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Old 04-19-2012, 11:15 PM   #1
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Tire pressure increase during towing.

My tires are load range D and are inflated to 65 PSI when cold. I check them with a good tire pressure gage and also have valve stem sensors on them.
My question is: When towing the pressure went up to 77 PSI. I stopped for gas and it went down to 74 PSI and then went up again.
Is this normal and is there there a maximum the pressure shouldn't go over?
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Old 04-20-2012, 12:03 AM   #2
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My personal assumption has been the Maximum rating of the tires themselves.. Anyone know a normal range of increase... ??

I believe the pressure increase is from the heating of the tire and air inside.. it cools while you are stopped..

Some of it is normal but could indicate to tire balance, out of round, or other source of wobble...
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Old 04-20-2012, 02:12 AM   #3
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Road friction makes all tires heat up and increase pressure. That's why for an accurate PSI check tires cold.
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Old 04-20-2012, 06:26 AM   #4
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The max pressure stamped on the side of the tires is max when COLD. Pressure above that while the tire is warm (from using it) is normal.

My trailer tires run at about 100 degrees I've noticed. I don't check the pressure on the road.
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Old 04-20-2012, 06:36 AM   #5
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Quote:
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Road friction makes all tires heat up and increase pressure. That's why for an accurate PSI check tires cold.
Not just road friction; also sidewall flexing. That's why under-inflated tires overheat faster, and explains why under-inflated tires are more likely to experience a blowout.
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Old 04-20-2012, 07:27 AM   #6
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Stan,
As the previous writers have indicated, this is normal. We inflate to 65psi cold, cruise at 65mph and depending on the outdoor temps the pressures will vary. On our current trip, with the outdoor temps in the high 70s to low 80s our rolling tire pressures, as indicated on our monitors, run around 72-74psi. Not to worry.
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Old 04-20-2012, 07:34 AM   #7
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Tire Temps

It is normal for the tire temps to increase with road friction and heat but how much is the issue. My particular TPMS has a pre set pressure warning of +/- 12% of the selected tire pressure. So for a 65psi setting pressures above 72.8 and below 57.2 gives a pressure warning. Speaking to others with various TPMS it seems the 12% value is pretty constant. The TREAD Act, which is the legislation that requires TPMS on all new cars, calls for a warning when the pressure drops 25%.
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Old 04-20-2012, 10:14 AM   #8
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The TREAD Act, which is the legislation that requires TPMS on all new cars, calls for a warning when the pressure drops 25%.
New CARS. My Airstream Interstate, which is based on a Mercedes Benz Sprinter 3500, doesn't have TPMS. The Sprinter 2500— with only four tires on the ground— has TPMS, but the 3500 with dual rear wheels does not.

I thought about buying an aftermarket TPMS, but I'd like to avoid the ones that screw onto the valve stem in place of a valve cap. All they'd have to do is loosen a quarter turn, and they would cause bigger problems than they solve. Plus, the ones I've seen all have a warning in the owner's manual: "Avoid using this product on aluminum valve stems." If I want to use them, then I'd have to remove them about once a month and apply a coating of dielectric grease to the screw threads on the valve stem, to keep the TPMS sending units from welding themselves in place. And once greased, the TPMS caps are less likely to stay tight.

That pretty much leaves me with aftermarket TPMS that replace the valve stem and have the sending unit inside the tire. This would require demounting the tires and also rebalancing them, so I'll wait until I've got a scheduled tire rotation coming up.
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Old 04-20-2012, 11:41 AM   #9
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We remove the TPMS sensors when we are not using the trailer. If you leave them on, they keep sending a signal and running the batteries down. That also solves any potential dielectric problem.

As for the OP, sometimes our TPMS monitor will go off because the pressure was correct in the morning, but temps go up so high during the day, pressure goes over the 12% limit. We stop and let some air out and that usually solves it. Our TPMS (Doran 360) needs to be re-programmed every year or two because the software becomes corrupted (I guess) and starts sending bad information. It has been posted that if you screw the sensors on too tightly, you can get false readings; this supposedly came from the manufacturer.

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Old 04-21-2012, 12:47 AM   #10
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Thanks for the information

That's good to know about the valve stem sensors. The alarm went off
for the first time for the pressure being high instead of low. I wasn't sure
how high might be too high.
Also on my Tundra TV the alarm went off several times and I finally realized it was because of the spare tire that I hadn't checked.
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Old 04-21-2012, 02:09 AM   #11
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For those that don't remember high school physics:

PV = nRT

P = pressure, V = volume, n = #moles, R = constant, and T is absolute temperature. Now the volume and number of moles and R are constant, so for cool and warm conditions. set equations equal and ignore the constants:

Pc/Tc = Pw/Tw

Re-arranging, and adding in constant to convert from F to absolute Rankine degrees:

Pc * (tw + 460)/(tc + 460) = Ph

If you inflate tires at 70F to 50 psi, you'll get:

F psi

70 50
80 50
90 51
100 52
110 53
120 54
130 55
140 56
150 57
160 58
170 59

Obviously, if you're running in the desert with 100F ambient temperature swings some attention to this is a good idea. Tire temps should not exceed 160-170F.


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Old 04-21-2012, 05:21 AM   #12
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Bart, that's what I was going to say!
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Old 04-21-2012, 05:59 AM   #13
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As for the OP, sometimes our TPMS monitor will go off because the pressure was correct in the morning, but temps go up so high during the day, pressure goes over the 12% limit. We stop and let some air out and that usually solves it. Our TPMS (Doran 360) needs to be re-programmed every year or two because the software becomes corrupted (I guess) and starts sending bad information. It has been posted that if you screw the sensors on too tightly, you can get false readings; this supposedly came from the manufacturer.

Gene
I think if you fill your tires with nitrogen would cure your problem Gene.
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Old 04-21-2012, 07:22 AM   #14
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I think if you fill your tires with nitrogen would cure your problem Gene.
Not really. Regular air is about 80% nitrogen already. Plus, nitrogen and oxygen both expand/contract similarly with changes in temperature. Boyle's Law is the same for all gases or mixtures of gases.

The benefits of using pure nitrogen really are limited to longer tire life because the rubber inside the tire doesn't oxidize. However, since the rubber on the outside of the tire continues to oxidize, it's not much help there, either. Save your money and use regular air.
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