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Photo by Josephine Flohr, Elephant at Camp, Namibia

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Photo by Josephine Flohr,
Elephant at Camp, Namibia



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  #1  
Old 5 Jan 2016
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Altitude and tire pressure?

I was wondering what effect large changes in altitude can have, if any, on my tire pressure? I just wonder as had some weird tire pressure readings lately, but have also been going from sea-level to over 4,000m. If I inflate (using hand pump) my tires at 4,000m, I assume the air is thinner - when I decrease altitude will the tire then have less air in it - or do you just have to pump more times to achieve the desired PSI? :confused

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Old 5 Jan 2016
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Tyre pressure is measured as an ambient pressure, so if the tyre is inflated at sea level to 2 bar, when you are at 4,000m the gauge will tell you that it's more than 2 bar.

If you inflate to what you think is 2 bar at 4,000m and descend to sea level the pressure will measure less.

And then you need to factor in temperature...
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Old 5 Jan 2016
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Old 5 Jan 2016
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In 4.000 metres attitude the difference of the pressure will be just 0.600 Bars, or in PSI +/-0.8 so means that a tire at an average of 28 PSI could be 1 PSI more or less, up or down….
As my friend Ted says: JUST RIDE IT!!!
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Old 5 Jan 2016
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Originally Posted by javkap View Post
In 4.000 metres attitude the difference of the pressure will be just 0.600 Bars, or in PSI +/-0.8 so means that a tire at an average of 28 PSI could be 1 PSI more or less, up or down….
I believe you've misunderstood what the 0.6 bar represents. It's a comparison against 1.0 bar at sea level, so it's a 40% loss and hence the difference for a tyre at 28 psi would be 11.2 psi.

Please correct me if I'm wrong.



This is the effect on a tube of Pringles at just 2,500m. I think at 4,000m it would have burst but I was hungry so I ate them and never found out
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Old 5 Jan 2016
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I've ridden from 0-4500 metres in one day. My tyres didn't look or act any differently. Never seen or heard of anyone adjusting them.

I'm sure the science is accurate but in the real world... Just ride.
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Old 6 Jan 2016
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The question was about unusual tyre pressure readings, not how the tyres acted on the road.
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Old 6 Jan 2016
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I thought this forum was about riding...
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Old 6 Jan 2016
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Seems right, altitude from sea level to 4K meters won't have a huge affect on cold PSI reading.

The other thing mentioned was HEAT, which can have an affect. And another factor NO ONE mentioned that also affects PSI is ... MOISTURE.
Moisture combined with heat can give a boost to PSI, not always desirable ... especially on a race track. Traveling? Just ride!

Typically, you set tire pressures when tires are cool. Once super hot, pressures can go up maybe 3 to 5 PSI on a tire that measured 30 PSI cold. The other thing that can cause eve more rise is moisture. Moist air is bad for an accurate,
consistent pressure.

Many Petrol station air hoses will emit A Lot of moisture with the air. (true here in USA, where you can often SEE the moisture in the air!) Racers use Nitrogen in their tires to mitigate this variable and maintain more stable pressures.

For a traveler, most times its a MOOT point. Just ride.
A 3 to 5 PSI rise is not likely to have great affect.
If it does ... stop and let out a few PSI ... but air back up later if required.
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Old 6 Jan 2016
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Quote:
Originally Posted by Tim Cullis View Post
The question was about unusual tyre pressure readings, not how the tyres acted on the road.
Hey Tim, News Flash! These two points are LINKED. You can't talk about one without mentioning the other. Pressures affect the ride and the ride (and bike) may need pressure adjustment depending on what the pilot "feels". The Pilots perceptions are KEY to it all.

If my bike "feels" weird (what ever the hell that means!) I might get off and put a gauge on the tires ... or maybe use my EYES and notice I've got a flat.

Seat of the pants "feel" is our last best hope for survival on a bike!
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Old 6 Jan 2016
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I understood that moisture in air causes no significant departure from the ideal gas law under normal practical driving conditions. When that is pointed out, all sorts of other reasons surface like causing rust on the rims or the tyres rot out on the inside which in itself is a load of rot or it rusts the steel cords or... or... .
The bit about using nitrogen in normal car and truck tyres gets shot down every time it is raised so I don't know why it can't stay dead.

As for the actual pressure inside a tyre. Couldn't you make an argument that if you go up in altitude, the air pressure is pushing less on the outside of the tyre so the tyre becomes effectively over inflated - but high altitudes are usually associated with lower ambient temperatures which reduce the running pressure, thus mitigating any altitude effects.

Then again ....

or maybe even .....

Just ride.
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Old 6 Jan 2016
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Quote:
Originally Posted by Tony LEE View Post

As for the actual pressure inside a tyre. Couldn't you make an argument that if you go up in altitude, the air pressure is pushing less on the outside of the tyre so the tyre becomes effectively over inflated - but high altitudes are usually associated with lower ambient temperatures which reduce the running pressure, thus mitigating any altitude effects.

Then again ....

or maybe even .....

Just ride.
Excellent point....

We need a official science boffin to solve this for us
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Old 6 Jan 2016
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Quote:
Originally Posted by *Touring Ted* View Post
Excellent point....

We need a official science boffin to solve this for us
They are too busy with the global/climate warming/change/cooling conumdrum.

Boyle's law applies though.

"the law is accurate enough to be useful in a number of practical applications. It is used, for example, in calculating the volume and pressure of internal-combustion engines and steam engines".
From Boyle's Law - HowStuffWorks
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Old 6 Jan 2016
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I had the three main gas laws—Boyle's, Charles' and Henry's—drilled into me from my days in BSAC (scuba).

Gases heat up as they are being compressed, one of the two reasons for immersing scuba tanks in water whilst being filled (the other being to mitigate the effect should a bottle fail). Gases rapidly cool when being deflated which is why those tiny CO2 canisters used to inflate a tyre have a mesh cover—the canister drops to subzero temperature.

Charles' law states the pressure varies directly in proportion to the temperature (°K) which is why you are told to measure tyre pressures when the tyre is cold, but many people don't realise this. When the hot tyre cools the pressure drops and the tyre is under inflated.

On another thread here or maybe another forum, someone was asking me how I was getting on with mousses in my KTM 690R (supposedly the equivalent of 0.9 bar or 13 psi). I had to say that I couldn't actually notice any difference between my 690 in Spain with mousses and my 690 in the UK with standard inner tubes which are probably at 2 bar.

So as Ted says, just keep riding.
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Last edited by Tim Cullis; 6 Jan 2016 at 11:52.
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Old 6 Jan 2016
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There is one situation where tyres need to be inflated to a non-resting value and that is when the vehicle is stored in a heated garage but will be operated in a very cold environment. I guess if it was stored in airconditioned garage but operated in summer desert conditions a similar allowance would need to be made.
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