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Old 04-11-2016, 12:51 PM   #1
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Tire pressure increase when towing

So, I managed to get the Maxxis 8008's put on, the Constancy 188's sold, the HW50C installed, then the TST 507 (6 sensor) system installed last week. (All told, about a $1200 week..lol)

Anyway, we left Friday morning for a short trip. (120 miles one way) It must have been aprox 68 degrees F when we left, and hit 77 F before we arrived. So, I'm watching the TST as I get on the road, and right away I notice the pressures begin rising, along with the temps which I fully expected. The temps topped out at aprox 84 degrees F, however the pressures climbed to 94 psi on the sunny side of the trailer, and the truck rear tires topped out at 88 psi. We started the trip with 80-82 psi in all the camper tires, and 79-80 in truck rear tires. On the return trip yesterday afternoon, one sunny side tire hit 95 psi.

I was a little concerned (and surprised) that the pressures went that high. Especially with the TST high pressure alarm set at 96. it never hit the alarm setpoint, but this was a mild day. Here in Louisiana, ambient temps will soon be in the 90's, so I'm anticipating a problem.

Now I need to decide whether to raise the high pressure alarm setting to 98, or to lower the trip starting pressure a couple psi and see if that helps. I'm not comfortable with only 1-2 psi before it alarms.

Is this pressure increase normal for those ambient temps?

I kept my speed to 60-65 mph, and we were not loaded heavy at all. Most of the trip was driving on Interstate hwys. Just looking for others experiences with the TST and pressure increases when towing.

Thanks
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Old 04-12-2016, 10:24 AM   #2
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Well, I just got off the phone with Eric at TST. He affirmed that what I was experiencing was completely normal. He stated that some tire manufacturers indicate a possible increase of even 25% from sidewall max cold pressure. I was experiencing a 19% increase in tire pressure which seems to be in the normal range. Eric stated that most of the tire manufacturers will not provide specific data but will only discuss the topic in general terms, which I suppose is to be expected.
Anyway, he agreed that raising the alarm setpoint to 98 psi would be fine. I will be interested to see what those pressures do when the ambient is 95 degrees and that black asphalt is 100 degrees or more.

So, fwiw, seeing a 20 to 25% increase in highway pressures even with moderate ambient temps is apparently okay.

I sent an email to the Maxxis regional rep, but haven't heard back from him yet.

dg
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Old 04-12-2016, 01:13 PM   #3
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Very normal.
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Old 04-12-2016, 01:23 PM   #4
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I have never seen an increase of more than 9 psi in my tires that includes AZ summer. It is usually 4-6 psi higher. My temp usually rises 10 degrees or so higher on the sunny side than shady side but only a couple of psi difference. I guess all tires are different.


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Old 04-12-2016, 01:24 PM   #5
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That is normal. I see similar increases. The key thing to look for is one wheel that is way out of line with the others. The sunny side will always be higher than the other side. After a few trips, you learn to trust the TST system and only occasionally check the monitor.
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Old 04-12-2016, 04:40 PM   #6
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Thanks for the feedback guys. I suppose the amount and rate of increase would depend on individual tire loading, speed, cold pressures, ambient temp, and whether the tire is in direct sunlight. Lots more variables than I have given thought to in the past. I admit that I was a little surprised at how much difference there was between the sunny side vs shaded side. I like the system so far and like getting data that I could not access before. But new data often raises new questions as it did for me...lol

Anyway, I'll continue to monitor it and learn.
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Old 04-13-2016, 11:37 AM   #7
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Smile Gas law, temperatur and tire pressure.

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.
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Old 04-13-2016, 12:10 PM   #8
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Asphalt in direct summer sunlight can get up close to 135> degrees. Several years back I did work at an automotive test track south of Fallon NV where they were testing many different asphalt mixes and they had thermocouples imbedded in the surface. It was hot enough to make our feet uncomfortable when standing still. I usually started the day around 6am and left around 2pm as it was to hot to get on the track.
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Old 04-13-2016, 12:21 PM   #9
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If I am leaving while it's cool and know I'll be traveling as temps increase throughout the day, I'll start a little low on PSI. Might set to say 62 instead of 65. I'm above 65PSI within a few miles typically. If there is a large temp swing, I might get to as much as low 70's using this method. I'm OK with this. High 70's would make me uncomfortable.
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Old 04-13-2016, 12:36 PM   #10
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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.

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. 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.

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.
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Old 04-13-2016, 01:23 PM   #11
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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.

What is you use nitrogen in your tires?


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Old 04-13-2016, 01:31 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, 01:50 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, 03:14 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, 04:43 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, 04:51 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, 06:13 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, 08:53 PM   #18
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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, 09:37 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, 10:43 AM   #20
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You are overthinking it. Set the tires in the morning and leave it alone.
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