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Ideal Temperature for Checking Tire Pressure


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#1 desmo

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Posted 13 September 2011 - 10:18 PM

How important is tire pressure?

I'm curious as to what the ideal temperature range for checking tire pressure, or is ambient temperature sufficient?

Most literature says to check tire pressure when cold. What is cold? In the desert winter months, nighttime is 20 degrees and daytime temperature is in the 70s.

I also read that tire pressure varies by 1 PSI for every 10 degrees F. That is a significant amount of pressure given such a wide range of temperature when your axle loads are near maximum tire loads!

What about high-altitude pressure readings in Denver, for example ... does it matter?

Is plus or minus 5 PSI significant?
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#2 CarlGeo

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Posted 13 September 2011 - 11:07 PM

Checking air pressure when cold. Means to check it before the vehicle is driven, like first thing in the morning before starting your trip.

Air tires according to tire manufacturers inflation tables and your load. You must weigh your coach to determine weight each of the tires is carrying. Air pressure must be the same for all tires on each axle, and each of the dual tires.

Also, do not check pressure after driving in hot weather, and finding it high, let air out. Do not remove air in that circumstance.

My cold pressure is 105 psi, but after driving on a hot day the pressure will be about 125 psi. All very normal.

Good Luck

CarlGeo
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#3 wolfe10

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Posted 14 September 2011 - 07:39 AM

Excellent advice from Carl.

As he suggests, cold is at current ambient temperature before driving-- be it 20 or 100 degrees F. Yes, as winter sets in, you will need to add air to maintain the proper PSI.

And with wheel position weights, use the heavier wheel position on each axle to go to your tire manufacturer's inflation table to determine the correct MINIMUM PSI for all tires on that axle. Many of us add 5 PSI to that minimum to cover minor changes in temperature and load.

While changes in ambient temperature have a significant affect on PSI, changes in altitude have very little affect-- just straight physics.
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#4 TBUTLER

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Posted 14 September 2011 - 08:17 AM

Checking the ambient temperature also involves making sure there are no outside influences on the tire temperature. A tire sitting in direct sunlight will heat up due to the sun. It's temperature is not ambient air temperature. If you adjust the temperature of the outside dual in direct sunlight to match the temperature of the inside dual in the shadow of the outside tire, the two pressures will in fact be different. I always check my tire pressures before sunrise to ensure that the sun won't cause variations. Right now, my generator is running which creates artificial heat around the front tires. If I checked them after the generator runs for a while, they would seem to have a higher pressure than they actually do as a result of this artificial warming. Likewise, if the engine is running to inflate tires for instance, it will warm the inside dual tire raising its pressure artificially. It is only a matter of a few pounds but in the case of dual tires that can be enough to cause lower fuel mileage and unnecessary tire wear or worse.

Seasonal adjustments aren't the only times tire pressure needs to be checked and adjusted as necessary. Yesterday I started driving in the plains, the temperature was 60 degrees in the morning. Now I'm above 5000 feet elevation, the outside temperature is 45 degrees. I need to check and adjust tire pressures here not because of altitude or seasonal change but because the ambient outdoor morning temperature is significantly different than the day before. That is one reason why we check tire pressures every day before we drive. The other reason is to discover slow (or fast) leaks which may be due to damage to a tire or valve which could result in failure if not detected.

How important is tire pressure? A failed tire can cause a serious accident and you could end up losing your life. At best, a failed tire will cost you money you could have spent elsewhere. Check your tires every morning before you drive as if your life depended on it. It does.
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#5 Guest_Wayne77590_*

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Posted 15 September 2011 - 11:49 AM

Wow, this subject comes up all the time, and most of the information is solid on.

One of the subjects covered at the Freightliner class in Gaffney, SC was on Tires.

First, 1 PSI for 10 degrees F is incorrect. (Sorry, don't mean to offend anyone)

For every 10 degrees of temperature change there is a 2% change in pressure.

So let's say you start out with 105 psi, as measured in the cold state (which means before it is driven 1 miles) When you stop for the night and you get up in the morning, the temperature is 10 degrees higher than where you were the previous morning. Your now tire pressure will be approximately 2.1 PSI higher, or 107 PSI. Don't fret. Don't change it yet.

Now also consider that for every 1000 feet of altitude change there will be a 0.48 PSI change. So now if you have gone from 0 to 2000 feet in altitude, you have another increase of 0.96 psi. Add that to the 107 for the temperature change and you are close to 108 psi. But don't fret. Stay with me.

