when I left Idaho Springs at dawn.
It was ~50F when I reached the summit around noon.
Idaho Springs was ~80 when I returned at 1pm.
I had no idea that was happening to the bottles until I was about to lift the bike onto the car rack.
I'm surprised they didn't fly out considering they were empty and I was doing 30-45mph for an hour on the descent.

prathmann
04-03-10, 04:48 PM
The barometer in Albuquerque, about 5200 ft, is 29.93 Hg, right now, while the barometer in North Aurora, about 600 feet, is 29.81, with almost the same weather conditions. The barometer in Frisco Colorado, at about 9,000 ft, is 29.79, although it is quite a bit colder there.... None of the above will have any significant difference in tire pressure (discounting any temp differences.)
Not at all surprising that the barometric pressures reported by the weather service show essentially no variation with altitude since these reports are artificially adjusted to indicate not the actual pressure at each location, but rather what the pressure would be if each town were at sea level. That's done so the reported barometric pressures are all comparable and people are used to seeing pressures above 30" Hg on nice days and to expect a storm if it starts dropping down toward 29" Hg.

But take an actual glass tube filled with mercury and measure the column height in Alburquerque and you'll find that it *never* gets anywhere close to 29.93".

And yes, I think we're all agreed that the small changes in effective bike tire inflation pressures with altitude are insignificant - that was clear from the initial (and correct) response of about 2.5 psi for going from sea level to 5000'. You're the one who chose to argue with that response.

TerryH
04-03-10, 04:57 PM
Thanks so much! How does one figure that out?

Standard atmosphere tables. For instance: http://www.engineeringtoolbox.com/standard-atmosphere-d_604.html

Wanderer
04-03-10, 05:01 PM
And, to think that I thought we were talking about the pressure inside the tire..... silly me!

Oosbahnd&Weefay
04-03-10, 05:06 PM
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.

Shimagnolo
04-03-10, 05:10 PM
Straight-forward discussion of altitude & tire pressure:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167

zonatandem
04-03-10, 09:29 PM
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 . . .