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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

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.


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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.


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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

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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