Submitted by xYoSoYx t3_z4nu9g in explainlikeimfive
I realize that when it gets cold the molecules slow down, which will reduce the psi inside the tire. But wouldn’t the outside pressure also reduce at an equivalent rate since the outside temperature has also gone down to the same temp, thus canceling out the psi difference in the tire?
If anything, i would think the temp inside the tire would be higher, because black tire absorbs more heat from the sun and would keep the tire more insulated.
Ahh okay. So they don’t just slow down and give less pressure - they actually condense into water, and give no pressure at all then?
I don’t think I ever would’ve thought of them turning to water.
Follow up - if you’re interested - how does this affect the pressure of the tire if you fill it up in the winter, and then it gets warmer out? Wouldn’t that make the psi too high?
I'm pretty sure they do not condense. Maybe I need to look into it, but I am almost certain it's just that all things contract when cold. This is why I'm some old houses windows and doors don't shut right in the cold.
This means the same amount of air takes up less space and you need more. When this air gets warm you may end up with a little too much pressure.
At 32 psi tire pressure (46 absolute) inside and 14 absolute outside, a decrease of 10% pressure would leave you with 41.4 absolute inside vs 12.6 outside. The relative pressure has dropped by, you guessed it, 10%. Now you're at a 28.8psi.
However, there are other factors at play, too. The atmosphere's pressure depends on both its temperature and volume, and its volume is not limited by any container, so a hotter atmosphere will expand but the pressure can stay the same.
In fact, the pressure does stay the same, because it is the weight of the air in the atmosphere that gives it pressure, whereas in a tire it is the tire walls stretching and keeping the air in that gives it pressure.
This all compounded by the nonlinear PV-T behavior of tire gas, especially water, which sees a huge pressure/volume drop as you approach 0 degrees c. Our atmosphere also has water in it, though, so without my first two points this wouldn't matter.
Very good explanation…thank you!
Because tire pressure is about the absolute pressure of the gas within the tire, not the difference between inside and outside. The gas within the tire has to apply enough force on the tire to keep it from deflating due to the weight and movement of the car. The ambient air pressure has nothing to do with this. Once the pressure drops within the tire, the gass can't exert enough stress on the tire to keep it in shape.
Had another thought after rereading your last statement about water…
Would this mean that going from an extremely humid climate to an extremely dry climate would also impact the tire pressure? Just less significantly than a change in temperature?
Just curious if there is some mathematical equations that would represent the multiple factors you’ve laid out to explain the impact on the tires psi.
You’ve pretty much answered my initial question, just the math nerd in me wondering if this is a thing, so had to ask lol.
I had not heard of/considered “absolute” pressure before - not my area of expertise - so that definitely helps me better understand this. Thank you!
Because PV=nRT. This is a formula from chemistry, also known as the Ideal Gas Law where:
P - pressure
V - volume
n - number of moles of the gas (where a mole is Avogadro’s number of molecules of that gas)
R - Boltzmann’s constant
T - temperature
So in a closed environment like a tire, the volume and moles of gas are constant. That means if the temperature goes down, with everything else being equal, pressure has to go down as well. Similarly if temperature goes up, then the pressure would necessarily have to go up.
Awesome - I’ve always loved Chem/Phsysics, so now that I know there is a formula for this, I am satisfied :) thank you for providing this!
It shouldn't. As I mentioned, outside air pressure is pretty much fixed. There's a bit of variation but almost none. None big enough to be significant compared to tire pressure.
Humidity within the tire matters when it's inflated. A tire inflated with hot and humid air will lose a lot of pressure as it cools, but a tire inflated with cold and dry air will not gain nearly as much pressure as it heats up. This is because the amount of air and water in the tire is fixed, but the water may expand or contract quite a bit with temperature. The outside humidity once the tire is sealed makes no difference.
That makes total sense, again - thank you!
Once gas contained inside a pressurized tube escape (inevitably it will escape, balloons shrink too) it cant re pressurize. Entropy dictates that the pressure inside a balloon or tire is trying to stabilize to atmospheric pressure. Only the tire or balloon wall is preventing that from happening. But they all slowly leak regardless which is why you have to re inflate anyway. Cold weather exacerbates this process. Cold temperatures drop pressure which reduces the effectiveness of a good seal on your wheel. Low pressure tires lose air faster than high pressure tires because the shape of the tire with pressure produces a better seal. Once the seal is compromised by temperature change it losses more pressure.
dbradx t1_ixru0di wrote
When it gets cold enough, the air in the tire condenses, i.e. transforms into water droplets and takes up less space in the tire. After driving for a bit, it heats back up and will often return to full pressure until it gets cold again.