I understand how it can decrease, continuous pressure would force that out, but I can’t wrap my head around why sometimes, I check the pressure and it’ll say like 40psi??? It should be 32???
In: Engineering
Friction between the rubber and the road, and internal friction in the rubber as it flexes, heats up the tire, and then heats up the air inside. If it’s summer and the road surface is hot, that’ll contribute too, and of course so will sunshine on the rubber.
Assuming the walls of the tire are fairly rigid, if your tire starts out at room temperature, every Celsius degree it warms up should raise the pressure by about 0.3%, or an extra 0.11psi if it started at 32.
Now, having said that, a jump from 32psi to 40 is an awful lot and I’m not sure heating can explain all that unless your tires are too hot to touch. Is there a chance your pressure gauge is inconsistent?
[Edit: Or have you ever used one of those spray-can repair kits that mends a flat tire? Those can leave residue in the tire that will evolve fumes.]
Depending on your manufacturer’s specs written in the door jam, it should be around 30 cold, meaning you haven’t been driving it. As you drive, the tires flex and get warm. That heat transfers to the air in your tire which increases pressure.
Air volume increases as the temperature increases. When you drive, your tires heat up, causing the air inside them to increase in temperature and expand.
PV = nRT is the “Ideal Gas Law” formula. Of course, this is under ideal circumstances… but close enough.
‘P’ is the pressure in atmospheres
‘V’ is the volume in liters
‘n’ is the number of particles in moles
‘T’ is the temperature in Kelvin [ (((F° – 32)x5)/5)+273.15 ]
‘R’ is the ideal gas constant (0.0821 liter atmospheres per moles Kelvin)
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In this situation, R and n **cannot** change as R is a constant and
n ( that’s the amount of gas you have inside a confined system) will stay fairly constant unless you have a leak.
That means that Pressure can only increase under two conditions:
1)Temp goes up
2) Volume goes down
Tires won’t expend much, so the biggest factor in pressure will be temperature (again, assuming you have no leak).
Friction from motion will heat up the tires and the contents inside… combined with hot air and hot asphalt, the temperature can rise quite a bit.
Nonetheless, the pressure should not rise by 7 psi. Maybe 2-4.
My guess, is that you likely filled the tire on a cold day, before the tire warmed up. You were mislead to believe that you had lost some air in the tire and “overcompensated” by adding more.
When the temperature rose back to normal. The “pre-driving” pressure was likely 1-3 psi over 32. Combined with the tire heating up for the reasons mentioned above, this could partially explain the mystery for why you are seeing such a big difference.
But, it still seems unlikely that you “always” fill up your tires on a really cold day.
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