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Old 10-16-2013, 05:40 AM   #15
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You should check your tire pressures each morning before you drive. As you travel south (warmer) you will find you let some air out. Back North ( cooler) put some back in.

The rule of thumb I use is this. Every 10F temp change causes about a 2 PSI change in tire pressure. Every 2k change in altitude causes a 1 PSI change in tire pressure.
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Old 10-16-2013, 05:48 AM   #16
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Leave things alone unless you are heading from colder temps to hot temps. If you are taking 4 days to get to AZ from Chicago during the colder times perhaps adjust at the end of the second day and again when you get south. Since you will probably be south for some time you're OK. When you head north again do the same.

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Old 10-17-2013, 10:04 PM   #17
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Now just when do we change the winter air to summer air.
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Old 10-17-2013, 10:09 PM   #18
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Quote:
Originally Posted by AFChap View Post
Inflate your tires to the correct pressure for the weight when cold/before driving, and then enjoy the drive. Don't try to over-manage the pressures. The tire pressure charts reflect the "when cold" pressure and have the fact that pressure rises with heat in mind. A tire that requires 95psi when cold is likely dangerously under-inflated if it has 95psi when the tire is warm/hot.
Exactly what the tire companies say to do!! All tire pressure are given before the tire is driven on, and it doesn't matter what the ambient temp is. Even 1 mile driven is too far according to them.
From the August 2010 Motorhome magazine article "Tread Carefully"
Quote:
Always try to check tires when they are “cold” and have not been driven for more than one mile.The stated load capacity for a given cold inflation pressure is based on ambient outside temperatures.

The pressure in a “hot” tire may be as much as 10 to 15 PSI higher than a cold tire pressure.

If the tires must be checked when they are warm, be sure to allow for an increase in pressure, and make sure the pressure of the tires on both sides of the axle are within a few PSI of each other. Never let air out of a hot tire.
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Old 10-17-2013, 10:28 PM   #19
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Originally Posted by TeJay View Post
Leave things alone unless you are heading from colder temps to hot temps. If you are taking 4 days to get to AZ from Chicago during the colder times perhaps adjust at the end of the second day and again when you get south. Since you will probably be south for some time you're OK. When you head north again do the same.

TeJay
Better to do it in the morning of the third day, that assures that the tire is cold and not still warm from the days drive. I takes many hours for the tire and air to cool down to ambient.
From the Auguts 2010 Motorhome magazine article "Tread Carefully"
Quote:
1. Check at least once a month and before any major trips.
2. On long trips, check every morning before driving.
3. Check before and after storage.
4. On short trips of a day or less driving each way, check before you leave and before you return home.
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Old 10-18-2013, 12:10 AM   #20
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Quote:
Originally Posted by Coma View Post
You should check your tire pressures each morning before you drive. As you travel south (warmer) you will find you let some air out. Back North ( cooler) put some back in.

The rule of thumb I use is this. Every 10F temp change causes about a 2 PSI change in tire pressure. Every 2k change in altitude causes a 1 PSI change in tire pressure.
This makes sense to me. When checking tire pressures in the morning before traveling, the tires are going to have considerably less air pressure at -20 degrees as compared to +100 degrees. Also, tires with exposure to the morning sun are going to have more air pressure than tires in the shade.

What I have been doing is paying attention to the tire pressures throughout the day. I know at the end of the day if all tire pressures have remained constant and equal. If they have, the next morning, regardless of temperature, I read the tire pressure monitor system to see if all tire pressures are relatively equal. If they are, I decide that we're ready to head out for the day.
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Old 10-18-2013, 11:11 AM   #21
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IMDSailor,

I use the "Fudge" factor. Weigh the four corners and determine the proper inflation for the tires. Let us say that your weight determines that you should have 100 psi for 4000 pounds of your weight(assumption), and then the charts say that you need 110 psi for 5000 pounds and that is also the MAX inflation for your tire (assumption). Setting your psi to the 100 psi does not leave any fudge room should your pressure drop, and setting it to 110 psi does not leave any room for a pressure increase.

So set it to 105 psi will igve you a fudge factor of 5 psi in any direction.

As stated, and let's do some math. As temperatures increase or decrease pressure will change by 2% for each degree in temperature. So a 105 psi and a 10 degree decrease will lower pressure to 103 psi, and an increase in temperature will raise it to 107 psi. Both setting are within the weight specifications for the tire. No change is necessary.

By the way, for car tires it is 1% but for the bigger and higher pressured tires it is 2%

Now consider altitude. For each 1000 feet difference in altitude the tire pressure will change by 0.48 psi. Let us assume that you drop 5000 feet in altitude. That will be a 2.4 PSI drop in pressure. At 105 psi the tire would now be at 102.6 psi and inversely if you increase altitude 5000 feet the pressure would gain 2.4 psi and the tire would be at 107.2 psi.

