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Old 12-10-2016, 06:17 AM   #1
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TPMS Pressure Settings

Just before our last road trip I installed a TST system and set the pressure limits. Outside temp was about 55 degrees when I set all the tire pressures, going with 112 on the fronts and 90 on the rears, based on Michelin's load tables. When I set up the alarm limits, I went with 85/130 on the fronts and 75/110 on the rears.

On the 200 mile drive, the air temp never went above 65 degrees and it was mostly in the driving rain, so the tires stayed cooler than normal. In fact I never saw tire temps more than 68 degrees. However towards the end of the trip I got a high pressure alarm from one of the rears showing 110psi. The fronts were running about 125psi and didn't alarm.

I would have thought a 20psi high limit was plenty for the rears, especially on a cool rainy day, but they were all 108-110 at the end of the trip. I'm thinking I'll bump the rear limit to 115, but I'm still concerned as we get to warmer temperatures that I'll see an even greater increase. We're new at this so I don't have any experience to draw from. Is a 20psi jump normal, and should I raise my limits? Of all the things I don't want to have problems with, tires are #1 on my list.

FWIW we have done some weight redistribution and will be weighing the coach again next week. I'm hoping to take a couple hundred pounds off the front axle, which means I'll be adjusting tire pressures again, and resetting the TST limits. Looking for suggestions.
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Old 12-10-2016, 08:00 AM   #2
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Yeah, 15-20 psi is a fairly normal increase. Tires heat up a lot with road friction.

However, I would double check the rear tire inflation vs weight, making sure you have adequate inflation. Even a slightly soft tire heats up faster and much higher than a properly inflated one.
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Old 12-10-2016, 08:35 AM   #3
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Contact TST for some guidance on settings with the TPMS. They have great customer service. Also setting your pressures at a 22 psi difference front to rear seems odd to me. I don't know the particulars for your coach but mine are set the same all around. My rear pressures do rise more than the front while traveling. I set cold pressure at 90 psi and the fronts go to 95 - 100 psi and the rears easily hit 108 - 110 even though OAT and TPMS temps seem cool (in the range you encountered). I suspect the rear pressures you set may be to low for the weight of your coach. By all means reweigh your coach and set the pressures accordingly. TST, as I said before, has great service and they will help with adjusting limits and can give you insight on how the systems works.
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Old 12-10-2016, 09:48 AM   #4
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Here is what I did. After weighing all six positions, I had the ride height adjusted to get the front tire weights close to the same. (the right was 900# heavier than the left) Then, with the new weights, I added 5 psi the the pressures Michelin recommended for the loads on a 60* morning before driving. The fronts--115. drives--100, and the tag--90. The fronts will increase to 138, the drives to 118, and the tag up to 104. Michelin told me that was all perfectly normal and not to be concerned with the pressure rise as long as the cold psi was at or above the recommended pressure for the load. I found that in 8000 miles and 9 months, going from 1000' up to 11,000' altitude and morning temps from 35* up to 75*, the cold pressures were never below the minimum and were never over the maximum cold psi for the tires. I did not add or release any pressure from the tires in that period. The front high pressure alarm is set at 145 psi, the drive at 124 and the tags at 110. The low pressure alarm is set at the Michelin recommended minimum for the load.
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Old 12-10-2016, 09:59 AM   #5
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tire pressure increase

Rear inboard tires increase in pressure because of the crown in the road !
look at 18 wheelers many have both inboard and outboard connected to equalize the pressure
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Old 12-11-2016, 04:45 AM   #6
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Quote:
Originally Posted by Gary RVRoamer View Post
Yeah, 15-20 psi is a fairly normal increase. Tires heat up a lot with road friction.

However, I would double check the rear tire inflation vs weight, making sure you have adequate inflation. Even a slightly soft tire heats up faster and much higher than a properly inflated one.
My rear axle is just over 20k pounds, putting the rears at 90 according to Michelin, but I think I might raise them to 95 and see what the next run is like. I'm also going to raise the rear high limit to 115. 20psi just surprised me on a tire with a very large volume. I have raced cars for many years and we'd see hot pressures going up 20 but that is after some serious track work.
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Old 12-11-2016, 04:49 AM   #7
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Quote:
Originally Posted by c92vette View Post
Contact TST for some guidance on settings with the TPMS. They have great customer service. Also setting your pressures at a 22 psi difference front to rear seems odd to me.
Thanks I will contact them, good to hear they have good customer service. I set the pressures according to Michelin's guide - front axle 12.3k and rear 20k. My front was very close to the GAWR while my rear has 4k pounds of headroom, which accounts for the difference. I have shifted some weight to the rear hoping to improve that, and we will weigh again next week - hoping to get corner weights this time.
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Old 12-11-2016, 05:39 AM   #8
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Mine alarmed a couple of times and I set the alarm a bit higher. I monitor the pressures as I drive so a couple of psi isn't a big deal but a big tire pressure increases will still alarm. The delta in the owners manual is a starting point as opposed to an absolute. Also, if I start the day a couple of psi higher than my desired value, I don't reset all the tires and the extra alarm range allows me to do this.
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