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Old 03-27-2014, 12:41 PM   #31
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I had a telephone conversation with a man of Vredestein/Appollo , and he told me that if a sertain spot of the tire reaches 170dgr Celcius/300dgr F ??? the rubber hardens because of sulfurbridges that build up and so the flexability is gone, and then tire damage begins.
But that dont happen when the tire inside is warmed up by the heat of the brakes transported trough the rimms to the tire inside.
The high temperature of 170 dgr C is courced by to much bending of the rubber, wich probably will happen at the sidewall edges where they make go over in the rimm and treath.
So at 200degr F the rubber wont get harder so no damage to tire, is my conclusion.
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Old 03-27-2014, 12:55 PM   #32
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I'll stick with TST/Michelin recommendations just to be safe. Will stop and let the tires cool if they get over 110 - 115 just to be on the safe side.
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Old 03-27-2014, 01:11 PM   #33
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I run mine at 75 front and 78 rear all the time, I get better milage and the last set of factories went to 68k miles, it is a littler rough in the seat but it is a 3/4 ton 4x4
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Old 03-27-2014, 01:17 PM   #34
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As the only tire engineer I know of on this thread I see three issues:
A. What pressure to use when empty
B What pressure to use when loaded (towing)
C What to do about TPMS

The only correct way to know A & B is to get your TV on a scale and learn what the actual axle load is. Since TV are normally pretty balanced side to side it is reasonable to assume 50/50 side to side split. So knowing the axle loads you consult the Load & Inflation tables for your specific tires from the tire manufacturer of your specific tires. This will give you the MINIMUM COLD inflation to set your tires at. BUT you should add a 10% margin so you are not chasing 1 or 2 psi pressure differences due to changes in Ambient.

Issue C is more difficult as we are talking about an OE TPMS. It's not easy to change the settings and even if you did I bet that after a couple of times you would give up. Since you should be running a TPMS on the TT I would suggest getting an aftermarket system that can monitor the TT & TV tires. 8 tire systems w/replacable batteries and 3 year warranty can be had for $360. This way you set your TV TPM at the low pressure and the aftermarket system with the TT numbers (tire max) and the TV loaded numbers.

Hope this helps.
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Old 03-27-2014, 01:25 PM   #35
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Quote:
Originally Posted by f1100turbo View Post
For example.
36 degrees this am.
Front tires 60 psi.
Drove 25 miles at 55 and 15 miles at 65.
Front tires 64 psi.

4 psi raise with ambient temp at 36 degrees.

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If you have completely dry inflation air the pressure rise is 2% per 10F. You can see the math HERE.
Now since the inside of a tire is never 0% moisture the pressure rise will be higher and can be non-linear at higher temperatures. BUT it is probably reasonable to assume a 2 to 3% change for 10F change.
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Old 03-27-2014, 01:26 PM   #36
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Also remember when you have a lower pressure set, there is more sidewall flex + increased temp because of the flexing and accelerated wear on the sidewall. More press = less flex = lower temp = longer life of the tire. RAM has a Hi/Low setting for tire press for unloaded/loaded running. I tend not to use it and am getting excellent tire wear from the OEM Firestone E rated tires. The door sticker is there for a reason not just to guess what you might need. Of course it is ONLY for the OEM tires. The TPMS in the truck will reset when switched to the lower press on the tires. A hassle to release press/increase press so I just run the "high standard". Tireman9 may agree or not on this but I do bow to his expertise on the subject.
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Old 03-27-2014, 01:35 PM   #37
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Originally Posted by OldCoot View Post
I'll stick with TST/Michelin recommendations just to be safe. Will stop and let the tires cool if they get over 110 - 115 just to be on the safe side.
I think you are being over cautious, but that's OK if you want. The temperature reading of external TPMS will be lower than the internal air due to cooling of air in valve stem. Also the air in the tire will be sort of an "average" of the hot and cool surfaces of the air chamber.

As a tire engineer, I know that different rubber compounds will "revert" i.e. de-vulcanize and get soft at temperatures ranging from 200F to 250+F but since different areas of a tire can have 40 to 50F different temperatures there is no way for you to know which compound is at what temperature.

I know that one TPM Mfg uses 156F as their warning temperature. It is also true that tires will run 20F to 60F above ambient so again no simple one number fits all answer as your personal load, inflation, speed and tire design will give different results than I see.

BUT my experience is that I run about 20F above ambient at 62 mph but my tires are underloaded for their inflation pressure.
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Old 03-30-2014, 06:20 PM   #38
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I just heard back from Michelin. I posed the same question to them (I'm the original poster). They said that the minimum should be 60 psi cold. After that, it is a matter of how much load I put on them. With the P tires I was getting a lot of sidwall flex even though they weren't overloaded per the scales. I just wanted a margin of safety with the E tires.
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Old 03-30-2014, 06:48 PM   #39
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I run 80 psi in mine. That is what they are designed for. I used to not believe in the "rated air pressure" thing, but I've been converted. Heat is the enemy of your tires, and running under rated pressure will contribute to excessive heating.
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Old 03-30-2014, 08:05 PM   #40
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I have read that you shouldn't use a lower pressure than 70% of the max rated pressure in any tire. That is 56lbs so I guess that is not too far off if Michelin says not lower than 60.
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