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Old 04-13-2016, 02:23 PM   #11
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Originally Posted by Dennis in Texas View Post
The nerd in me wanted to get out. So here goes...The Combined Gas Law predicts a rather substantial increase in pressure according to the following formula. Where P=pressure, V=volume and T=temperature (degrees Kelvin).

P1V1/T1 = P2V2/T2

since the volume will not change to any significant degree,

Thus we use:
Gay-Lussac's Law
P1/T1 = P2/T2

the pressure must increase as the temperature increases.

What is you use nitrogen in your tires?


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Old 04-13-2016, 02:31 PM   #12
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Originally Posted by hbillsmith View Post
Nice thread. Here's where I'm thinking. My Maxxis 8008 call for 80psi cold pressure on their chart that indicates an 'E' rating (65mph) requires 80psi cold. Lower psi, reduces the ratings down to D or C and the resulting load carry ability.

Maxxis Load Inflation Chart

If I'm to accept that the maximum PSI should not exceed 25% of 80psi, then my maximum should not go over 100psi. So if I'm in Texas and the RV is parked outside on hot pavement, the cold-pressure is going to be at a good start point and I'm probably OK getting out on the 100+ interstate. YES

But if the 5er is parked in a shaded storage bin or maybe for a couple of weeks camping on grass or gravel, the starting 80psi cold will be unusually cold for the hot Interstate I'm about to get out on. In these cases, I will drop the starting cold temp to 70psi and watch it grow as I continue out on the Interstates. NO NO NO This works for me and when the ambient temp outside is 99deg to 103deg, I'll see my TST tire temps go up to about 98deg to 100deg. No I would expect the TPMS to show a temperature of +10 to +50F over ambient depending on ambient, tire load and speed.

Haven't hauled across the Mojave desert yet but if I ever see those towing temps go over 100, I think I'll let some air out.
NO!!!! do not drop your cold inflation pressure!!!!. That will make things worse and could lead to a tire failure.

Tire inflation will normally increase at about 2 or 3% for each increase in internal temperature of 10F.
External TPM are cooled by the external ambient air so are not giving you an accurate reading of the contained air temperature and can be off by 20 to 30F. I have tested this by running internal TPM vs external TPM on my RV.


The TST guy may be correct that some folks are seeing significant pressure increase but those that are are probably overloading their tires (over 50% of RVs have one or more tire in overload) and probably are also driving in excess of 65 mph, which is the max speed rating on most of the trailer tires on the road today.

What are the actual loads on each of your tires? Not the total and not the load on each axle but on each tire?

Is your digital pressure gauge accurate to +/- 2psi or better when checked against a certified gauge? If not get a good gauge. When I test slip-stick gauges many are at or beyond 5 psi error and some as much as 10 psi off. Some are just hard to read and IMO are better suited as tent pegs.

I strongly suggest you read my tire blog. I have a number of posts on temperature and pressure and even the mathematical proof of the pressure vs temperature formula

With LR-E tires you should always run 80 psi "cold". Cold being when tires have not been driven on or in direct sun for at least a couple of hours.

When properly loaded, tires are designed to handle the normal increase in either temperature or pressure observed here in the US. I am not aware of any published limit stating a max increase of 25% in pressure. However if I was seeing that level I would suspect some other issue such as overload or overspeed.
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Old 04-13-2016, 02:50 PM   #13
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Originally Posted by Tireman9 View Post
NO!!!! do not drop your cold inflation pressure!!!!. That will make things worse and could lead to a tire failure.

Tire inflation will normally increase at about 2 or 3% for each increase in internal temperature of 10F.
External TPM are cooled by the external ambient air so are not giving you an accurate reading of the contained air temperature and can be off by 20 to 30F. I have tested this by running internal TPM vs external TPM on my RV.


The TST guy may be correct that some folks are seeing significant pressure increase but those that are are probably overloading their tires (over 50% of RVs have one or more tire in overload) and probably are also driving in excess of 65 mph, which is the max speed rating on most of the trailer tires on the road today.

What are the actual loads on each of your tires? Not the total and not the load on each axle but on each tire?

Is your digital pressure gauge accurate to +/- 2psi or better when checked against a certified gauge? If not get a good gauge. When I test slip-stick gauges many are at or beyond 5 psi error and some as much as 10 psi off. Some are just hard to read and IMO are better suited as tent pegs.

I strongly suggest you read my tire blog. I have a number of posts on temperature and pressure and even the mathematical proof of the pressure vs temperature formula

With LR-E tires you should always run 80 psi "cold". Cold being when tires have not been driven on or in direct sun for at least a couple of hours.

