Tire pressure increase when towing

[Medic97]

As a new TST user about to embark on a long trip, I certainly have appreciated everyones comments on this thread. Especially those of Tireman. Thank you all for a great discussion and a great learning experience dealing with tires and safety.

[Davidg]

Originally Posted by hbillsmith

Its the 75mph adj that is confusing. The 8008 (16" with Hispec wheels) have a max of 80psi and provide load of 3420. Since you cannot exceed 80psi, I was told a decrease of load by 10% was the acceptable safe alternate. So the 3420 max is reduced to 3078. Thus 4 tires with trailer weight evenly distributed would allow 13,680 trailer weight. A CAT weighing is required since some of the trailer GVW lies in the tongue carried by the truck payload.
EDIT: Escapees does individual weighings at each wheel for those of us that cant seem to figure out a load balance.

Okay, so when I weighed my 29RE, both axles were carrying 9460 lbs. (I honestly thought this weight would be higher) As of today, I do not know how that pressure is spread between the individual tires, so for my calculations, I used 11000 lbs to gain a reserve capacity. 11000/4 = 2750 per tire. According to the Maxxis load/inflation chart for my tire, the correct pressure would only be 55 psi. (yeah right) So, again, using their load/inflation chart and their guidelines in the email, I could add 10 psi to that inflation pressure and run 75 mph, or I could add 20 psi and run up to 85. Those numbers would allow me to run 85 mph with 75 psi in the tires. I will never tow that fast but that's what Maxxis states.

As I have posted before, my intentions are to run at a cold inflation of 80 psi. I am a little concerned with how high the pressures get after towing on the interstate for a few hours, but it's something I keep an eye on. It will be interesting to see how high the inflation pressures get when hot weather gets here. I'm sure that black asphalt in July/August will be well over 100 degrees F. So, I will be forced to do one of two things. (or a combination of) 1. raise the high temp alarm on the TST, and/or 2. lower the cold pressure in the tires.

Because I have changed more than a few blowouts on the side of an interstate, this is a very interesting topic for me, and I am learning a lot from several of the forum experts.

[gectisme]

Originally Posted by Medic97

As a new TST user about to embark on a long trip, I certainly have appreciated everyones comments on this thread. Especially those of Tireman. Thank you all for a great discussion and a great learning experience dealing with tires and safety.

X2. My TST system is arriving next week so this is very good and well-timed!
Thanks All.

[hbillsmith]

Forgot to mention, I do use the TST monitor. I have my high temp alarm set at 105deg. and I have my high pressure alarm set at 92psi. Just an FYI. I do adjust my driving habits according to outside conditions and ambient temps. It it's 105deg outside, I usually just drive at 65mph. If it's raining on a mountain road, go ahead and pass me 'cause I'm probably at about 35 or 45mph (or less).

[Davidg]

Originally Posted by hbillsmith

Forgot to mention, I do use the TST monitor. I have my high temp alarm set at 105deg. and I have my high pressure alarm set at 92psi. Just an FYI. I do adjust my driving habits according to outside conditions and ambient temps. It it's 105deg outside, I usually just drive at 65mph. If it's raining on a mountain road, go ahead and pass me 'cause I'm probably at about 35 or 45mph (or less).

Those are probably good settings, but be aware that the temperature reading on the TST and the actual tire temp are usually a world apart. I have a well built Klein infrared temp guage that I carry with me. Whenever we stop, that's the first thing to come out, and I'll check the temp of all 8 tires and hubs. During the summer, tire surface temps will easily hit 135 degrees F, and the hub a little cooler. As I understand it, the actual tire temp will be a couple of mm below the surface of the rubber at the outside corners. This is where the tire is flexed as it rolls. Before I stop, I'll check the TST which has been running around 84 deg F, and then I'll check with the Klein and temp will be around 108 +/-.

Thanks for sharing the data from your TST. That is good to know.

[lbrjet]

105 seems awfully low for a high temp alarm. What is the default?


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

[Blackhat6mike]

I was told by TST that the default high temp was set to 158*. That is where mine is set.

[Davidg]

158 is also where mine is set, and that temp appears to be the lowest setting possible, but I understand there is a way around it. I just haven't gone to that trouble yet.

Having used mine a few times now, I've not seen the temps above 87. Granted it's not summer time yet, but the sensor is insulated from the rim by either a rubber valve stem, or by thick rubber washers on a metal stem. And, the sensor is out there spinning pretty fast at 60 +/- mph, so, even if it were to see the tire temps, it's being cooled off at a fast pace anyway.
In addition, the tires do not typically get warm where they seal against the rim. Tires are warmest at the side corners of the tire where the tread cap sits. About the only thing that might warm the sensor up would be a failing bearing, but that's not usually a long curve to failure. Once it starts, it's a fast, and self destructive event. Not always, but usually.

So, given all that, the temp sensor is almost useless as a predictive failure tool. At least that's my thought.

The pressure sensors however, are very useful technology.

[Tireman9]

I would not get all wrapped up with TPMS temperature readings as in my experience they can be low by 20 to 50F.
Here is some info on real life temp measurement.
As you can see even expensive external IR guns do not provide the facts.

TPM sensors on metal stems are reading the temp of the air that has been cooled by outside air. Air is a great insulator so heat from the tire is not getting transfered to the TPM sensor.

I did a quick, low speed test and published the results. Note the cold ambient.

Use the TPMS to monitor PRESSURE as it was designed to be used.

[Davidg]

Originally Posted by Tireman9

I would not get all wrapped up with TPMS temperature readings as in my experience they can be low by 20 to 50F.
Here is some info on real life temp measurement.
As you can see even expensive external IR guns do not provide the facts.

TPM sensors on metal stems are reading the temp of the air that has been cooled by outside air. Air is a great insulator so heat from the tire is not getting transfered to the TPM sensor.

I did a quick, low speed test and published the results. Note the cold ambient.
Use the TPMS to monitor PRESSURE as it was designed to be used.

Yep, and almost all the data in my last post came from Tiremans website. It's an excellent source of information to educate yourself. I learned a lot, yet I barely scratched the surface.

Tireman, thanks for making this information available. Helped me a lot.

[lbrjet]

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

[Tireman9]

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.

[lbrjet]

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

[Davidg]

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?

[racer4]

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.