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Discussion Starter · #1 · (Edited)
I just installed some $20 Chinese TPMS sensors in my tires on my 2008 R1200GSA after adding the RDC module, and I was dismayed to find they read about 5 psi low. But after more thought, I realized, this actually makes sense.

I live at an elevation of 7,600 feet (2316 m). At this elevation, the atmospheric pressure is actually 3.7 psi lower than at sea level. A tire pressure gauge actually measures psig (psi "gauge"), which is relative to the atmosphere. The way I see it, a TPMS sensor is inside the tire, so it has to be made to measure absolute pressure in psi, not relative pressure in psig. There's no way a pressure sensor that is wholly contained inside the tire can physically make a relative measurement like an external gauge can.

See page 9: https://static.nhtsa.gov/odi/tsbs/2015/MC-10192638-9999.pdf

In addition, the current temperature in my garage is about 80 F (300 Kelvin), although cold inflation pressure is specified at 69 F (293.7 Kelvin), which is a difference of 2.1%. This equates to a pressure increase of 0.67 psi for a 32 psi (nominal) tire. However, TPMS is supposed to be temperature compensated, so it would subtract the increase. Adding these two numbers together gives 0.67 + 3.7 = 4.4 psi. So according to physics, you would actually expect the gauge to read 4.4 psi higher than the TPMS sensor in the wheel, which is pretty damn close to the ~5 psi I measured.

Alright, so with that out of the way, this raises a couple of questions. First, if you live at high altitude, is it OK to have your tires inflated 3.7 psi higher (e.g. 35.7 psig instead of 32 psig). Second, at what pressure loss will the BMW RDC system throw a warning? If it throws a warning with >3 psi loss, then RDC will not work properly at such high altitudes (assuming a tire should be inflated to psig, not psi).
 

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The standalone Chinese TPMS sensors I use on my 2007 GS is accurate or at least matches mt Slime digital pressure gauge within 9/10th of a PSI. The Slime gauge goes to 10th's whereas the TPMS display is whole numbers. So 36.9 PSI on the slime gauge would read 36 PSI on the TPMS. Good enough for me.

As a note the TPMS does not make elevation corrections i simple reads pressure.

BMW TPMS alarms when a 4% pressure drop is noticed.
 

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I just installed some $20 Chinese TPMS sensors in my tires on my 2008 R1200GSA after adding the RDC module, and I was dismayed to find they read about 5 psi low. But after more thought, I realized, this actually makes sense.

I live at an elevation of 7,600 feet (2316 m). At this elevation, the atmospheric pressure is actually 3.7 psi lower than at sea level. A tire pressure gauge actually measures psig (psi "gauge"), which is relative to the atmosphere. The way I see it, a TPMS sensor is inside the tire, so it has to be made to measure absolute pressure in psi, not relative pressure in psig. There's no way a pressure sensor that is wholly contained inside the tire can physically make a relative measurement like an external gauge can.

See page 9: https://static.nhtsa.gov/odi/tsbs/2015/MC-10192638-9999.pdf

In addition, the current temperature in my garage is about 80 F (300 Kelvin), although cold inflation pressure is specified at 69 F (293.7 Kelvin), which is a difference of 2.1%. This equates to a pressure increase of 0.67 psi for a 32 psi (nominal) tire. However, TPMS is supposed to be temperature compensated, so it would subtract the increase. Adding these two numbers together gives 0.67 + 3.7 = 4.4 psi. So according to physics, you would actually expect the gauge to read 4.4 psi higher than the TPMS sensor in the wheel, which is pretty damn close to the ~5 psi I measured.

Alright, so with that out of the way, this raises a couple of questions. First, if you live at high altitude, is it OK to have your tires inflated 3.7 psi higher (e.g. 35.7 psig instead of 32 psig). Second, at what pressure loss will the BMW RDC system throw a warning? If it throws a warning with >3 psi loss, then RDC will not work properly at such high altitudes (assuming a tire should be inflated to psig, not psi).
Never seen this happen. We routinely tour the Himalayas in the Ladakh region of India, We go up to 19.000 feet and routinely are at 14,000 plus feet. The TPMS has been rock solid at 36/42.
Sometimes it fluctuates 35/41 and then back to 36/42.
Have not seen 5 psi off. Maybe the sensors are off?
 

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Discussion Starter · #4 ·
Yeah of course it doesn't
Never seen this happen. We routinely tour the Himalayas in the Ladakh region of India, We go up to 19.000 feet and routinely are at 14,000 plus feet. The TPMS has been rock solid at 36/42.
Sometimes it fluctuates 35/41 and then back to 36/42.
Have not seen 5 psi off. Maybe the sensors are off?
Dont think so. This is basic physics. Read the attached publication from Nissan that is posted on the NHTSA website. It mentions that elevation tends to be counteracted by a drop in temperature, so you often dont see the effect if you dont fill up your tires at elevation.
 

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Yeah of course it doesn't


Dont think so. This is basic physics. Read the attached publication from Nissan that is posted on the NHTSA website. It mentions that elevation tends to be counteracted by a drop in temperature, so you often dont see the effect if you dont fill up your tires at elevation.
I agree but then why arent the TPMS doing that on our bikes? And its not one GS....we are a group of 7 GS bikes.
 

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Im not denying what you are saying but have not had anyone complain about this much of a drop in the BMW bikes.
Maybe others living at elevation can comment.
 

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After reading your post and looking for info. about the TPMS sensors, I conclude that they dont have acces to the atmospheric pressure, so they should be calibrated with an 'absolute' value, probably 0 psi at sea level.

In altitude, their reading will probably not be correct, a good quality gauge should be used to insure proper inflation (gauges are reading pressure relatively to the atmospheric pressure, the inflation should not be compensated for the altitude). The reading by the TPMS will be lower than the relative pressure. In altitude, the air pressure is lower outside the tire, the pressure inside should be also lower.

I'm not recommending using a corrected value on the TPMS to inflate you tires, I suggest getting a good gauge and use the recommended pressure.
 

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My sensors died years ago. BMW charges too much for what I consider info that isn’t really needed. Develop a feel for your tires and carry a tire pressure gauge that you can buy at any auto part store. You’re getting way too wrapped around the axle on this - KISS
 

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Discussion Starter · #12 ·
My sensors died years ago. BMW charges too much for what I consider info that isn’t really needed. Develop a feel for your tires and carry a tire pressure gauge that you can buy at any auto part store. You’re getting way too wrapped around the axle on this - KISS
Some people enjoy working on mechanical and electrical things. There is a reason TPMS is a federal requirement for cars, and actually more critical for motorcycles. Also, TPMS is simple, simpler than measuring your pressure with a gauge all of the time.

But nobody is forcing you to have TPMS on your motorcycle. I choose to have it, however. Which is why BMW sells it.
 
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