Tire Pressure

We live at the 5,000 foot elevation. If I put 50lbs of pressure in the TM tires at 5,000 feet when I get to sea level the pressure will be less than 50lbs. Does anyone know if there is a formula I could use to compensate for drop in tire pressure. Most of my traveling is done at sea level to about 1,000 feet. If I travel in mountains it is not an issue.

Originally Posted by freein05

We live at the 5,000 foot elevation. If I put 50lbs of pressure in the TM tires at 5,000 feet when I get to sea level the pressure will be less than 50lbs. Does anyone know if there is a formula I could use to compensate for drop in tire pressure. Most of my traveling is done at sea level to about 1,000 feet. If I travel in mountains it is not an issue.

Freein05, there kind of is a rule of thumb for this, but you have to keep all of the other variables constant (or know how to compensate)

The main variable in tire pressure is temperature. Since air is a gas, it expands when heated and contracts when cooled. This is why you check/adjust tire pressures at cold temperature (which is morning temperature)

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (it goes up with higher temperatures and down with lower).

There is a difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country of about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle's tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle's tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.
If the vehicle is parked in the sun, the sun's radiant heat will artificially and temporarily increase tire pressures.

OK, now if we measure the psi in the tire at the same temperature but just change the elevation, then the rule of thumb is that atmospheric pressure increases about a half pound per 1000 foot of elevation. A tire with 32 psi at mean sea level will measure around 34.5 psi at 5000 foot above sea level, given the same temperature.

I am sure the engineers we have on the forum will be able to explain it better.

[cali camping]

Originally Posted by wmtire

Freein05, there kind of is a rule of thumb for this, but you have to keep all of the other variables constant (or know how to compensate)

The main variable in tire pressure is temperature. Since air is a gas, it expands when heated and contracts when cooled. This is why you check/adjust tire pressures at cold temperature (which is morning temperature)

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (it goes up with higher temperatures and down with lower).

There is a difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country of about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle's tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle's tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.
If the vehicle is parked in the sun, the sun's radiant heat will artificially and temporarily increase tire pressures.

OK, now if we measure the psi in the tire at the same temperature but just change the elevation, then the rule of thumb is that atmospheric pressure increases about a half pound per 1000 foot of elevation. A tire with 32 psi at mean sea level will measure around 34.5 psi at 5000 foot above sea level, given the same temperature.

I am sure the engineers we have on the forum will be able to explain it better.

I'll be the first to chime in. Atmospheric pressure DOES NOT increase with elevation. Standard day atmospheric day pressure at sea level is approximately 14.70 psi. This is the pressure acting on us and is not to be confused with barometric pressure readings. Atmospheric pressure DECREASES with altitude which is why we all struggle to breath when we go to the mountains! It is also why gasoline engines lose power with an increase in altitude. Nasa has put together a rather cool website here:

http://www.grc.nasa.gov/WWW/K-12/airplane/atmosi.html

It shows the atmospheric pressure drop to be approximately 5 psi during a climb to 10,000' from sea level. Remember this is the outside pressure not the tire pressure. The tire is sealed and the pressure variable would be negligible at best. The greatest change to tire pressure will occur because of changes in temperature to the air inside the tire. That temperature change will occur due to tire flexing during rotation and heat energy transfer from the brakes.

Any one else care to add to this?

[Wavery]

A lot of people think that the pressure inside the tire increases at altitude because they have seen pictures of balloons getting bigger at higher altitude and assume that is due to the pressure inside the balloon increasing. The fact is, there is little to no change of pressure inside the balloon. The balloon expands because the outside pressure is lower in relationship to the pressure in the balloon.

Likewise, if you take a balloon under water, the balloon will get smaller as it goes deeper. There is no change of pressure inside the balloon. The balloon is getting squashed by the weight of the water.

We have all heard of oxygen tanks exploding in aircraft. That is not due to an increase in pressure on the inside of the tank. It is due to the lack of pressure outside the tank. That makes the tank itself have to work harder to keep the contents from rupturing the tank. Conversely, A submarine at 3000 ft below the sea, may be crushed by the outside pressure but at sea level the hull has no problem.

