Tire pressures during temperature swings

DarkSkySeeker wrote:

myredracer wrote:


Actually the Ideal Gas Law is PV = nRT where "n" is the number of moles (a measure of the number of molecules of gas). :R

It's been so long since I was in High School or College Chemistry. :C

Gawd, I so hated chemistry in university! Almost didn't make it into engineering school. Studied my arse off and did a make-up exam and aced it. Now look what happens, lol... :)

A friend who owns a tire store says a tire that is under-inflated runs hotter due to increased sidewall flex. This heat is what causes blow outs on tires that are within their normal life span. Old tires may blow out due to cord failure no matter what the pressure or heat. Newer tires may blow out due to previous cord damage from a big pot hole or running over a curb. Unless a viable tire is way over-inflated when cold it won't blow out when on the road because it runs cooler at higher pressure. The small increase in pressure due to higher ambient temperature are not enough to cause failure.

myredracer wrote:


Actually the Ideal Gas Law is PV = nRT where "n" is the number of moles (a measure of the number of molecules of gas). :R

It's been so long since I was in High School or College Chemistry. :C

Harvey51 wrote:
The physics of pressure and temperature is PV = RT

Actually the Ideal Gas Law is PV = nRT where "n" is the number of moles (a measure of the number of molecules of gas). :R

"R" is a universal constant and the value of R depends on the particular gas. "T" is the temperature in Kelvin (called the "absolute" temp.). 70F for ex., = 294K.

It gets a bit more complicated because air is a mixture of gases - oxygen, nitrogen, argon, helium, CO2 and a few others. As well, air has water vapor in it and pressure can also be affected by the dew point. The volume of a tire will also be affected somewhat by the atmospheric air pressure (think weather balloon).

Gay-Lussac's law is more applicable for tires because n & R don't significantly affect the above gas law equation and P1/T1 = P2/T2 can be used. If you started your day at 70F and the tire air temp went to say 140F (333K), the increase in pressure would be 333/294 = 1.133 times (13 percent increase). Simply not much to worry about. Note that tire air temp. will be somewhat higher than ambient. And don't forget that the max. pressure on the sidewall of a tire is cold pressure.

Having said all that, if you really need to know the affect of temp., all you need to know is the general rule of thumb approximation, a 2% increase in pressure for every 10F increase in temp.

The physics of pressure and temperature is PV = RT
V is volume of air in the tire, R a constant, T the temperature in Absolute degrees, 273 plus the Celsius temperature. Where summer is 27 C higher than winter, T is 10% higher which causes Pressure to be 10% higher by the formula. I think best to do some morning adjustments before driving during spring and fall.

Its fine.

"Cold" psi is when the tires have been in full shade for min. 2-3 hours. If you are setting the psi as per the sticker on door jamb, you'd have even less to worry about. You just don't hear about tire failures on TVs in comparison to ST tires on trailers (which should be run at max sidewall psi) which is often from being under-inflated. Losing air in a tire is the thing to worry about, not excessive pressure.

We have E rated tires on our F250 and I run 75 front & 80 rear (for better handling) and have towed many thousands of miles all over the western US and never an issue. At the end of the towing season I let some air out.

You could always read Roger Marble's (retired tire engineer) RVtiresafety.net blog which has tons of excellent info. for RV-ers.

fj12ryder wrote:
Don't overthink the whole thing. Check your tires in the morning, adjust as necessary. Do the same the next morning, and the morning after that. Trying to keep tires at a mean pressure while actually rolling down the road is just a waste of time and effort. 10 psi pressure changes as tires heat up and cool down is perfectly normal.

Basically yes. The pressure should be set on cold tires and it’s expected to rise with driving down the road- and should not be reduced. (The ideal OPERATING pressure is higher than the cold pressure.

If there’s big temperature changes in the WEATHER, tires ideally should be adjusted- but should be cold when you do it.

Every day sounds a bit much unless you are doing the snowbird thing where you’ll get some possible significant changes.

Elevation gain or loss can also effect psi, as can barometer readings. Generally speaking.....unless as noted by on person, . You are at load limits....not worth worrying about.
Marty

Aside from different loads or seasons (for traction, not temperature per se), tire pressure is set it and forget it.
The one guy who’s pressure went up 15+ psi was partly due to ambient change and part due to tire heat from driving.
Only time I could see adjusting pressure based on temp is if you Dropped 80-100deg ambient and were at your load limits. Then you might want to add 10psi