Tire pressure increase during towing.

[Stan Drury]

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?

[chromis]

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...

[Lumatic]

Road friction makes all tires heat up and increase pressure. That's why for an accurate PSI check tires cold.

[Skater]

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.

[Protagonist]

Originally Posted by Lumatic

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.

[ZoominC6]

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.

[fltlevel510]

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%.

[Protagonist]

Originally Posted by fltlevel510

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.

[Gene]

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.

Gene

[Stan Drury]

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.

[barts]

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.


- Bart

[Hibby]

Bart, that's what I was going to say!

[r carl]

Originally Posted by CrawfordGene

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.

[Protagonist]

Originally Posted by r carl

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.

[kittrik]

My trailer is 3500lb on a single axle. On the highway the pressures increase 6lbs. When bouncing down back roads the pressures will go up 2lbs. I assume the sidewall flex and friction is the cause of the increase.

[fmr.capt.evi]

Until recently I owned a class-B, built on a Chevy 3500 chassis. When towing, it was not unusual for tire pressures to increase 15% or so (above 80 psi cold), before they'd stabilize. Going too fast on a hot day, they'd approach 20% over, and I'd have to back off to silence that annoying alarm.

[ROBERT CROSS]

Originally Posted by Protagonist

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.

Yep...just a way for some of the BB tire stores to get paid for the free mount/balance you just got on your new tires.

Dryness is the only real benefit, I do use a dryer on our home compressor.

Bob

[Gene]

[QUOTE=barts;1136529]For those that don't remember high school physics:
n = #moles/QUOTE]

Bart, I have trouble remembering high school. Moles? Won't they die if someone puts pure nitrogen in the tires?

Thanks for the table—gives me a good idea of how much pressure increase to expect. Is that tire temp or air temp? I guess tire temp because you have to go to Venus to get temps like 170˚—more like 400˚ there.

I notice that after sunrise—an hour or so, the tires on the sunny side increase in pressure 2-3 psi if the covers are off.

Gene

[Excella CM]

It's not just good sense, it's the law. Boyle's Law in this instance.

[barts]

Some reasons that (dry) nitrogen works well in tires are:

Nitrogen leaks through rubber less quickly than oxygen, apparently, so tires inflated with pure nitrogen will lose pressure more slowly than those filled with air. That said, I'd guess this is mostly not an issue for us trailering types, as we're careful about tire pressures already.

Water vapor will increase in pressure more quickly than dry air or nitrogen.n Unless special air driers are installed, the usual compressor/tank setup will deliver air saturated with water vapor; having dry gas inside helps reduce the pressure swings.

- Bart