Let's add the altitude and temperature drops/increases together. Temperature drop 10 degrees, pressure no at 103, add the altitude drop of 2.4 psi and the pressure is at 100.5 psi. that is still withing the specifications that I originally posted. The same is true for increases. Temperature increase of 10 degrees and tire pressure is now 107 psi. Add the altitude increase of 2.4 psi and the pressure is now at 109.4.

That's the fudge factor.

Happy trails.
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Old 10-18-2013, 11:19 AM   #22
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We are working this tire pressure thing to death. The tire pressure gauge does not know if you're in Miami Fl.or Quebec Canada . Also the tire pressure gauge doesn't know if it's -5* or if it's +100*. It's only going to give you the pressure that's in the tire. The manufacturers have always stated check your tires when they are cold. If you're driving your RV day and night it would be hard to check your tire pressure cold . But I would not let air out or put air in as long as i'm driving unless they look low. Once I stopped and the tire has gotten cold then check the tire and inflate or let air out . But the tire pressure gauge could careless if you're in north or south , east or west. Just check them when they are cold and put the amount of air in each tire per the weight of the RV. That's what the makers of the chassis and the makers of the RV and the tire maker.
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Old 10-18-2013, 11:44 AM   #23
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I have pulled out of Calgary when it was -15F to drive to Arizona. The tires were all down in pressure. I inflated them to slightly below my usual levels - I normally run them a little harder than spec.

I checked them for temp. and pressure when I stopped a few hours later.
After 4 hours of driving, the tires were still at a temperature where I would normally check them for "cold" fill pressures, and the pressures were also good.

Two days later I was in warmer climates and let a little air out because the were now above my normal cold pressures, and after a few hours of driving the tires were now warm to the touch but the pressures were what I expected.

There is generally a good relationship between your cold temp and where the tire temps will be by mid day after hours of friction, and it has proven true in my experience that the relationship holds. You need more actual air at low temps to make the required pressure, but your tires won't heat up that much during the day. As the days warm you will probably remove some air to make the cold pressure, but your tires will be hotter and experience higher pressure during the day.
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Old 10-18-2013, 01:06 PM   #24
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It does not matter what gas/air you put in your tires, if it is dry, pressure fluctuations will be at their minimum. This means, from the day the tire is mounted it should be filled from a dry air source. I have a 12V high-volume 150 psi compressor. Several times after inflating a farm equipment tire I have disconnected the air hose and had a small amount water run out of the hose. I also remember demounting a tire to discover a few ounces of water inside. Dry air is the key.
Someone mentioned nitrogen, it is a dry air by design; the reason it is used_it is the most common gas available. Argon (much more expensive) could just as well be used because it is also dry and an inert gas. The same effect is achieved with a quality air dryer on your air compressor.

Oh, and yes__ I too think you are over-thinking this. If you wish to investigate deeper, visit the Tire and Rim Assoc, and Rubber Manufacturers Assoc. websites.
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Old 10-18-2013, 04:39 PM   #25
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O the pressure!

Sorry I couldn't help myself.
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Old 10-18-2013, 07:00 PM   #26
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I just hit mine with the thumper and if they all sound the same I am down the road.
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Old 10-18-2013, 08:30 PM   #27
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O the pressure! Sorry I couldn't help myself.
Hahaha, love it.
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Old 10-18-2013, 11:02 PM   #28
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It would behoove owners to read the information contained in Roger Marble's Blog. He spent 40 years developing tires for a major manufacturer.

His blog is here

Here is an excerpt from one of his information pages:

"One other option for those only a few psi low. Drive to the nearest service station at slightly reduced speed (10 mph under the speed limit would be max) and follow these instructions on how to inflate a hot tire.

1. Record your cold inflation.
2. Calculate how many psi each tire would need to reach your goal cold inflation.
3. Drive at reduced speed, hopefully no more than 10 miles, to the service station with air available. You might want to call ahead to be sure they have enough space or long enough hose to reach your rig. Not all service stations can accommodate a Class-A with a toad.
4. Measure your now warm inflation pressure
5. Add the psi needed from step #2 above plus 3psi to learn your temporary "warm" tire inflation
6. Inflate your warm tires to the temporary goal inflation calculated in step 5.
7. Confirm you have the needed inflation the next morning after the tires are at ambient temperature and adjust accordingly.

If you follow these steps I think you will find that your tires have the proper inflation or 1 or 2 psi more so you can set the inflation at your exact goal cold inflation using your digital gauge."
------------

Air pressure is not rocket science, just physics.
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