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Old 03-02-2016, 07:16 AM   #15
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CapriRacer:

So I set the 15" Michelin LTX (P) 235/75R15 XL tires to 44 psi when it was 35 degrees in the morning. Later that day the air temp was around 75. The TPMS was reporting tire pressures had increased to 51 psi (maximum side wall pressure is 50 psi) while running 55 mph. Three tires carry an average of 1,250 pounds and the fourth is carrying 1,376 pounds due to the design of the 23D. I have seen less pressure increase when the starting air temperature for the day is fairly constant going thru the day.

Should I have stopped and let two or three psi out of the tire. I use a digital LongAcre tire pressure gauge?

First, it is never a good idea to let pressure OUT of a hot tire. Not only is the elevated pressure telling you something, it might be keeping you from a catastrophic failure.

Second, the max pressure listed on the sidewall of a tire is a maximum COLD usage pressure. It is expected that the hot pressure will buildup and perhaps be more than the max pressure listed on the sidewall.

Third, when doing these calculations, you have to remember that ambient temperature affects things. In this case, the ambient temperature increase from 35 to 75 should have resulted in about a 4 psi buildup, so the starting cold pressure should have been recalculated from 44 psi to 48 psi, and the operating pressure buildup to 51 psi is only 3 psi - and since that is less than 10%, that is good.
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Old 03-02-2016, 07:35 AM   #16
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Capri,
Can you comment on the effect of low ambient temperature in the morning and high ambient temperature in the afternoon, as is often the case in the desert SW? I have often left out in the morning with the temperature 38 degrees and afternoon highs may be near 100 degrees. How does this relate to the 10% rule of thumb? The higher temperature must account for some of the pressure increase, yes?
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Old 03-02-2016, 09:19 AM   #17
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Is there a table somewhere or a rule of thumb to use to estimate the tire pressure rise due to just the day getting warmer? An off the cuff observation of post 15 is that there is a 1 psi increase in tire pressure for each 10 degrees Fahrenheit. I presume that the tires in question are in the shade.

Thus a day that started at 55 and went to 115 (like in the south west) one would expect the shaded tire pressure to increase 6 psi.

The Dill TPMS does show higher temps and pressures for the tires on the sunny side of the trailer when driving down the highway. I can see similar results on the built in TPMS of the Dodge 2500HD.
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Old 03-02-2016, 11:07 AM   #18
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As long as we are asking questions, and I live in the South West like some other posters do.

Does the road temp make a difference? So in June the ambient can get to 115 (or a little higher) however the asphalt can be 10 or 15 degrees higher than that. (May be more however I don't check that) Since it is the road surface is where my tires dwell is that an issue for any concern as far as inflation?

My current strategy is to check and fill if needed the tires to any and all vehicles very early in the morning. Even on a 115 degree day that starts out at over 100 the driveway and tires have not been heated up so I use that time to check. Typically I check all vehicles once a month when I am in town. As the seasons change there is some adjustments in pressure based on ambient temps.

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Old 03-02-2016, 07:58 PM   #19
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I run mine at 70psi with a weight of 7500lbs.
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Old 03-02-2016, 08:52 PM   #20
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I'm running 80 psi on my 30' Classic Slideout. Weight with normal camping load is about 8600-8700 lbs. With fresh water tank full, I'm at the 9100 lb limit. I using the Michelin 16" LTX MS2 tires. This particular trailer was the heaviest trailer that Airstream built on a tandem axle. Personally I'd rather error on the higher side with the air pressure. I've not seen any negative issues structurally, running at this pressure.

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Old 03-02-2016, 09:07 PM   #21
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If I had Jack's trailer I would do the same he's doing given the higher weight of the trailer. I'm running 15" load range E tires with 80psi max inflation pressure and run them lower as my 70s trailer needs a smoother ride and weighs 7500 with full propane and water tanks and fully loaded.
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Old 03-03-2016, 07:34 AM   #22
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Capri,
Can you comment on the effect of low ambient temperature in the morning and high ambient temperature in the afternoon, as is often the case in the desert SW? I have often left out in the morning with the temperature 38 degrees and afternoon highs may be near 100 degrees. How does this relate to the 10% rule of thumb? The higher temperature must account for some of the pressure increase, yes?
Thanks
Quote:
Originally Posted by switz View Post
Is there a table somewhere or a rule of thumb to use to estimate the tire pressure rise due to just the day getting warmer? An off the cuff observation of post 15 is that there is a 1 psi increase in tire pressure for each 10 degrees Fahrenheit. I presume that the tires in question are in the shade.

