[ammolab]

And if you live at 7000 ft like me?....the inside the tire sensor can’t compensate for the reduced air pressure surrounding the tire. My tires at a sensor reading of 32psi are firmer than if I was in Houston at the same sensor reading.

 

[Panzermann]

I believe ‘cold’ means before riding, as the tyre temperature rises substantially above ambient when in use. As you say, @Dougl, ambient temperature is not of great significance.

 

[Dougl]

And if you live at 7000 ft like me?....the inside the tire sensor can’t compensate for the reduced air pressure surrounding the tire. My tires at a sensor reading of 32psi are firmer than if I was in Houston at the same sensor reading.

The tire is presumably rigid so it shouldn’t expand like a weather balloon with increasing altitude and the pressure inside won’t change. However your tire gauge has a diaphragm or transducer which works against ambient pressure. It reads higher at higher altitude. However, the TPMS should read absolute pressure. As I live at 5000 ft, this would explain why the TPMS reads 2-3 lbs lower than the gauge when the tires are “cold” and the ambient temperature is about 20C.

 

[Dr. Strangelove]

@Dougi
Damn it's been a long time but what you reference, sorta, is the Ideal gas law. What this law states is that a given volume of gas (the air in your tires) will increase as temperature goes up. The volume does not stay the same. It increases slightly, but since the tire is a confined space with limited expansion, as the temp increases so does the pressure. The number of molecules in the tire remains constant, but they spread out more, and bounce around more exerting a higher pressure.

@r0ckrat
I don't disagree with you, but I think this is a fuzzy area as you may not reach optimal temp on your ride. I know tires get "hotter," but how much? (IN the Patent you sent there is a temp sensor, but does that exist in ours?). I am just throwing this out there because this topic has come up ever since TPMs on BMW existed...like over 10 years? and often what the conventional wisdom is, is to say screw the tpm, I am just going to use my XYZ digital gauge that Joe at XYZ mechanic told me was the best...or something like that. Admittedly TOTALLY unscientific and probably approaching Luddite. TBC, that is NOT me. Like y'all, I think about this also (not a lot because I have a life, but ... )
What I do is I check the temperature of the garage. If it is nearly 68F I use my "trusty pencil gauge" and fill to 36/42 and my OBD will show close to 36/42 and sometimes right on the mark.
If it is warmer, the usual circumstance (New Orleans) I will inflate to read a bit higher, say 38ish/43ish and my OBD usually shows pretty close to 36/42. The garage rarely gets colder than 64 and at that I'll go with ~~~35/40+. Works for me, but that's a result of a lot of trial and error. I always check the OBD TPM when I am just starting out to note how close my gauge is to the OBD. And yes, when cold I don't do any acrobatics until the engine temp is normal

@Panzermann
I notice some minor changes in the readings as I am underway, but never really paid attention to the changes, they are slight, but to the actual number. If it's close to fine, I switch back to RANGE.
I think PSI may be more accurate than bar in that there are nearly 15 psi_s in 1 bar. The bar readout is in tenths so it is a grosser number, so between 2.0 and 3.0 bar there are 15 psi. IOW it is possible for small changes in PSI to go un-reflected in your bar readout. The differences would be slight

BTW, this is the BEST discussion of this topic I have encountered on any BMW forum and it comes up at least once a year.
And I guess the advice to the most anal of us is to inflate the tires to 36/42 as displayed on the OBD.

 

[r0ckrat]

@Dr. Strangelove , from what O have read, here, in the user guide, and online, our TPMS does have a temp sensor of some sort.

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[Dr. Strangelove]

I watched the tire pressures with recent rides and as the tires heat up the reading does change but only a bit, maybe 1-2 psi, nothing drastic. 36 may go to 38 and 42 to 43, 44. In the winter here where I store the bike--and inflate the tires-- is in the mid 60s, sometimes 68 +/-
The closer the ambient temp is to 68, the more correspondence I see on the TPM.

 

[Panzermann]

When tyre, bike and car manufacturers say ‘cold’ @Dougl, they are not referring to ambient temperature, they mean with the tyre not having been driven to any significant extent, or hasn’t been sitting in the sun. A tyre that has been driven (particularly at high speed) will heat up well above normal summer ambient temperatures. 20 deg C just happens to be the standard to which Shrader calibrate their sensors, rather than a target.
The following is an extract from my car manual (also German, but in a different dialect of Germlish to the BMW Motorrad manuals).

 

[Dougl]

When tyre, bike and car manufacturers say ‘cold’ @Dougl, they are not referring to ambient temperature, they mean with the tyre not having been driven to any significant extent, or hasn’t been sitting in the sun. A tyre that has been driven (particularly at high speed) will heat up well above normal summer ambient temperatures. 20 deg C just happens to be the standard to which Shrader calibrate their sensors, rather than a target.
The following is an extract from my car manual (also German, but in a different dialect of Germlish to the BMW Motorrad manuals).

