In my opinion, Tire Pressure Monitoring Systems (TPMS) are of little value when it comes to temperature warning for tires. Same for Infrared Temperature (IR) guns.
I have written a number of times on the TPMS temperature reading really being a reading of the metal wheel and not the rubber tire, as metal conducts heat while rubber is an excellent insulator. So your TPMS can be used to provide warning of potential problems with wheel bearings, breaks and other metal mechanical parts.
These posts use data from my ongoing, direct comparison of internal vs. external sensor TPMS. My data suggests that there is no meaningful difference in pressure reading based on the test of 12 sensors.
When seeing people claim the TPMS temperature reading was useful for predicting impending tire failure, the engineering DNA in me kicked in and I devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from TireTraker and one set of 6 internal sensors is from TechnoRV, who provided the internal TST system. My thanks to both companies for their support in my efforts to help educate the RV community about tires.
The question is: How do I make the test both fair and useful? For the pressure test, I decided to eliminate as many variables as possible. So I got all 12 readings from the same air chamber at the same time and compared them all against my personal digital hand gauge that I have checked against an ISO certified laboratory gauge.
Note: My hand gauge reads to 0.5 psi, which is way more precise than anyone needs for checking tires in normal highway use.
Here is the test fixture I made.
Six internal sensors are inside the tube and six external sensors are on the outside. In addition, there is a pressure regulator, a safety pop-off, and a reference dial gauge that allowed a visual check. Also, there is the test port for my handheld digital gauge that I have confirmed accurate to +/- 0.5 psi against an ISO Certified master gauge.
Results of comparison test of TPMS
Here are the results of my comparison test. The target pressure is 80.0 psi, as reported by my handheld digital gauge.
- Set A: 1 reading of 78 psi, 5 readings of 79 psi
- Set B: 2 readings of 78 psi, 2 readings of 79 psi, and 2 readings of 80 psi
I also recorded the temperature:
- Set A: 4 readings of 66 F, one each of 64 F and 68F
- Set B: 4 readings of 69 F, and 2 readings of 68 F
I have recorded the internal vs. external pressure difference in reported running pressure with the following results:
I do not consider any of the differences in the readings of the internal vs. external sensors pressure to be significant or meaningful for a TPMS. At ambient, I observed 3 to 5 psi differences when all 12 sensors were measuring an identical pressure, which is not too far off the claimed accuracy for TPMS.
BUT …
The problem with TPMS temperature readings is that they are not able to read the hot spot of a tire. Here is a graphic representation of tire temperature differences.
Heat can kill a tire
Heat can kill a tire, but tires simply do not fail based on their average temperature. However, they can fail if a single spot exceeds the ability of the rubber to maintain its strength. With this large temperature spread and the fact that the hottest spot in a radial tire is about 3/8″ to 1/2″ away from the air chamber and the internal TPM sensor is reading the average air temperature in the air chamber, I would ask why anyone would believe that a TPMS “High Temperature” reading is a reliable method of warning of an impending tire failure.
I started this post by saying I did not think that a TPMS or an IR gun were reliable tools for predicting an impending tire failure based on the reported temperature. I will cover IR guns next week.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s 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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This is probably mostly a case of, “if there is data it must mean something”. I don’t use temp to monitor for a failing tire, mainly because things like solar heating can already make a difference. Put another way the subtle change as a tire delaminates is below the noise floor of my data.
I use the temps as a logic test when I first roll out . It’s a closed container and it’s close enough that the ideal gas law (pvnrt) should apply. If a tire is a few psi low and has a lower temp … All good. If a tire psi is high, with higher temp, all good. It proves to me the TPMS is working, the laws of physics still apply, and I don’t fuss with pressures with my pencil gauge if the pressure/temp changes correlate within reason.
Our VESAFE TPMS monitor, with screw-on sensors, worked well last fall when a left rear bearing failed and triggered the 98° high temperature alarm. We were able to pull over in town and make repairs. I carry extra bearings and races, but needed to replace the drum and backing plate as well. I believe the TPMS system saved a potentially ruined axle, physical separation of wheel from axle, and possibly being stranded somewhere inconvenient. I do carry an infrared gun and use it when ambient air temperatures are above 90ﾟ.
You say in this article “So your TPMS can be used to provide warning of potential problems with wheel bearings, brakes and other metal mechanical parts.” Do you think a wheel bearing that is starting to go bad due to inadequate lubrication will be detected by the TPMS early enough to save it?
Depends on how far above ambient the metal parts get. There has to be enough heat generation to get into the 150Fto 180F and above range I would think. I have not tried to run a test by pulling a trailer with bearings with no lube so can only guess.
We use the ‘screw on’ TPMS and had a wheel bearing failure last fall. It did not provide any warning, so I would say it is not useful for detecting bearing failures. At another time it did provide a low-pressure warning on one wheel so it paid for itself in that situation.
If you’re using external TPMS that would make sense, as Roger said rubber is a great insulator so if you had rubber valve stems the TPMS would’ve been insulated from the heat of the wheel transferring heat from the bearing. Had it been internal mounted in contact with the wheel it may have shown a higher temperature than the other wheels. That’s really the only way to know if there may be a problem is comparing one reading to the others. If one reading is showing 30-60 degrees warmer than the others I would be checking why. If the pressures are the same then it probably is a mechanical reason.
That is the perfect use case for an IR temperature “gun”. Get in the habit of “shooting” the wheel hubs as soon as you stop because they cool down fast. You’ll soon learn what is normal for your RV. If one hub is noticeably hotter than the others, and hotter than it has been, you may have a bearing problem or a dragging brake.
IR guns, like a TPMS, are more valuable as a trend indicator than as an absolute number-checker. If everything is behaving the same as it has in the past, and everything is more or less similar (accounting for the direct sun) you’re probably OK. It’s when you get an outlier that you should investigate further.
So, at what temperature does the rubber start to fail?
If you mean Fail in a few minutes it would be in the 210F and higher. People seldom think to long-term heat ( 150F+) affects the rubber over the longer-term by weakening the flex of the critical components. Science indicates that for each increase in temperature of 18F the “aging rate” (rate at which the rubber loses strength) doubles. Here is a post from my tire blog that provides more details. Here are a number of posts on tire aging with further links