Thus a day that started at 55 and went to 115 (like in the south west) one would expect the shaded tire pressure to increase 6 psi.

The Dill TPMS does show higher temps and pressures for the tires on the sunny side of the trailer when driving down the highway. I can see similar results on the built in TPMS of the Dodge 2500HD.
These 2 questions are kind of the same, so I'll address both in a single post.

First, yes, it is common for people to measure the inflation pressure in the cool of the morning, then later when the temperature has risen considerably.

Rule of Thumb: For every 10 degrees F change in ambient temperature, the pressure in a tires changes 3%.

So far a passenger car tire, 3% is about equal to 1 psi for every 10 degrees.

- AND -

A tire measured at 65 psi at 55 degrees would measure at 115 degrees, 3% times 6 (60 degrees) = 18% more or 12 psi. That needs to be accounted for when doing pressure build up.

So a tire operating at those conditions could run as high as 85 psi.

That is a little concerning, but only somewhat. First, I think the conditions stated above are a bit extreme (overstated), so the actual temperature change is quite a bit lower then that. Also, the tire is designed to withstand much more than maximum stated on the sidewall.
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Old 03-03-2016, 07:39 AM   #23
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As long as we are asking questions, and I live in the South West like some other posters do.

Does the road temp make a difference? So in June the ambient can get to 115 (or a little higher) however the asphalt can be 10 or 15 degrees higher than that. (May be more however I don't check that) Since it is the road surface is where my tires dwell is that an issue for any concern as far as inflation?

My current strategy is to check and fill if needed the tires to any and all vehicles very early in the morning. Even on a 115 degree day that starts out at over 100 the driveway and tires have not been heated up so I use that time to check. Typically I check all vehicles once a month when I am in town. As the seasons change there is some adjustments in pressure based on ambient temps.

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I am not aware of any research that anyone has done regarding road surface temperature. Plus I think the difference between the ambient and the road temperatures is not nearly as great as it is between morning and afternoon ambient.

In other words, my gut tells me this is not an issue that needs to be worrying folks.
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Old 03-03-2016, 10:21 AM   #24
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All these air pressure theories sound like a lot of hot air!
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Old 03-05-2016, 12:43 AM   #25
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You know, tire industry standards are pretty consistent for recommended cold inflation pressures for your trailer tires. Whenever replacements are used the recommended inflation pressures need to provide the amount of load capacity the OE tires provided by inflation. Anything less than that is considered under-inflation.

Remember, the starting values were set by the vehicle manufacturer. Without certified modifications to your trailer's GVWR the tire inflation pressures set by the vehicle manufacturer for the OE tires will always - in the eyes of the industry - be valid.
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Old 03-05-2016, 03:01 PM   #26
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I have a related question as I will soon be programing my TPMS. Based on a recommended inflation of 80 psi, at what pressure would you guys recommend setting the alarm on the monitor?
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Old 03-05-2016, 03:52 PM   #27
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Tire inflation

Quote:
Originally Posted by JVfromGrnBay View Post
I have a related question as I will soon be programing my TPMS. Based on a recommended inflation of 80 psi, at what pressure would you guys recommend setting the alarm on the monitor?

An uninformed opinion based on what I've understood from the tire guys here - if a 15% increase in pressure suggests a problem, that's 92 PSI - you might want to pick 90 or 91 to give you advanced notice - and I'd personally pick 75 PSI on the low side just to catch things. Also - if you have a temp sensor, 150 degrees is also a good warning ceiling.

I've noticed on mine that pressure increases about 10% (I start my 15" p-rated Michelins at 50 PSI cold and they run up to 55 PSI, even on hot days) and temps typically get to about 30 degrees above ambient. This has been consistent since installing the TPMS.

Good luck!
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Old 03-06-2016, 08:45 AM   #28
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Last Thursday, I was running the 15" Michelins at 44 psi at 65 degrees set that morning. By the afternoon and 85 degrees, I see 50 to 51 psi and the temperatures approached 130 degrees on the sunny side of the trailer. I was driving at 55 mph.

So the pressure rise is within limits (just at or above the sidewall rating of 50 psi). The twenty degree rise in air temperature should bring the starting number to 46 psi for calculations and a 5 psi over that number is within limits.

Since the TPMS reported sunny side tire temperatures of 130 degrees or 45 degrees above ambient, should I raise the starting tire pressure to a higher number to keep the temperature rise lower?
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