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Tyre pressure stability (air, nitrogen, temperature etc)
- Thread starter Mr Miserable
- Start date
For anyone who forgot or maybe never knew, early large aircraft wheels were mostly magnesium, with large Auden high temperatures on landing the wheels would burn. Not good! thus nitrogen, because it is inert. I still use nitrogen in my cars, probably out of habit since I used to fill my cars from the aircraft supply. Nonetheless I do always use nitrogen.
Whatever, nitrogen works for me.
As winter approaches the tyres retain their pressure - no yellow lights for me.
(Properties of gases - seemed to take up a lot of time on ETPS syllabus)
As winter approaches the tyres retain their pressure - no yellow lights for me.
(Properties of gases - seemed to take up a lot of time on ETPS syllabus)
Glan gluaisne
Active Member
Together with partial pressures, and their effect on human respiratory function at altitude . . .
Glan gluaisne
Active Member
As the volume is fixed by the physical dimensions of the wheel and tyre, it's another of the Gas Laws that applies, Gay-Lussac's law.
Charles' Law covers the case where the volume changes with temperature, whereas Gay-Lussac's law covers the case where the volume is fixed and the pressure changes with temperature.
For those of you interested in digging into this subject further, here's a white paper and a letter on the subject of effusion and permeation as it relates to O2 vs N2 in tires. The short answer is that if the two are completely dry, there's no appreciable difference between O2 and N2 in volume change due to temperature at the pressure and temperatures seen in automobile tire usage. The difference comes from moisture that is typically present when tires are filled with ambient air and the fact that O2 is 3-4 times more likely to pass through tire compounds than N2 is. These moisture/permeability issues do make a difference.
The main advantage of nitrogen is it's generally a more consistent and pure gas, and yes while all gasses to expand and contract with temperature, the other components like water in standard air from a regular garage compressor can result in the tyre warming to a higher temp than if it was running nitrogen meaning that the pressure increases. This is more noticeable in racing than daily driving, but for general daily driving it won't make much difference, just keep your pressures accurate in winter and summer.
HenryT
Active Member
There's part of this that's incorrect. If the air has some moisture in it, it will increase the pressure quicker and so the tire will actually run cooler. Race car drivers use nitrogen because it's perfectly dry so they can tune the suspension to get that last 1/100th of a second reduction in lap time. However, in normal highway use, the tires have to flex more with nitrogen to get to the equilibrium pressure (where the heat buildup from flexing is equal to the cooling from the air passing over the tire) so they will run hotter than a tire with air and a bit of moisture. Nitrogen inflation for on-road and above ground applications is generally non-optimal.
LongRanger
Active Member
Just fill your tyres with a puncture sealant can. Much less temperature effect on pressures and very rarely needs topping up 
Glan gluaisne
Active Member
I've lost count of the number of times people have argued with me when I've mentioned that parachutes descend faster through cloud than they do in dry air. For some reason, despite the fact that it's pretty damned obvious that clouds are lighter than air, so less dense, some people seem to assume that water vapour has to be denser than air.
AndrewGR
Member
Sorry about thread drift but... I don’t think that is right. Clouds are visible because they contain liquid water or ice which is much heavier than air. If they were less dense than the air around them they would be rising - which they sometimes do. - just watch a growing CuNim! I believe there is a story about someone who bailed out in a CuNim and took over an hour to reach the ground. Of course, in thermals below cumulus clouds the moist air is not saturated and, being less dense, does rise - as glider pilots know.
Glan gluaisne
Active Member
I worked on parachute development trials back in the early 1980s (weapon retardation parachutes) and the terminal velocity definitely varies with humidity. Back then (pre-personal computer era) we used a temperature/humidity nomogram to determine terminal velocity for any set of environmental conditions, as it made a significant difference to forward throw, and hence where the release point needed to be for any given target position on the surface.
Upward air movement will also make a difference, although that depends on the velocity of the object. In our case, terminal velocities for parachute retarded stores were usually around 70kts to maybe 120kts, so a lot faster than someone coming down on a sport parachute.
AndrewGR
Member
Hi Glan. Very prepared to accept your temperature / humidity / terminal velocity observations. Just wondering if they covered the situation in clouds where, effectively, the humidity is greater than 100%?
Glan gluaisne
Active Member
It was just the effect that water vapour had on air density, so not the situation where there's very tiny liquid water droplets suspended by rising air movement. Because water is changing phase within a cloud, from gas to liquid, the air has to be close to being fully saturated with water vapour, so will be less dense than dry air. Not sure that the water droplets have much impact on the drag of anything falling through a cloud, I'm pretty sure it's primarily the effect of the high humidity that reduces the density a bit.
There isn't much liquid water in a cloud, either, IIRC clouds are usually only about 0.1% liquid water, the rest being air, saturated with water vapour.
Sean.
Active Member
Sparkeur
Member
Ahhh. Every proper car forum needs a “nitrogen” thread...
Unlikely however, that we will progress to Nitrous Oxide...on this forum.
