Should target tire pressure be changed for high elevation?

[RebornRider]

I understand the difference between gage pressure and absolute pressure.
I understand that the gage pressure for a particular tire inflation will be higher at high elevation than at sea level.
I understand that metric units make much more sense than psi, horsepower, slugs, and BTU.
But I grew up with these dang imperial units, so that's how I think. Sorry!
Let's get started.

• Assume I have an accurate tire pressure gauge.
• Assume I inflate the tire to 20 psi at a location where atmospheric pressure is 14.7 psi.
• The absolute tire pressure is 34.7 psi (14.7 + 20)
• I take the tire to an elevation where the atmospheric pressure is 11 psi.
• Assume the tire doesn't leak, so its absolute pressure is still 34.7 psi.
• But the tire's gage pressure is now 23.7 psi (34.7 - 11)
• Since the absolute pressure didn't change, my assumption is that the tire will feel the same as it did at sea level.
• But I've never tested this carefully.

My question for the guys that can feel small differences in tire pressure: Do you adjust your target inflation pressure when you ride at high elevations? e.g. change from a target of 22 psi at sea level to 25 psi at 11,000 feet?

FWIW, I live at roughly sea level, but I go to Mammoth Mountain (California) twice a year, which reaches 11,000 feet at the top.

 

[steve_sordy]

It's a while since I've been at high elevation, but I'm sure that you will be able to feel the difference. I set my tyres to my normal pressure while inside my cool garage. When I was out in the hot sun the tyres felt a bit harder than usual. I checked with the same gauge and they were 2psi higher. If I can easily feel 2psi, I'm sure that you can feel nearly double that.

Having the same absolute pressure is an illusion. What matters is the internal pressure above the surroundings. That is what gives the tyre its support and its feel. So, your assumption is wrong. And you should use the same gauge pressure at whatever altitude you are at. And don't forget to check your air suspension too. 4psi may not affect the shock much, but it may have a noticeable effect on the fork.

Remember, if the absolute pressure was 14.7 psi, the tyre would be flat at sea level!

 

[RebornRider]

Having the same absolute pressure is an illusion. What matters is the internal pressure above the surroundings. That is what gives the tyre its support and its feel. So, your assumption is wrong. And you should use the same gauge pressure at whatever altitude you are at. And don't forget to check your air suspension too. 4psi may not affect the shock much, but it may have a noticeable effect on the fork.

Remember, if the absolute pressure was 14.7 psi, the tyre would be flat at sea level!

I think you may misunderstand gage vs. absolute pressure. A flat tire is at 14.7 psi absolute at sea level, the same as everything else at sea level unless it is sealed and artificially inflated. 14.7 psi absolute is 0 psi gage at sea level.

Here is the first Google hit in a search for gage versus absolute pressure. Read it and then get back to us.

Difference Between Gauge and Absolute Pressure Measurement

Pressure is as the force applied to an area. There are two common different systems of pressure measurement, absolute pressure and sealed gauge pressure

esi-tec.com

 

[RebornRider]

I’ll say with 100% certainty that at 11,000ft you’ll feel the beginning of hypoxia.

Edit to say that, yes, that's my Levo and yes, I took that picture.

 

[RebornRider]

I lived in Littleton CO for 6 years and hiked with a group that was into bagging 14ers. I remember hiking to the top of Longs Peak (14,259 according to The Internet) a couple of times, but I don't remember which others I was in on. I'm the little guy between the ladies.

 

[Tonybro]

Let's get started.

• Assume I have an accurate tire pressure gauge.
• Assume I inflate the tire to 20 psi at a location where atmospheric pressure is 14.7 psi.
• The absolute tire pressure is 34.7 psi (14.7 + 20)
• I take the tire to an elevation where the atmospheric pressure is 11 psi.
• Assume the tire doesn't leak, so its absolute pressure is still 34.7 psi.
• But the tire's gage pressure is now 23.7 psi (34.7 - 11)
• Since the absolute pressure didn't change, my assumption is that the tire will feel the same as it did at sea level.
• But I've never tested this carefully.

My question for the guys that can feel small differences in tire pressure: Do you adjust your target inflation pressure when you ride at high elevations? e.g. change from a target of 22 psi at sea level to 25 psi at 11,000 feet?

There's another variable you are missing...

Variability of atmospheric pressure (weather). It will vary from low in (generally) poor weather to high (generally) good weather and that could account for one or two PSI either way.

That's why aircraft get given the atmospheric pressure for where they are taking off and landing so the altimeters work and read accurately.

 

[steve_sordy]

I think you may misunderstand gage vs. absolute pressure. A flat tire is at 14.7 psi absolute at sea level, the same as everything else at sea level unless it is sealed and artificially inflated. 14.7 psi absolute is 0 psi gage at sea level.

Here is the first Google hit in a search for gage versus absolute pressure. Read it and then get back to us.

Difference Between Gauge and Absolute Pressure Measurement

Pressure is as the force applied to an area. There are two common different systems of pressure measurement, absolute pressure and sealed gauge pressure

esi-tec.com

I have not misunderstood the difference between absolute and gauge pressure. Reading your link has only confirmed that view.
I agree that a flat tyre is at 14.7psi absolute at sea level. Which part of what I posted is it that you disagree with? Maybe it could just be a difference in emphasis of how what I wrote is read.

 

[yorkshire89]

I just set my normal tyres pressures at the lowest point you are going to be riding in a certain area, usually where I start the ride.
I couldn't really notice any difference from the top and bottom of the trail/mountain.

I'd also have to remember to reset my fork pressures, this had a much bigger effect on the feel of the bike in a high altitude area after moving away from sea level

 

[Slowroller]

I'll adjust my tire pressure in the local I'm riding in. So, yes, routinely adjust for higher elevations. The only time I don't is when I'm starting high, but finishing low, then I'll set it for a lower elevation. In response to "lol, it doesn't matter that much", if your ride involves 5-7k ft in elevation change, it sure does.