When properly loaded, tires are designed to handle the normal increase in either temperature or pressure observed here in the US. I am not aware of any published limit stating a max increase of 25% in pressure. However if I was seeing that level I would suspect some other issue such as overload or overspeed.
Tireman, you know infinitely more than I do on this subject, but, in my bullheaded stubbornness I will continue to drop pressure by 2-3 psi if I check early morning and know for a fact that temps will be rising quickly over the next 1-2 hours. I don't run at 100% of load rating and don't exceed 65 mph, so a slight drop in pressure for 15 minutes or so should not be an issue. Here in Texas, you could leave for a trip at 7:00 a.m. at 40 degrees and easily be traveling in afternoon sun pushing 80's at times. That ambient air temp coupled with the increase in temp from friction could easily push a tire to a significantly higher pressure. I would NOT correct my pressure for an anticipated friction based increase in pressure- only when I know a significant increase in ambient air temp will be occurring. I also would not do this if I knew my tires were carrying 100% of their capacity. Well, I would never run ST tires at 100% capacity, but that's for another thread.
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Old 04-13-2016, 04:14 PM   #14
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Okay, I shot an email to Maxxis with the following data and the associated question.

================================================== =

At the time I installed the new Maxxis tires on my fifth wheel, I also purchased a TST 507 tire pressure monitoring system, and put 80 psig in each tire using (two for comparison) NAPA digital tire pressure guages. Last Friday, we went on a 120 mile trip.(one way) The ambient temps ranged from 68 degrees F when we left, and topped out at 77 degrees F on the interstate. I was maintaining a speed of 60 - 63 mph, and tire pressures slowly increased to 95 psi. Tire temp held steady at 84. I was surprised and a little concerned that pressures increased almost 20%.

Is this normal for the 8008 tires?

Thanks

David

==================================================

And this is the response I received directly from Maxxis;

David,
That is pretty common. Due to the tire construction materials (steel belts namely), coupled with the heat of the pavement and friction, a tire will build up quite a bit of additional inflation pressure. You may see it even exceed 95 PSI when the ambient and road temperature increases moving into summer.

Respectfully,

--------------------------------------------------------------------------------------------

Looks like Tireman is spot on.

David
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Old 04-13-2016, 05:43 PM   #15
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Quote:
Originally Posted by Dennis in Texas View Post
The nerd in me wanted to get out. So here goes...The Combined Gas Law predicts a rather substantial increase in pressure according to the following formula. Where P=pressure, V=volume and T=temperature (degrees Kelvin).

P1V1/T1 = P2V2/T2

since the volume will not change to any significant degree,

Thus we use:
Gay-Lussac's Law
P1/T1 = P2/T2

the pressure must increase as the temperature increases.
And the pressures must be in absolute (add about 15 psi at sea level).
In any case:
80 psi (95 absolute) @ 70 deg F (294K)

heated to 90 deg F (305K) will increase gauge pressure to 83 psi
heated to 135 deg F (330K) will increase gauge pressure to 92 psi

rough rule of thumb for every 10 deg F increase in tire temperature, pressure increase about 3 psi.
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Old 04-13-2016, 05:51 PM   #16
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Thanks Tireman. Gotit. It's 80psi cold and monitor but do not get concerned about high temps. I will continue to be on the alert for a single tire that may grossly exceed what the others are doing and thus inspect for issues. I understand the normal variance attributed to the sunny side vs. the non-sunny side.
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Old 04-13-2016, 07:13 PM   #17
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You bought the tire monitoring system let it do its job it will tell you it you have a problem
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Old 04-13-2016, 09:53 PM   #18
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Originally Posted by DSQR View Post
And the pressures must be in absolute (add about 15 psi at sea level).
In any case:
80 psi (95 absolute) @ 70 deg F (294K)

heated to 90 deg F (305K) will increase gauge pressure to 83 psi
heated to 135 deg F (330K) will increase gauge pressure to 92 psi

rough rule of thumb for every 10 deg F increase in tire temperature, pressure increase about 3 psi.
So, if I start at 80 psi, and after an hours drive I'm at 94 psi, then tire temps must be approx 137 F ? That being the case, then the TST temp reading isn't worth the battery power it's consuming.
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Old 04-14-2016, 10:37 AM   #19
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Contained Air Temperature (CAT) and temperature of a sensor wipping around in outside are can be 20F to 30F or possibly more different. The TPM can only read the temperature it is exposed to.

Air is a good insulator and I doubt there is much air turbulence or movement of the air in the valve stem.

Now a metal stem might conduct some heat from CAT to outer end but it would also cool the stem too.

As I said I have measured and observed the significant difference between CAT and external readings.

Tire PRESSURE Monitor Systems are primarily for measuring and reporting and warning of PRESSURE changes.

Tire Temperature is a secondary issue and people spend too much time on that topic.
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Old 04-14-2016, 11:43 AM   #20
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You are overthinking it. Set the tires in the morning and leave it alone.
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