Let us say for this purpose that your are running 255/70R22.5 tires. The Minimum for a single tire is 4190 pounds, and for a dual is 3970 pounds. Let's do a chart
255/70R22.5 rated for 75 mph

psi-----------80-----85------90-----95----100-----105-----110-----115------120 (load range H)

Load Single--4190---4370----4550---4675---4895----5065----5205----5400-----5510(H)
Load Dual--- 3970---4110----4275---4410---4455----4610----4675----4915-----5070(H)

Now, let us assume that your weight for tires across a single axle is 9600 pounds. It is better to weigh each tire, or duals as a group, individually and then use the highest value for all tires on that axle.

In this scenario, this would place a weigh of 4800 pounds on each tire on a single axle with only two tires, let us say it is the front axle.

Look at the chart. For this specific weight example, the minimum you should carry in a single tire would be 8595 pounds at 100 psi. Not yet, not yet, wait for it. Let us say that you went to the scales, weighed and this is what you came up with, so now you put 100 psi in each of the front tires. You go home, pick up DW, and head on down the road. Let me ask you this, and I dare you to post it, how much does DW weigh? DW gets in the MH, with her purse, make-up kit, 70 pound dog and says, let's go. Let us just assume that DW weighs 120 pounds, but everything else she is carrying adds another 180 pounds. Do now the weigh on that passenger side, if everything she brought in is on that side, is 8895 pounds, and if it were to have been distributed across the axles evenly it would put the weight on the tires at 5045 pounds. That now means that the minimum you should have for tire pressure is 105 psi. Don't stop and pick up groceries or supplies as that again changes your weight. If you leave your tires at 100 psi cold, based on the original weight your tires are now under inflated.

Also consider that you have a temperature drop and an altitude drop and it becomes an unsafe situation, but there is a solution.

Based on your original weight you can run at 100 psi, but you take a risk. So let's figure in a fudge factor. Your tires, based on the weight of 4800 pounds per tire, can be run from 100 psi to 120 psi, with the 120 psi being load range "H". If you set them to the highest setting you will not hurt them, but you will experience a slightly bumpier ride. What you do not want to do is run them under inflated, or over inflated. So here is the solution.

In this specific case (just an example), set your tire pressure to 110 psi with a load range of 5205 pounds per single tire. That is a difference of 310 pounds per tire between the 100 psi and 110psi setting. That is 620 pounds across the axle. you can go out and buy some groceries and supplies and still be within range of the proper pressure, and you, not DW can eat at McDonald's every day.

Also consider the temperature and altitude differences. A 30 degree increase in temperature will change the pressure from 110 to 116 psi, and still within your load range. And a 30 degree decrease in temperature will decrease you pressure to 103 psi, still within the load range of 100 psi for 4800 pounds. You can do the math on Altitude, but as you can see from this you do NOT have to change your tire pressure every time you stop if you are within your load range as prescribed by your tire manufacturer.

When to check your tires:

  • Before each trip
  • Every morning during long trips
  • Before you leave and when you return home on short trips
  • Before and after storing your vehicle

So when you check your tire air pressure and you are still within range, you are still set to go without having to add or delete air from the tires. Just know where you are going, what your ranges are, and should you fall into a -50 degree ambient temperature change, adjust accordingly - and turn the heater up. I don't think you will fall into that category unless you are in Antarctica or similar. The whole meaning is that you do not have to add or remove air for every single trip if you stay withing the ranges indicated by the tire manufacture. Please do get the coach weighed, even if it is single axle weights at one of the CAT scales. When you can, get the individual corner weights and then readjust you pressures.

Edited: And remember, checking the tires when they are cold means to check them in the early morning before leaving on a trip. We normally do not depart a campsite until around 9 o'clock. Using the method I indicated above, when I check the tires and there is a pound or two difference between the outside dual and the inside dual it is most likely because of the sun hitting that tire. I do not change the air pressure in either tire. The inside dual will catch up with the outer one in a few miles. Also, when it is cold/cool outside, the outside dual may indicated a lower temperature than the inside dual. The inside dual has been protected by the heat of the MH during the evening hours and may show a higher pressure. Again, I do not change pressures. I'm sure their may be other things to consider, and maybe a tire person can come along and post them. The important thing:

Be safe.

#6 desmo

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Posted 15 September 2011 - 03:10 PM

Thanks to all for the information, especially Wayne's analysis.

Desmo
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