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Old 04-21-2016, 10:25 PM   #51
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Your test was more of a low miles test, not a low speed test. I don't see any of the TPM temps off by 20-50 degrees.

2010 F250 5.4L 3.73
2011 Flagstaff 831FKBSS
Equal-i-zer 4pt 12K


2011 Flagstaff 831FKBSS
2010 F250 4X4 5.4L 3.73 LS
EQUALIZER E4 1200/12000
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Old 04-22-2016, 09:50 AM   #52
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This thread may have drifted a bit so I will try and answer the Temperature question here then start a different thread on speed.

Tire temperature is important but the problem is in learning what the actual temperature is. The average temperature is not a good indicator and that is about all you can get by reading the Contained Air Temperature. While many seem to believe that their external TPM is giving them a reading of the CAT, I can tell you that the TPM will always read 10 to 50F cooler than the CAT and that the CAT is always cooler than the hottest location in the tire.

I read that many TPMS have set as the high temp warning point at about 158F. That is all well and good if you are measuring the actual CAT but I do not know how an external TPM will sense that temperature especially if it is on a metal stem or at the end of an extension hose.

I would always be more concerned about the pressure measurement as that is not going to be affected by cooling of the valve stem or extension hose. We know that for our purposes pressure does follow the "Ideal Gas Law" and it doesn't matter if you use air or Nitrogen the results are essentially the same unless you are measuring your pressure to the 0.1 psi.
So knowing that tire pressure will change by about 2% for each change of 10F in temperature of CAT we can get a reasonable estimate of the CAT and will note that out TPM is showing a lower temperature change than indicated by the pressure change.

I would not use the temperature reading from a TPM to set tire pressure. If I got a high temperature reading but the pressure reading was reasonable I would suspect a mechanical problem such as bearing or brake issue.

A motorhome properly loaded with the inflation set based on actual measured load on the tire and consulting the load Inflation tables will not result in an over-heated tire. NOTE Trailers should use different method to set pressure due to the Interply Shear issue of trailer axle alignment and should set the cold inflation to the tire sidewall pressure.

If the above isn't enough detail for you you can visit my blog where I have 20 posts that have Temperature as part of the topic.

I write a blog on RV tire application and safety.
Also give seminars on tires at RV events across the US. 40 years experience as tire design & quality engineer for major tire mfg. Freelander 23QB on Chevy chassis is my RV
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Old 04-22-2016, 11:22 AM   #53
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I see the range has changed now from 20-50 to 10-50. I will give you the benefit of the doubt that you simply mistyped the 20 in the previous post and meant 10 all along.

I agree with most of what you say, but would love to see some data supporting that 50 number. I know in my case my TPM is not off by anywhere near 50, but has a high range of less than 20 and is usually pretty accurate. On our 4000 mile trip last summer, I had my wife made notes several times along the way of TPM pressures, temperatures, the ambient temperature and the sunny side of the trailer. Before every trip I would jot down the initial pressure and tempurature, both tire and ambient. The true ambient temp at the time she made the readings may have been off a little since they came via internet using the nearest town. By having many sets of data points to analyze I could then calculate what to expect my TPM readings to be on the next leg of the trip based on anticipated ambient temperature changes.

I am sure that all TPM's act a bit differently and are maybe not quite as consistent as mine is. By doing this study I know what is normal for my TPM and tire combination.

2010 F250 5.4L 3.73
2011 Flagstaff 831FKBSS
Equal-i-zer 4pt 12K

2011 Flagstaff 831FKBSS
2010 F250 4X4 5.4L 3.73 LS
EQUALIZER E4 1200/12000
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Old 04-22-2016, 02:26 PM   #54
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Join Date: Jul 2015
Location: Louisiana
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Interesting posts. Tireman, do tires go through a "break-in" period or miles? On our first trip with the newly mounted Maxxis tires, the pressures got up fairly high, which was the trip that started this thread. Cold pressure was 80 psi in high 60's. As ambient temps reached 77 F, the tire pressures reached 93-95 psi, just below alarm at 96 psi.

Okay, fast forward a couple weeks; we made another trip where temps are warmer by 4-5 degrees, yet tire pressures are consistently lower. Specifically, with 81 degree ambient temps, I am now seeing 90-92 psi tire pressures, which I am a little more comfortable with. Yes I checked cold pressures which varied from 80-82 psi.

So, what am I missing?
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16
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Old 04-22-2016, 03:55 PM   #55
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Lots of info in this thread.

I will throw out a statement made by CapriRacer, RV.Net Open Roads Forum: Revived Trailer Tire Thread (formerly on the 5th Wheel Forum that I filed. CapriRacer says he is a tire engineer, you can find more here: Barry's Tire Tech

CapriRacer: "If the tire pressure build up is less than 10%, it's OK. If the pressure build up is between 10% and 15%, then continue to monitor until you are sure it is not above 15%. If the pressure buildup is 15% or greater, add load carrying capacity (and one of the ways to do that is to add pressure). For ST and LT tires, NEVER use more than 10 psi over the sidewall pressure."

If you haven't read the forum thread linked above, there is informative info in there.

I put credence into what Tireman9 and CapriRacer say. If I remember correctly, they know each other.

Chris and Pat
2016 Ram 3500 Cummins Aisin Dually
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