By Roger Marble
I saw a post on an RV Forum about a tire “blowout” on an RV that had a Tire Pressure Monitoring System (TPMS).
The RV owner said they had a TPMS but still had a sidewall failure. He also said he set his “‘baseline’ PSI, which is the threshold for the low-end pressure reading, and then the upper limit alarm will tell you if you’re exceeding normal pressures and headed for a blowout. Same thing for the temperatures as well….set the baseline and then it alerts you if the tire temperature is above normal and too high. I used my unit last year on 2 long-haul trips to MI and it performed well. Gave me the peace of mind knowing I could keep an eye on the tire pressures and temps.
“I do recommend the unit. I purchased the unit which can support up to 22 sensors, but got the TPM system with 6 sensors on Amazon, then bought 2 additional sensors for a total of 8 – 4 for my TV and another 4 for my TT. My trailer has dual axles. I mounted the Wi-Fi extender on the front of my TT just behind the battery box and use the clip-on leads from the battery to power it. The Wi-Fi unit is in my truck and I tuck it in the center pocket in my cab. Works well.”
Not all TPMSs let you set the low pressure warning level
I pointed out to the poster that not all TPMSs let you set that actual low pressure warning level. Some use a percent or number of PSI below the “set pressure” aka “baseline.”
I recommend that the low pressure warning level be no lower than 5 psi below the minimum pressure needed to support the measured load on the inflation for the tires on that axle. With some TPMSs, this will require some extra calculation and work, as you will need to set the baseline higher than your actual “cold” inflation.
Example of how to set the low pressure warning level
If tire minimum inflation needed to support the measured load was 70, your low warning level should be no lower than 65.
If your system has the warning at -25% from “baseline,” then you need to set your baseline to 87 psi (75% of 87 = 65). Some systems use the pressure in the tire when you first connect the TPMS sensor as the “baseline,” so you would need to pump up your tires to 87 before first connecting the TPMS sensor. Once the TPM system is fully connected you could then lower your 87 to 80 (70 x 1.15).
The 1.15 is a suggested margin of inflation so you are not pumping up your tires if the ambient temperature drops. Remember that a drop of 10°F will result in about a 2% drop in the “cold” tire pressure.
Yes, this is complex. But that is a problem with the TPMS manufacturer not giving the user full control of the warning and baseline inflation levels.
Please learn how your TPMS is programmed. Your instructions should tell you how to set the “Low Pressure Warning Level.” But if it doesn’t, you will need to do the above math.
Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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My TPMS instructions were clear on how to set the low pressure but not the high pressure. What is the formula for that?
No formula as undamaged tires can handle significant increase in pressure. If you are concerned and really want a number I would set the High Pressure warning at 150% of your “cold set pressure”. BUT unless you have some malfunction I don’t see how you could get that high.
“… and then the upper limit alarm will tell you if you’re exceeding normal pressures and headed for a blowout.”
That’s not even close to accurate unless the owner has way over-filled their tires, is it?
“High Temperature” may have some value on a trailer where it might indicate a defective wheel bearing or a hanging brake but to me, it’s pretty much worthless on a motorhome.
What are your thoughts on those two alarms, Mr. Marble?
I absolutely agree on the Low Pressure setting and that alarm’s value.
High Pressure… See comment above. I have covered my feelings on the Temperature reading in a post in my blog RVTireSafety.net
Thank you for the article and explaining how to use the TPMS correctly. We are about to install and use our first TPMS and still have a lot to learn.
You can get your questions on TPMS and tire pressure and load on my blog.