TPMS (Temperature Considerations)

My TPMS quandary (nothing to do with pressure)... 

The questions are ... 

(i) Of what is the temperature being read by the sensor? and 

(ii) How should we interpret the reading and to what are corrections required?

Most Tire Pressure Monitoring Systems (TPMS) provide temperature readings in addition to air pressure readings. After using a TPMS for half a year on my 2019 Yamaha Tracer 900, I've decided that I find the temperature readings odd and have started to wonder what exactly is being read for its temperature and how such information should be applied in maintaining my motorcycle.

By way of background, the TPMS model I use is the wireless, motorcycle-specific CAREUD M3 that I purchased on Amazon for around $50. I have no complaints or issues whatsoever with its operation; I just decided that I don't understand the temperature monitoring feature. 

As can be seen in the photo above of my Trace barr, the TPMS Monitor is mounted on the handlebar. There is an internal rechargeable battery in the monitor that is charged on a USB Charging Cable. The Sensor, as shown below, is mounted on the wheel valve stem. The sensors are powered by replaceable button batteries and are secured to the valve stem with a security nut; tightening tool included in kit.

What does the research say?

Research on this topic as it specifically relates to TPMS consistently seems to point to the temperature of a tire as being an extremely important feature to measure; the hotter the "tire," the more likely that it will fail. They actually don't come out and say it, though. It's just inferred. I don't disagree with that anyway. I think the issue of heat and tires just goes without saying. In fact, I found this quote about tire temperatures in the Bendix Blog entitled, Why Temperatures Matter!

"Most experts consider 195 degrees Fahrenheit as the "Line in the Sand" when it comes to tire temperature. Beyond that point, the temperature will start impacting the tire life. At 250 degrees, a tire will start to lose its structural strength and experience tread reversion."

My overall impression on the TPMS-specific information articles is that the articles are asking me to "assume" that the TPMS temperature readings are those of the tire. I don't think that's the case and, if not, back to my lead-in questions!

What does my TPMS manual say about the temperature?

Well, here's what it sez ...

No questions here. The CAREUD TPMS owner's manual explicitly states that the temperature reading is that of the tire.

Essentially, the CAREUD model is designed to trigger a warning when the "temperature of the [front] tire" reaches a presumed dangerous level. The CAREUD temperature trigger level is preset to 66 degrees C (150 degrees F); I left it at this level. For the record, both ends of the motorcycle are subject to continuous measurement and displayed for temperature and the temp trigger level is separately adjustable on either end.

Let's take a closer look at that "tire temperature" representation and put it to the test.

A closer look into a TPMS Temperature Reading?

The initial odd and confusing observation that brought me to these questions was that the temperature readings I was seeing for hot, active tires was very disparate between the front and the rear, and quite significantly. The temperature for the front wheel would actually hover around the outdoors ambient temperature after long, fast riding while the rear temperature would reflect the "expected" heat increase from friction with the road surface. I noticed other motorcyclist TPMS-users commenting about this very same observation.

To better illustrate, I took and documented readings from my TPMS on a recent ride that included a good deal of highway travel. Here's the info and comparo:

A couple notes:

• Is the temperature reading on the monitor accurate? Well, I've tested it to and it's accurate to ambient temps several times during this process. For heated temps, I took a cold (46F) sensor off the valve, sat it on a flat thermometer and brought it up to 80F with my shop hair drier. When reinstalled, the monitor reading tied out with the thermometer.
• Tread reversion being the cause of overheated tires, the hot tire temperature readings with the infrared thermometer were take on the rubber tread at several locations around the tire and averaged. However, readings were all in-line.

Results....

First and foremost, the "seemingly" low front temperature reading on the monitor compared to the much higher rear temperature is present in this test. The "hot" front temperature is exactly at the ambient air temperature. 

Ambient Temperature 68F

Monitor Cold Front Temperature 66F

Monitor Hot Front Temperature 68F

The second test result, established by use of an electronic infrared thermometer, clearly illustrates that the TPMS temperature reading simply can't be the actual tire temperature. The monitor and the thermometer are just two very different numbers.

Monitor Temperature, Front 68F. Infrared Thermometer, Front 85F.

Monitor Temperature, Rear 87F. Infrared Thermometer, Rear 110F.

Based on this information, I think it's reasonable to draw a couple detail conclusions at this point.

As it relates to the disparate temperature readings being experienced between the front and the rear temps...

... the only sensible answer is that the front tire/wheel and TPMS Sensor are right out there in the open taking the full brunt force of air as the motorcycle steams forward. It's like blowing on a spoon of hot soup. The rear tire does not experience the same brunt force because it is largely tucked back behind the frame, the engine and the exhaust. Further, air blowing through the frame and making it to the rear wheel is actually being heated by the hot engine and exhaust. Overall, on this point, the locations of the wheels relative to the motorcycle design is the natural explanation for why the front temperature reading is at or near ambient temperature and the rear is significantly higher.

As it relates to the "what" is having it's temperature read, well ...

... it's obvious that the Monitor isn't displaying the tire temperature, or the correct tire temperature. This conclusion was drawn by simply comparing the readings of the handheld infrared thermometer obtained on the actual hot tire carcass to the monitor's readings. The temps between the monitor and actual carcass are far too disparate; 25% too low on the front and 26% too low on the rear. What good it that? 

Honestly, such a conclusion wasn't necessarily hard to deduce since, as can be seen in the photo, the sensor isn't even connected to or even close to the tire. Rather, it's sitting up away from the tire and wheel on the valve stem. As such, the sensor is subject to all kinds of temp-influences ranging from the brunt affect of the tire rolling into the wind (front) and the heated air flowing off the engine and exhaust (rear).

I'll go as far as saying that radiant heat rising off the road has more influence on the sensor's temperature than does the tire itself.

So, on question number (i) in the lead-in , which was ... 

... Of "what" is the temperature being read by the sensor.

The "what" is nothing more than the sensor itself, and the sensors (front and rear) are obviously being significantly influenced by lots of "external" factors other than tire temp such as the ambient temperature (front), radiating road heat and the heat from the engine and exhaust (rear). That's it!

Perhaps, hopes that the actual inner tire air that works its way up into the valve stem influences the sensor enough to replicate tire temp. Unfortunately, those hopes were dashed by simply testing the temperatures as I've done.

On question number (ii) in the lead-in, which was ...

... How should we interpret the readings and to what are corrective actions required?

Well, there's really no need to interpret the readings, because the temperature readings of the sensors themselves serve no real purpose. Saying the sensor reads tire temperature is like putting a thermometer in your tank bag and saying that it's reading your body temp. Bottom line, it appears that the intention of the temp readings were to provide early warning of tire failure from excessive heat. It doesn't appear as though that's going to happen with the valve mounted TPMS sensor. 

Now, having said the temperature readings serve no purpose, they are there, so how about we all agree that, in exercising an abundance of caution, if the temperature warning is triggered, we stop and have a look at what's going on?

And, the Blue Knob Ventures conclusion is ...

I like having a TPMS to keep an eye on my air pressure. Cutting out those few minutes checking air pressure with a traditional pressure gauge has great value to me. However, the temperature display on a TPMS Pressure/Temperature monitoring system (if as designed like my CAREUD TPMS) simply has no relevance or value to the motorcycle or its operation or its maintenance ... whatsoever. Sorry, but it's useless information. 😊 

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Other TPMS issues:

Is a TPMS really useful on a dirt bike or dual sport? My views in this BLOG POST.