Air pressure is determined by the temperature of the air in the tire. Wheels and rubber may just delay the air temp.
Also, for a given temperature change the amount of thermal expansion or contraction of steel, aluminum, or rubber is a very small fraction of the amount for air (if that’s what @ChrisTheTireWhisperer is getting at). So the pressure changes due to air volume changes dominates the system.
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Yes, but I was also aiming at the rate of how cold a steel vs alloy wheel gets, and how that affects tire pressure as well.
On year, I had alloys on a car, and my wife’s has steel rims. Her cold tire pressures fluctuated very little on a week-to-week basis, but my car, with alloys, the pressure was up n down like the biggest roller coaster at the Six Flags park! Both our cars then specified 30-32psi cold.
I have rarely experienced air loss due to temp. TPMS is great, TPMS on one car will even show the tire pressure accurately as far as I can tell. Last 4 years maybe 4 times I have had to add air. I am probably more representative of the driving public. rarely check oil level or tire pressureetc.
Temperature is temperature. Wheel material is irrelevant in that regard. Alloy wheels may be porous, however, leading to air loss.
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Not just pressure loss.
When the weather warmed up, and I hadn’t checked our pressures in a while, my wife’s car was barely 1psi over. Mine on alloys was 2psi over!
How do the tire sizes compare?
Certainly sounds like a scientific study .
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The will both will get just as cold as whatever the ambient temp. is. Please explain how one could get colder than another.
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Different materials, Pvt!
So when it’s 18f outside, which rim is colder, the steel or the alloy?
If you put the same size/brand/model tire on the same size steel or aluminum wheel, I bet they would experience the same pressure changes with temperature. But you may have a lower profile tire on the aluminum wheel compared to the tire on the steel wheel. That why I ask: what are the two tire sizes?
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Aluminum conducts heat better than steel, but the alloy wheels have a thicker cross section so they may have more thermal mass. Conductivity and thermal mass will affect how fast the wheel will cool down or heat up, but both will eventually get to the same ambient temperature.
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I don’t believe tire size, and therefore volume of air, matters. Pressure drop or increase with temperature remains the same.
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That was my initial thought, but he’s seeing some difference between the two, so I’m trying to come up with something that would explain it. I can imagine a 235/30x21" tire/wheel having a slightly different reaction to temperature change than a 185/65x15" tire/wheel.
On all 4 tires? or just one side of vehicle? if just one side it could be the sun was hitting that side. if you are parked next to each other one side of your car would be in the sun. where your other side and all 4 of her car would be in the shade. just a thought.
+1
I monitor my tire pressure very closely, and when my vehicle is parked outside, the tires on the “sunny” side are always 1-2 psi higher after a couple of hours.
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I expect CTW is wondering if there’s any pressure effect owing to the rim materials’ dimension changing with temperature.
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Nope. Both 195-65R15.
Unless a Chinook is coming through, either wheel will keep up with the ambient temperature change. You’re barking up the wrong tree here.
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