Thanks for all of the help. I will check the tire pressure in the morning when it is cool. I am leaning towards the temperature at the time of filling the tires as the cause of my problems.

Our trip next week will be interesting. We will leave our home at 5,000 feet and cool morning temperatures. We will travel down to the Central Vally of California about 300 feet and hot temperatures and than end up at Lassen National Park 6,000 feet and cool temperatures. Maybe I am just worrying too much about tire pressure. I think what I should do is fill them to 50lbs in the morning when it is cool and check them again before I leave Lassen.

[ShrimpBurrito]

Originally Posted by freein05

Maybe I am just worrying too much about tire pressure. I think what I should do is fill them to 50lbs in the morning when it is cool and check them again before I leave Lassen.

I think you hit the nail on the head. IMHO, you should be checking air pressure before you leave for the day, and then again when you stop for gas, even if there is no elevation change. If you're doing that, you'll catch any non-catastrophic changes in air pressure no matter what the cause.

Dave

[Bill]

I agree with Dave (Shrimp Burrito). And more important, the tire manufacturers agree with him. Set the pressure in the morning, and leave it alone. It is good to check it as you are travelling, to make sure you haven't got a big decrease from a slow leak, but don't try to tweak it pound-by-pound. And as Chip and Kat pointed out, elevation is not a big factor - temperature of the tire body is the real determiner of what is going on. Like many of us, you should get used to touching your fingertips to the sidewall of the tires every time you stop. If the sidewall is cool or warm, that's fine. If it hot, check the inflation. You'll get your fingertips calibrated quite soon.

Bill

Originally Posted by Bill

I agree with Dave (Shrimp Burrito). And more important, the tire manufacturers agree with him. Set the pressure in the morning, and leave it alone. It is good to check it as you are travelling, to make sure you haven't got a big decrease from a slow leak, but don't try to tweak it pound-by-pound. And as Chip and Kat pointed out, elevation is not a big factor - temperature of the tire body is the real determiner of what is going on. Like many of us, you should get used to touching your fingertips to the sidewall of the tires every time you stop. If the sidewall is cool or warm, that's fine. If it hot, check the inflation. You'll get your fingertips calibrated quite soon.

Bill

I agree. Here is Goodyear's take on inflation pressure and altitude (or actually on why the tire gauge reads different pressures at different altitudes, which Freein05 experienced). They explain it much better than I did, with graph's to boot. If you will look on page 4 of the pdf link, there is good information (and graph) on just how much underinflation will cost you in mileage (which will apply to your tow vehicle and trailer).

http://www.goodyear.com/truck/pdf/ra...tread_S5_V.pdf

They also give their take on nitrogen for filling tires (which was discussed in another thread), on the last page.



EDIT: Here is Tire Rack's even better explanation:

http://www.tirerack.com/tires/tirete...urrentpage=111

[Speckul8r]

So, I check the tire pressure b4 leaving, 90 degrees outside in the shade, get to the mtns. and the next morning the outside temp is 48 and the tire pressure is ~46 psi. I should again inflate to 50 psi?

[ShrimpBurrito]

I would again inflate to 50 psi, if that is your target pressure. As the ambient temp increases and the temp of the tire increases from the road friction, the air pressure will rise slightly. But you may also lose some air as well. If you don't, the pressure may remain a few pounds high....maybe 55 psi.

But I would rather error on the side of being a little over inflated vs. under inflated, as long as the pressure remains below the rated max pressure of the tire. Higher pressure means less heat and more load capacity.

If your tires start wearing more in the center of the tread vs. the edges of the tread, that's a good sign you need to decrease pressure. You're just wearing your tires out prematurely.....although frankly, that may not be so critical because you will likely replace them because they are old, and not because they are worn out. (again, as long as you are below the max rated pressure.)

Bobby (wmtire) will hopefully chime in.

Dave