Tyres, Brakes and Suspension » Trying to understand TPMS behavior.

I'm hoping someone else seen this and can explain why the TPMS acts like it does.

When I start the bike and take off, I get a pressure reading after a few hundred feet as expected. However, from there the pressure given never changes and I often ride with the pressure showing on the top of the TFT. The value never changes until I stop and shut off the bike. When I restart the bike even after a short period, I'll typically get a somewhat different value. Like I might have been getting 42.5 for the rear and that will drop to by up to 1psi and sometimes even go up.

So first, I don't believe that the pressure doesn't go up as I ride and the tires warm up. Next, I find it hard to believe the pressure will change that much after stopping for 2 minutes. It seems like BMW is manipulating the data. Any thoughts?

Our 1250 TPM readouts act a little different compared to the 1200s we used to own.
On the 1250 I rarely see a change in the pressure while riding. On the 1200 the pressure would increase slightly after riding 5 miles or so, then stay the same.

On the 1250 most of the time the pressure is the same after a fuel stop but a few times I have noticed a pressure change less than 1 PSI.
Both of our bikes act the same.

Another odd thing is the TPM reading shown on the Nav V is 0.5 PSI different than displayed on the TFT.

One more. When I first take off in the morning the TPM reading will show after a couple hundred feet and my speed is over 20 mph.
Once in awhile after a gas stop it will take 1/2 to 3/4 miles before the TPM will display.
Both bikes have done this since new. Hope the bikes did not get bad TPM batteries from the factory.

Zitat von Paul911 im Beitrag #1

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I'm hoping someone else seen this and can explain why the TPMS acts like it does.

When I start the bike and take off, I get a pressure reading after a few hundred feet as expected. However, from there the pressure given never changes and I often ride with the pressure showing on the top of the TFT. The value never changes until I stop and shut off the bike. When I restart the bike even after a short period, I'll typically get a somewhat different value. Like I might have been getting 42.5 for the rear and that will drop to by up to 1psi and sometimes even go up.

So first, I don't believe that the pressure doesn't go up as I ride and the tires warm up. Next, I find it hard to believe the pressure will change that much after stopping for 2 minutes. It seems like BMW is manipulating the data. Any thoughts?

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Zitat von Mr. 36654 im Beitrag #4

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Zitat von Paul911 im Beitrag #1

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I'm hoping someone else seen this and can explain why the TPMS acts like it does.

When I start the bike and take off, I get a pressure reading after a few hundred feet as expected. However, from there the pressure given never changes and I often ride with the pressure showing on the top of the TFT. The value never changes until I stop and shut off the bike. When I restart the bike even after a short period, I'll typically get a somewhat different value. Like I might have been getting 42.5 for the rear and that will drop to by up to 1psi and sometimes even go up.

So first, I don't believe that the pressure doesn't go up as I ride and the tires warm up. Next, I find it hard to believe the pressure will change that much after stopping for 2 minutes. It seems like BMW is manipulating the data. Any thoughts?

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When dealing with an air measurement, it's common to reference the measurements to "Standard temperature & pressure" or STP. In my experience, that's been 15C (59F) and sea-level atmospheric pressure. So, the TPMS is sensing pressure and temperature and reporting the gage pressure (pressure above atmospheric) at standard conditions, (Pressure / Temp)_measured = (Pressure / Temp)_standard conditions.

As you note, the tire heating will increase the tire air temperature, but that heating isn't instantaneous. The heat is generated by the flexing and rubbing of the tire cords. Thus, some time is required for that energy to migrate thru the tire carcass and warm the captured tire air.

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Zitat von Lee im Beitrag #3

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One more. When I first take off in the morning the TPM reading will show after a couple hundred feet and my speed is over 20 mph.
Once in awhile after a gas stop it will take 1/2 to 3/4 miles before the TPM will display.
Both bikes have done this since new. Hope the bikes did not get bad TPM batteries from the factory.

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Zitat von wessie im Beitrag #2

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Yes, BMW manipulates the data. It is explained in the manual.

The ECU applies a conversion factor to compensate for the tyre warming as you ride. The TPMS has a temperature sensor as well as a pressure sensor so the pressure reading is adjusted to compensate for the difference in the tyre temperature and the "ambient control temperature". I can't remember the latter but it is fixed, probably 20C.

The apparent drop in pressure after parking is probably explained by differential cooling of the tyre and wheel, compared to the sensor. The tyre will likely be cooler than the sensor, which is somewhat insulated in comparison, leading to the ECU making a slight error in the calculation.

For me, I do not rely on the TPMS for any degree of accuracy. It is there to warn me of a major drop in pressure due to a puncture. Change the display to a less worrisome variable as it will still warn you if there is a large drop.

