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Old 01-22-2015, 03:43 PM   #1
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Couple questions about tires

These may well have been asked before, but can't find any reference on here.

We have 275 80R/22.5 tires on our coach.

Any time air pressure is mentioned, it is always "cold". So, just what temp. is "cold".

Here in San Antonio, if I check the tires in the morning, the temp might be 40, but if I wait until mid afternoon, it just might be 75. So, If the pressure should be 100 psi and I set the pressure first thing in the morning, by afternoon, the pressure will be off. I have never seen a pressure chart that said "around 100 psi", so thinking they are talking a specific pressure.

On another note, those same recommendations suggest checking weight, and go so far as to suggest weighing each corner, and adjusting pressure accordingly. But, at the same time, they say to keep all tires on the same axle at the same pressure. Seems to be a bit contradictory.

And finally, do truckers adjust their tire pressure after unloading, or even after dropping half their load, then driving down the road a few hundred miles with half the weight? Having been around trucks and CDL licenses for awhile, I know checking tire pressure is certainly part of the pre-trip, but I don't recall any mention of adjusting pressure as the load changes, and certainly no mention of checking corner weight.

I ask all this because I just don't see most RVers being that focused on tire pressure. Even still, half a tank of fuel can weigh almost 500 pounds, most of which is on a single axle. Is that enough to be an issue?

I am thinking of finding a reasonable medium weight per axle, and going with that. Other than fuel load, there should not be much of a change, unless we stop at Golden Corral for lunch.

Any thoughts would certainly we welcome.

Ken Gasbarri
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Old 01-22-2015, 03:52 PM   #2
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When I was a trucker I thumped the tires with my club or a hammer every morning. If they bounced the hammer back, they were good to go. We might check air pressure a couple times a year, and no, we certainly did not adjust for how much weight we were carrying.

I had trailer tires with close to a million miles on them and then they would start to blow out just from age. When that happened we would replace the whole set.

I inflate my tires to the cold setting and find that no matter what the weather is they will stay pretty close to that as they warm up during the drive. Up or down a few pounds really isn't a problem.

I have a Truckers Supply TST 507 system that I use on the Motorhome now. I check tire pressure in the morning from the drivers seat with a cup of coffee. All I am looking for is that one tire that doesn't match the rest.
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Old 01-22-2015, 05:01 PM   #3
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Cold tire pressure has nothing to do with the temperature of the air. Cold tire pressure is measured before your tires have moved for the day. Hence everyone says to check them in the morning, because it is in the morning that people will begin their days travel.
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Old 01-26-2015, 03:04 PM   #4
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Quote:
Originally Posted by DraginRat View Post
Any time air pressure is mentioned, it is always "cold". So, just what temp. is "cold".
As others have said, "cold" generally means that the tires haven't been run... and that the sun hasn't been on them (black tires heat up fast in the sun, so the air in the tire will heat, and expand, too... changing the pressure reading). I would assume that most tire manufacturer inflation charts were based on some "baseline" temperature... like 70 or 75 degrees, but I'm not sure what the exact number is. Most don't worry about that aspect of it and just check their tires first thing in the morning before they've been driven or heated by the sun.

Quote:
Originally Posted by DraginRat View Post
On another note, those same recommendations suggest checking weight, and go so far as to suggest weighing each corner, and adjusting pressure accordingly. But, at the same time, they say to keep all tires on the same axle at the same pressure. Seems to be a bit contradictory
The idea is this: weigh all corners of your coach, then for each axle, inflate all tires on that axle to the pressure needed to carry the largest weight measured for any tire on that axle. So, for instance, if you weighed the two sides of your front axle as 6,800 pounds for the driver's side and 7,000 pounds for the passenger side, you'd inflate BOTH tires to the pressure needed to carry the 7,000 pounds so that they are equal. Many advocate adding 5-10psi to whatever pressure your chart says for the weight... as a safety margin to account for variation due to different loading conditions, etc.

This video might help some, too:

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Old 01-26-2015, 06:49 PM   #5
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Quote:
Originally Posted by DraginRat View Post
I am thinking of finding a reasonable medium weight per axle, and going with that. Other than fuel load, there should not be much of a change, unless we stop at Golden Corral for lunch.

Any thoughts would certainly we welcome.

Ken Gasbarri
The tire pressure charts give the minimum air pressure for the tire to carry that particular load. Adjust your pressure to at least enough to support the maximum weight you have at the time. I also usually add 5psi to the chart pressure as a cushion for air lost checking the pressure or other variables...
Over-inflation causes a rough ride, under-inflation can damage the tire.. A tire run 20% underinflated is considered to have been run flat and needs to be inspected or replaced.
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Old 01-29-2015, 07:40 AM   #6
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I believe 65 to 67 degrees F is best ambient to set air pressure and then I check with my TST 507 in the am with my coffee before moving on.
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Old 01-30-2015, 07:48 PM   #7
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Cold inflation pressure means having driven less than a mile after sitting for 8 hours. I forget which tire mfgr's literature that came from.
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Old 01-31-2015, 10:52 PM   #8
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Both temp and pressure will increase above nominal as the tires are driven. Whether you start from an ambient of 40 degrees or 75 degrees doesn't matter-- it is all accounted for in the manufacturer's calculations.

A good source of information is Roger Marble, sometimes on this forum. He is a retired tire engineer. He also runs a blog under the name "tireman9". Check it.
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Old 02-03-2015, 04:23 PM   #9
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I agree with the above: it's in the table numbers somewhere as long as you haven't driven the coach. FYI standard atmospheric temp is 59F. That never shows up in the charts I have (Michelin). Always check before departure, but if the tires in the sun are a couple lbs higher, no problem as long as the lower side meets minimums. Don't chase the difference. Just don't go low!
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Old 02-03-2015, 05:29 PM   #10
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Never have i heard so much about nothing... Cold means not driven. Not refrigerated, not iced down, not only on a freezing winter morning.

Cold. like it's been sitting all night. Check the air and drive the wheels off. This isn't a NASCAR race.
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