New TPMS Parameter Setting Needed

[SpaceNorman]

I recently replaced all 8 tires on my coach - replacing the original Goodyear G670's with a set of Michelin XZA2 Energy tires. Both sets of tires were/are 295/80R22.5 size. I ran my old Goodyear's at 100 psi across the board. According to the "Load & Pressure" data for my new tires presented on the Michelin web site - a 13K axle load should have the tires inflated to 100 psi, a 20K dual axle load should have the tires inflated to 85 lbs and a 10K axle load should have the tires inflated to 80 lbs. The folks at the tire shop said - pump 'em up to 120 psi across the board (which is the maximum cold inflation printed on the tire sidewalls). I elected to take what I consider a "compromise" position - and am running at 110 psi across the board. A little harder than the inflation called for based on individual axle load - but less than the max cold pressure that the tire is rated for.

My TPMS (a Doran 360 system) is still set for my old Goodyear tires. Over the Memorial Holiday weekend, we took a trip up to Traverse City - roughly 250 miles in each direction - ambient temps during each drive were in the mid 60's. In the course of each drive - tire pressure increased 15 - 18 psi from the 110 psi starting point - setting off "high pressure" alarms on my TPMS system (which is set to alarm if/when tire pressure exceeds 125 psi).

I believe that I need to increase the threshold at which my TPMS system sounds "high pressure" alarms. I searched the Michelin web site for data regarding max pressure for warm tires - and found nothing. I contacted the Michelin help desk - who either could not (or more likely, would not) provide a "max" number. Their advise was to contact my RV manufacturer. Of course, the dealership AND the tire store both recommend I contact Michelin. Bottom line ... I'm on my own in terms of "picking a number".

Unfortunately, my TPMS system does not provide a temperature reading (or "high temp" warning) - just the tire pressure and a "high pressure" warning. Since the change in pressure is driven by the rise in tire temperature - I feel like I can live with this "shortcoming" by judiciously setting my "high pressure" warning thresholds. My gut feel says that I should probably set the "high pressure" warning threshold at 130 psi (which is 20 psi over my starting "cold" inflation). Doing so would have avoided all the alarms I dealt with this weekend.

My questions to the forum are: What sort of pressure increases have you observed between a "cold" tire and a "warm" tire? What "high pressure" threshold do you set in your TPMS system?

[JFXG]

Norman.... I have the same tires, XZA2, albeit just a touch smaller-- 275/70/22.5. I've been running at max gross for 5 years-- 12K Lbs front axle, 19K Lbs rear. Michelin charts call for 110psi front and 95psi rear, and that's where I run them, maybe plus 3-5. I wish I could shed a little weight. At 110psi, every little pavement crack feels like I'm running over a 2x4!

I've noticed about a 15+% temp induced pressure increase at highway speed, perhaps a bit more in high summer. I have my TST TPMS high threshold at 130 front and 110 rear. It seems to work out well. Pressure mostly rides within a Lb or two, occasionally triggering.

[vettenuts]

I have had mine exceed the recommended threshold (20%) as well and I shut off the alarm and watched for the maximum pressure that the tires reached. I then set my limits slightly higher. This also gives a little leeway if the day is warmer and my initial pressure is a few psi higher than my desired cold pressure. The 20% is just initial guidance and my manual does say it may need some adjustment.

[Crasher]

I have my high pressure for the front set at 148 psi. The fronts are the heaviest loaded tires and start at 115 (5 psi over the recommended) and get as high as 140* on a hot day. A Michelin tech told me that if the tires are set at the correct cold pressure don't be concerned about the pressure increase after they warm up. The closer the tire is to it's max load, the higher the psi will increase when warm. After a few trips, you will know what the average increase will be. Set the high limit 5-10 psi higher than that and enjoy the ride.

[Gordon Dewald]

Norman - folks at the tire store are likely correct if you are driving an OTR vehicle. They inflate to maximum because the next load will likely be at the maximum. Because we have a coach that is loaded to what it is IMO we can adjust the tire pressure to accommodate the best combination of capacity, ride and handling.

I believe that a large component of the tire design has to do with tire contact patch which includes sidewall flex. The tire contact patch can be determined by dividing the information on the tire (max load/max cold pressure). Since we have a constant load on the tire s determined by corner or axle weights I believe we can reduce the air pressure according to the charts which will give us the design contact patch.

Having the tire at the correct pressure will give us the best handling, traction and ride.