Tyre pressure at high altitudes?

[JulesW]

Can anyone tell me, out of idle curiosity, do tyres noticeably overinflate with a significant rise in altitude e.g. 2000m ?

[SvNewbie]

According to Wikipedia the pressure difference between 0 and 2000m is around 20kPa, according to Google that works out as just under 3 PSI. So just about noticeable if your riding but no where near dangerous.

http://en.m.wikipedia.org/wiki/Atmospheric_pressure

[JulesW]

Cheers SvN ;)

[jambo]

This is interesting, I'm wondering how much of this pressure change balances out against the temperature change?

You loose about 0.65 degrees C, per 100m, so would have lost about 13C in ambient temperature over 2000m.

If the volume remains constant inside the tyre, a temperature drop will give a pressure drop, you might find that the drop in outside pressure and temperature level out, unfortunately I don't have time to work this out at work today. I like these kind of queries.

Jambo

[Luckypants]

You might be on to something there Jambo. IME there is no discernible difference in pressures at 2000+ metres and sea-level. I had thought the reason for this was daily tyre pressure checks on a trip where this kind of altitude gain is possible, not had the chance to go from sea-level to 2000 meters in one hit!

This is certainly not a topic which gets any coverage in Euro-trip forums, so anecdotally I'd say the change is negligible.

[SvNewbie]

I was thinking about that too Luckypants. Assuming a perfect tyre (no leaks) the actual volume of air inside the tyre hasn't changed. If you brought it back to sea level the pressure would surely read the same.

Is the tyre pressure you'd measure with a tyre pressure gauge actually relative to the atmospheric pressure? I think that is the case, in which cases you would see a pressure change. Otherwise, the pressure inside the tyre would remain constant.

[Red ones]

Surely the tyre pressure is dependant on the quantity of air you put in the tyre, the volume of the tyre itself and ye temperature of the air in the tyre. The pressure inside the tyre can only change if the temperature drops, the tyre size changes or the quantity of air is altered.
The tyre size is likely to be fairly constant and unless you have a leak the quantity of air is also constant therefore the only variable is the temperature.

Boyle's Law.

Taken to an extreme the tyre would pop if the external pressure on the tyre is very low but I doubt many of us would be trying to ride in a vacuum but this is where there is no pressure to hold the tyre together.

[SvNewbie]

The absolute pressure is constant (ignoring the temperature change and the fact that the tyre would deform if the pressure was high enough), however the gauge pressure is relative to the outside pressure. So if you took the valve out of the tyre and let it go completely flat, then reinstalled it, the pressure would read 0, as it is the same as atmospheric pressure.

[yorkie_chris]

You are measuring a relative pressure with any gauge you have. Gauge pressure... geddit...
Otherwise they'd read -1 bar when not connected to a tyre, ish.

important thing is relative pressure. You want the tyre to exert a force on the road... to do a thought experiment... if you went to hell and the Pabs was 3 bar... your fully inflated tyre would be flat.

[Red ones]

My tyres didn't go flat when I went to Luton.

[ophic]

My tyres didn't go flat when I went to Luton.
On a car they turn to bricks

[Red Herring]

I find when I go over 2000m the ice and snow on the road make the tyre pressures on my bike pretty irrelevant...

[Biker Biggles]

My tyres didn't go flat when I went to Luton.

I went to Suffolk and mine did.But thats below sea level so the pressure is very high.Strangely only one tyre went flat,and it could have been related to the nail sticking out of it.

[Jason H]

This is roughly what would happen to your tyre pressure based on a temperature drop of 0.65C and a pressure drop of 1.2kPa per 100m.

Not a huge difference really.

[wideguy]

As the tire pressure drops, the carcass gets more flexible, which heats it up if you're riding twisty roads, so the tire pressure goes back up.
I wish my favorite mountain roads weren't covered in snow and ice.