I've always read it's important to check tire pressures "cold" before driving the car. While the furthest thing from an engineer, I've always assumed that is because of the relative density of air dependent on temperature. That driving creates friction and hence, heat. That this heat causes expansion, and therefore more pressure. But how does air temperature affect tire pressure? Some mornings cold can mean 80F, and others 25F. Wouldn't this also have a significant effect on tire pressure? Is there a chart, or other method that allows for adjustments? What IS the significance of changes in air temperature to "cold" tire pressures?

 

A few months ago, I went up to the mountains for a weekend with my wife. We took her A4 with the OEM sport package Dunlop SP 8000E's on it.

Our home elevation is 25 feet above sea level. We spent the weekend at 7000 feet above sea level. The A4 sat undriven for two days and nights while we hiked, etc. Overnight temps were in the 30's, and daytime temps were probably in the 50's.

We got in the car on Sunday afternoon to drive home, and about 25 miles down the mountain, one of our tires blew out. When I took it in for repair, the tire shop said that the tire ran too long at a depressed pressure.

I figure that the combined effect of altitude (thinner air), reduced temperature, and lack of use for two days depressed the tire pressures to an all time low.

So, yes, I agree with you; I think that outside temps do have an effect on pressure, but only if your tires have had a chance to adjust to those temps.

 

<ul><li><a href="http://www.tirerack.com/tires/tiretech/general/pressure.htm">tire pressure</a></li></ul>

 

What you measure is called gauge pressure. It is the difference between the pressure in the tire and the external pressure. As the external pressure decreases, the gauge pressure increases.

You lose about 1 psi per 10 deg F drop in temperature. You probably gained 1 to 2 psi from the altitude.

Sounds like it was seriously cold and/or you had a leak and/or you hadn't checked the tire pressures in a while.

- Tom

 

The only answer seems to be an air compressor or air in a can or the good old manual tire pump, in the garage before the car is moved, first thing in the morning.

 

but the cold certainly will cause the tire to deflate (the gas inside the tire will contract, but the actual number of "air" molecules will not change). Elevation will, however, affect tire volume...as the outside air pressure goes down (at altitude) the inside air will expand. The tire is a sealed, pressurized chamber. Gas in an enclosed space will expand at elevation, but the pressure within the chamber does not change (if it were to be allowed to expand as much as it needed to)...it expands because it is trying to fill the pressure void created by the lower outside air pressure...but of course, it is restricted by the tension of the walls of the tires and the pressure increases.

The cold temps and elevation actually oppose each other with relation to tire volume...if the elevation is great enough, the tire could explode due to high volume inside...if the cold is great enough the tire could deflate significantly.

If your tire wall blew out because of alleged low pressure, it was certainly because the wall had been weakened by a very prolonged time at low pressure. Most people drive around, unknowingly, with one or more of their tires seriously deflated for months and months and don't have blow outs...I don't think 2 days could do it. Perhaps the blow out was due to the high elevation causing expansion, then when you started driving, the tires heated up more causing more expansion and KABOOM!

 

Hmmm...yes, that makes sense. That's why the bag of chips is always puffed up when we get up there.

 

I need 2000?!

 

Yeah, that's kinda funny

 

But not to the exclusion of watching the road. Actually, I check it most days just after I come out of the driveway and spot check it on hot afternoons or after fast driving to see how things are changing. The front tires heat more than the rear (more load!) and I often notice a change from one side to the other from the sun (even while driving). A lot of cornering in one direction (say right turns) will heat the outside front tire more than the inside tire relative to the turns. The tires will go from 37 cold in the morning to 42 in the afternoon after driving.

- Tom