TPMS and Tire Pressure Observations

  I recently completed a road trip with my RV (2017 Thor 23u on E450) pulling a toad (98 Jeep Wrangler). Based on advice from this group, I put in an EEZTire TPMS system for both the RV and toad. This was my first trip pulling a toad along with having a TPMS system. It was quite interesting and educational watching the dynamics of tire pressure throughout the trip.
  As per spec, the TPMS alarm thresholds were set to -10% and +20%. Before leaving, tire pressures were very carefully set cold as per the door stickers. Specifically: RV Front: 75psi, RV Back: 80psi, Wrangler: 32psi. All +/- 1psi.
  Once I hit the road, I found it very interesting how much the pressure went up. I even got to the point of a couple of high pressure alarms. On rare occasions, the RV rears hit 96psi to trigger the alarm. They never went over that. The Wrangler had a case where one tire hit 39psi (but not over). The fronts went up but not quite enough to trigger an alarm. It should be noted that all alarms were during times of higher outdoor ambient temperature. When it was cold out, pressures were lower. This was an eye opener for me to see the radical pressure change as a function of driving and the temperature. I asked a tire shop and they said this behavior was normal. They also mentioned that they see people making the mistake of letting air out of a hot tire to match the cold temp spec.
  The other thing that I saw that surprised me was the difference in morning cold pressure based on local temperature. Original pressure was set at home at around 40 degrees F. at one point, I was at an ambient temp in the morning of around 60. Tire pressure was up accordingly. Later, I was down into the 20?s overnight. Pressure was low.
  I think one action I learned from this is to be sure to check pressure each morning cold and adjust as per local environment. Should I adjust for where I will be during the day? That is, I start cold and will be in a hot environment later in the day.
  As I said, this experience was very educational for me. Before TPMS, I never realized the dynamics of tire pressures.

Jeff

You can adjust daily if you like, but it is more practical to set the pressure to handle the range of temperatures expected. You don't have to worry if it gets a bit hotter than expected, but you don't want to be too low on an unusually chilly morning. Even then I wouldn't worry about 1-2 lbs. If you set the psi when it was 40 F., you need not worry unless you get into weather that is below that at the time you begin driving.

Pressure varies with temperature at the rate of 1.8% (percent, not lbs) per 10 degrees F. If a tire was set to 50 psi at 40 degrees, it will lose 1.8% x 50 = 0.9 psi when the temperature drops to 30 F.

Tire pressures on a given axle should be close while under way.  If they are not then look for what is wrong.

I have found there is very little difference on a given axle while under way for more than 20 miles.  Even tires on the sunny side do not differ much from the ones on the shade side while under way.

I use 95 psi on all 22" tires. Been doing that for years and never had a problem.

I really had fits with mine in the hight desert this past summer
My alarm set points are
Front: 70psi (63 low / 84 high)
Rear: 80psi (72 low / 96 high)

I don't remember exactly what criteria I used when setting those high and low limits, but that's where it is.... seems to work well enough around my home stomping grounds.  I try to start the day (or the weekend usually) with the tires cold and within -1 to +2 of my set point.

So I had a situation where it would be so cold in the morning out in ABQ that I would be well below the set point.  I don't recall if it was ever to the point of low alarm, but I want to say that it was.....  Following the guideline that I've been taught my whole life, your set point should be set at the cold morning temps.... so I aired up the tires.
Well by afternoon the ambient temps were so high, that you can guess the results.  beep beep beep beep....

I never did find a happy set point with such a wide temp swing.... it was always too high or too low....and I really tried to avoid that too low end of the range.... a big headache on the trip

blw2 said:

So I had a situation where it would be so cold in the morning out in ABQ that I would be well below the set point.  I don't recall if it was ever to the point of low alarm, but I want to say that it was.....  Following the guideline that I've been taught my whole life, your set point should be set at the cold morning temps.... so I aired up the tires.
Well by afternoon the ambient temps were so high, that you can guess the results.  beep beep beep beep....

Click to expand...

Same. I've come to learn that, within reason, one has to accept a happy medium with all this info we now have from our TPMS. I think it's safe to assume that the 18 wheelers out there aren't airing the tires each morning and letting air out as they arrive in the desert. Currently living in the desert it's typical to have cold nights, and I often start the morning below my target pressure. What I look for is one tire different than the other(s) on that axle. I'll start rolling perhaps 10psi low with the alarm silenced and within a couple miles I'm back near target pressure. We all drove for years without TPMS. While I love lots of info (hence my VMSpc) sometimes I think close enough is close enough.

I attended a tire seminar by Michelin at a WIT rally a couple of years ago.  Of course the question about tire pressure  rising as you?re driving came up. He said not to worry about the pressure rising, that is built into their tables. He said to be especially concerned about pressures dropping. My tire pressures rise about 10-15 psi on a hot or cold day front and back. I adjusted my Eeztire TPMS to 20 psi above my cold tire psi. No more alarms.

Yeah, that's why I wish I had an analog TPMS indicator.  Too easy to get caught up in the digits....