TPMS Accuracy– A variation on the Theme

[smswy]

Yes, I read most all of the other stuff via Search and found the jockeying back and forth for dominance of opinion entertaining. And yes, I will do what I think best. But you might enjoy the story, or not.

Colonel, I want the truth! You can’t handle the truth. Yes, I can.

Up to my 30,000 miles service yesterday in Tucson, on my less than 2 years old 2023 R1250 GS, I have always tried to set front and rear tire pressure at 36F/42R (without making much fuss about 1deg psi adjustment for +/- 10deg changes in air temperature). And I have tended to manage this using the TFT pressure readout after being underway as my guide and a check on the 36F/42R. And if each tire increases 0.5-1.5 psi with heating I don’t worry about it, especially since 75% of the time I am carrying a 70lb load of camping/travel gear. And, on my last set of Dunlop Trailmax Mission tires I got 20,500 miles from them and they were not quite to the wear bar before I put new ones on. So at least in terms of tire longevity my method seemed OK.

The day before I left Prescott Valley, AZ to ride down to Tucson for the small service I checked my tire pressure and the front was only 35.5 psi and rear 41.5 psi based on TFT (and temps were over 85deg). I use a portable Dewalt compressor and I have to screw the end onto the valve stem. It is always a pain to unscrew and I tend to lose at least 1-2 psi during the process, so I usually set the Dewalt (which may be inaccurate itself) at 39 psi. I did that this time and after I rode the bike up and around the streets for some time the TFT showed 37.3 psi for the front and that it was about 0.2 psi over spec (as a function of the temperature change I guess). I thought, good enough. I added a little air to the rear and the TFT was about 42.8 psi.

The Variation on the Theme:
After service between 9 and 10 am in Tucson (hotter and greater air pressure) the assistant service manager went over the job and stated they adjusted the tire pressure as well. After heading back towards Oro Valley I looked at the TFT and the front tire was 33.9 psi and the rear was 40.1 psi. Seemed 2 psi less than the 36F/42R recommendation. I thought maybe they did this since the Dunlop TMM tires have a very stiff sidewall. When I stopped for fuel in Oro Valley I called the service department and left a message, since they did not answer, about the tire pressures. I asked if they’d call back and leave a voice message since I would be riding. They did and the assistant service manager said “Your TFT systems are pretty much there just to give you a notification for sudden loss of pressure. If you are using your TFT to gauge where your actual psi is at it’ll be inaccurate every time. It’s always hand check with your Dewalt or another little hand device, but never go off your TFT”.

And here I thought German engineering is precise. And if the assistant service manager is actually correct, it begs the question why then have the Tire Pressure screen (3rd to the right past the My Vehicle) that shows the Spec at a given temperature and how much below or above spec the tire pressure is.

 

[m_stock10506]

I’m not sure if you have a question buried in your somewhat entertaining post. I run external TPMS sensors on all three of my bikes. I have found them to be fairly accurate and with repeatable results. They don’t do temperature compensation, so I just watch the display for a 3-5 psi increase once I have gone about twenty miles. If you set the pressure initially and the tpms agrees with the initial setting then the display is either accurate or you can determine that it varies by a small amount either high or low. That comment by the tech is ridiculous.

 

[smswy]

I’m not sure if you have a question buried in your somewhat entertaining post. I run external TPMS sensors on all three of my bikes. I have found them to be fairly accurate and with repeatable results. They don’t do temperature compensation, so I just watch the display for a 3-5 psi increase once I have gone about twenty miles. If you set the pressure initially and the tpms agrees with the initial setting then the display is either accurate or you can determine that it varies by a small amount either high or low. That comment by the tech is ridiculous.

Thanks for your methodology. There was no buried question per se. The post is mostly a follow-on from other TPMS discussion by various “experts”. But I thought it interesting that the service department seemed to have lowered my air pressure. And that they more of less trashed BMW TPMS. Although I will let it rest, I wonder if I contacted BMW and shared the service department assistant manager’s (not the actual mechanic tech) response with them, how their tech group might respond.

 

[lee]

But I thought it interesting that the service department seemed to have lowered my air pressure. And that they more of less trashed BMW TPMS.

It's interesting the assistant service manager forgot or did not know the recommended tire pressure is compensated to 68 deg F.
I actually like this set up and both of our bike TFT displays agree within 1 PSI of my tire gauge when the air temp is near 70 F.
If the temp in my garage is 60 or 80, I adjust the pressure up or down by 1 PSI.
We had this setup on our 2016 bikes and again on our 2022 bikes.

 

[MCMXCIVRS]

I don't fret about a variance of a couple psi from the recommended pressures on the TPS readings. I set my tires pressures using a digital gauge and usually aim for about a half to one psi over the desired number which allows a bit more time for the natural loss of pressure that always happens. I re-inflate when they drop more than a couple psi below the target. Whatever the display reads is just a guideline that the tires are within an acceptable safe range and of course the warning feature in case of loss of pressure.

All this sciency and mathy stuff of calculating temperature and altitude effects on the tire pressure is too much work for me and takes away from just getting out and having a great ride.

 

[omega man]

I don't fret about a variance of a couple psi from the recommended pressures on the TPS readings. I set my tires pressures using a digital gauge and usually aim for about a half to one psi over the desired number which allows a bit more time for the natural loss of pressure that always happens. I re-inflate when they drop more than a couple psi below the target. Whatever the display reads is just a guideline that the tires are within an acceptable safe range and of course the warning feature in case of loss of pressure.

All this sciency and mathy stuff of calculating temperature and altitude effects on the tire pressure is too much work for me and takes away from just getting out and having a great ride.

Me Too!
OM

 

[patm]

If you were to install a non compensated TPMS on your bike, you would see the pressure rise when the tire warms up. This might alarm you and you may reduce the pressure to "fix" the issue. By using a compensation algorithm BMW protects you from just that.
In reality, with the tft you don't know what the exact pressure is when the tire warms up.
The increased pressure isn't a bad thing, it's expected. Tire manufacturers specs take that into account. That is why they tell you to check the tire pressure on cold tires.

Check your tire pressure before the ride, use whatever measurement tool you want, a guage or the TFT.

 

[lee]

Check your tire pressure before the ride, use whatever measurement tool you want, a guage or the TFT.

When leaving on a trip, look at the TFT tire pressures as you drive down your street and remember those numbers.
If you see the pressure drop 2 or 3 PSI on the trip, add air.
On our bikes we can go 2.5 to 3 weeks before the pressure drops 2 PSI.

 

[smswy]

I don't fret about a variance of a couple psi from the recommended pressures on the TPS readings. I set my tires pressures using a digital gauge and usually aim for about a half to one psi over the desired number which allows a bit more time for the natural loss of pressure that always happens. I re-inflate when they drop more than a couple psi below the target. Whatever the display reads is just a guideline that the tires are within an acceptable safe range and of course the warning feature in case of loss of pressure.

All this sciency and mathy stuff of calculating temperature and altitude effects on the tire pressure is too much work for me and takes away from just getting out and having a great ride.

I do agree. I like to check before I head out and don’t worry about it so much on the trip, especially if the pressure as shown on the TFT does not vary from 36F/42R by more than +/- 1.5 deg. I do notice that the rear pressure tends to creep a little higher than the front, perhaps due to heating when carrying me and a 70lbs load.

 

[lee]

One thing I noticed, both of our dash readings on our R1200RSs would change a little the first 5 or 10 miles.
On our R1250RSs TFT there is very little is any change.