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Zitat von Paul911 im Beitrag #6

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Zitat von Lee im Beitrag #3

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One more. When I first take off in the morning the TPM reading will show after a couple hundred feet and my speed is over 20 mph.
Once in awhile after a gas stop it will take 1/2 to 3/4 miles before the TPM will display.
Both bikes have done this since new. Hope the bikes did not get bad TPM batteries from the factory.

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Mine does the same exact thing. In fact, I just got back from a short evening ride and stopped at the gas station. Sure enough, it took about .5 mile to register and the front registered first.

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Zitat von Lee im Beitrag #5

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Zitat von Mr. 36654 im Beitrag #4

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Zitat von Paul911 im Beitrag #1

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I'm hoping someone else seen this and can explain why the TPMS acts like it does.

When I start the bike and take off, I get a pressure reading after a few hundred feet as expected. However, from there the pressure given never changes and I often ride with the pressure showing on the top of the TFT. The value never changes until I stop and shut off the bike. When I restart the bike even after a short period, I'll typically get a somewhat different value. Like I might have been getting 42.5 for the rear and that will drop to by up to 1psi and sometimes even go up.

So first, I don't believe that the pressure doesn't go up as I ride and the tires warm up. Next, I find it hard to believe the pressure will change that much after stopping for 2 minutes. It seems like BMW is manipulating the data. Any thoughts?

---

When dealing with an air measurement, it's common to reference the measurements to "Standard temperature & pressure" or STP. In my experience, that's been 15C (59F) and sea-level atmospheric pressure. So, the TPMS is sensing pressure and temperature and reporting the gage pressure (pressure above atmospheric) at standard conditions, (Pressure / Temp)_measured = (Pressure / Temp)_standard conditions.

As you note, the tire heating will increase the tire air temperature, but that heating isn't instantaneous. The heat is generated by the flexing and rubbing of the tire cords. Thus, some time is required for that energy to migrate thru the tire carcass and warm the captured tire air.

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Since the TPM system is temperature compensated to 68 F you don't see as much change in the dash pressure reading when the tires heat up as you do in a car that does not have temperature compensation in the TPM system.

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Zitat von Mr. 36654 im Beitrag #9

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Lee,

I double-checked the manual and you're correct. BMW is using 20C (68F) as their standard condition.

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Zitat von Lee im Beitrag #10

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Zitat von Mr. 36654 im Beitrag #9

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Lee,

I double-checked the manual and you're correct. BMW is using 20C (68F) as their standard condition.

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At first I thought the temperature compensation thing was all smoke and mirrors but I have found it works for us.
I like I can ask Debbie what her tire pressure is over the intercom. She usually grumbles because she does not like to bother with dash info like the gas gauge or tire pressure but If I ask nice she'll tell me the tire pressure :)

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thanks to all for some great info. I would like to add that when I had a major unrepairable rear puncture I felt the familiar looseness in the rear (feeling like the wheel was coming off) then coasted to a stop and only then did the TPMS register a major pressure loss. I have had other punctures (slower) that were caught by the TPMS but for major ones you might feel them before the TPMS and need to ride accordingly as the warning on the TFT may well come too late to avoid a possible crash

IMG_2784.jpeg - Bild entfernt (keine Rechte)

Zitat von Mr. 36654 im Beitrag #11

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Zitat von Lee im Beitrag #10

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Zitat von Mr. 36654 im Beitrag #9

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Lee,

I double-checked the manual and you're correct. BMW is using 20C (68F) as their standard condition.

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At first I thought the temperature compensation thing was all smoke and mirrors but I have found it works for us.
I like I can ask Debbie what her tire pressure is over the intercom. She usually grumbles because she does not like to bother with dash info like the gas gauge or tire pressure but If I ask nice she'll tell me the tire pressure :)

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My bike came thru w/o TPMS, keyless, or the chrome muffler. I'm happy about the muffler, ambivalent about the keyless, but do have some regrets about the lack of the TPMS. As you say, it's more convenient and provides a safety advantage.

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Zitat von WillC im Beitrag #12

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thanks to all for some great info. I would like to add that when I had a major unrepairable rear puncture I felt the familiar looseness in the rear (feeling like the wheel was coming off) then coasted to a stop and only then did the TPMS register a major pressure loss. I have had other punctures (slower) that were caught by the TPMS but for major ones you might feel them before the TPMS and need to ride accordingly as the warning on the TFT may well come too late to avoid a possible crash

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You know I really wanted to find what did that and, since I had an hour before my friend got to me with a trailer, I searched half a mile back to see if I could find it and no luck. That was a newish tire too!