View attachment 109720

That’s why I put cold in quotes. Today, my tires are actually cold, as the temp is 7C. The sun is out though, so I’m going for a ride.

 

[Dougl]

Not much going on so here goes again. Is there any bike, auto or tire manufacturer other than BMW Motorrad that tells you to correct your cold tire pressure to 20C (STP)? My car tire vendors (Big O, Discount Tire) never heard of this. I had been using the indicated temperature corrected pressure to inflate my cold tires but the last time I got the bike back from a service, they had deflated the tires and set the pressure with their gauge to 36F/42R which read lower than the temperature corrected pressure on the TFT. Why doesn’t BMW simply output the actual pressure and allow us to worry about temperature? Do tire manufacturers design their tires for use at 20C? If so, I can see BMW’s point. If not, wtf?

 

[r0ckrat]

Do tire manufacturers design their tires for use at 20C?

No, but they do design them with a baseline starting temp, and know fairly accurately how much pressure increase to see when in "normal" use. So if you set them to the temperature corrected "baseline" then they can fairly accurately tell you what the internal pressure will be at any given internal air temperature. If you set the pressure to the 20C "cold" pressure on a hot (25 or 30C) day, the internal pressure will be lower than what the tire manufacturer designed it to be used at.

 

[Panzermann]

Any manufacturer using Schrader sensors will be using temperature compensation - the sensors do the work, I’d reckon.

As discussed earlier, tyre and bike manufacturers specify recommended pressures ‘cold’ so having the sensors report compensated pressures makes sense. Otherwise the displayed pressure readout wouldn’t make sense - the rider would have to know how much to discount the pressure reading to a cold reading to know if inflation was at spec.

 

[Dougl]

I set it 1.5 lbs higher than recommended and the bike reads more than 1lb low. It wants more, and more, and it makes me crazy. I compartmentalize it though.

 

[r0ckrat]

So, the instructions in the manual state to ride until the TPMS turns on, then compare that value to a gauge (any gauge, it doesn't have to be especially accurate other than in reading a change in pressure). Then add or remove the psi/bar difference. For example:

• Rear TPMS reads 2 psi high at 44 psi
• Your gauge reads 48
• Remove 2 psi so gauge reads 46
• Next time the TPMS turns on, it should now read the correct 42 psi

Dougl, in your case, since the bike now says 1 psi low, increase pressure from current gauge reading to 1 psi higher on the gauge, and the TPMS should be "happy". If that's 2.5 psi higher than the "recommended" tire pressure, it's because the internal air temperature of the tire is higher than the 20C baseline temperature.

 

[Dougl]

I’m skeptical about any 20C “baseline”. I understand what BMW wants me to do but I’m not sold on it.

 

[r0ckrat]

Fine. Ignore it. But the math works out, and has been proven.

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[Dougl]

Fine. Ignore it. But the math works out, and has been proven.

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Temperatures in the US range from over 100F to -50F. How can there be a baseline?

 

[Lee F]

I have electric pump that I set to 37 or even 37.5 to get tire pressure to show 35-36.
At gas stations is same story, the value indicated at pump is not what bike's pressure monitor shows.

I would not trust the readout on alot of the electic pumps or the readout on a gas station air pump.
It's better to use a good gauge.

 

[r0ckrat]

Temperatures in the US range from over 100F to -50F. How can there be a baseline?

Replace "baseline" with "control" for the math.

At 65 f if you put 40 psi in a box, you know it"s always going to be 40 psi at 65 f. Math also knows what psi it will become at 80 f or 100 f or 20 f. That math has been proven and the calculations are built into the sensor.

Your tire pressure gauge does not know the temperature inside the box, it only tells you what the psi at the valve is. So if it's 80 f inside the box, and you fill to the stated recomended 65 f pressure for the box, then you aren't putting in enough air.

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[Dougl]

I’ve been an atmospheric chemist/physicist for 40 yrs so I have some familiarity with the ideal gas law. What I don’t understand is if the tire manufacturer tells me to inflate an unridden (cold) tire to 36 psi, my bike is telling me to inflate it to something else.

 

[r0ckrat]

"unridden" and "cold" are not the same thing. "Cold" is a very loose term, and means different things to different people. I didn't know that you were arguing the difference between tire manufacturer's recommended pressure setting method and BMW's recommended "adjusted" pressure.

If you really want to understand, you are going to have to contact the two manufacturers - the tire and the bike - and ask them how they came to their decisions on what the pressure should be.

Me? I'm going to use the bike's recommendation, as it's worked great for me so far and allows me to be able to use gas station pumps on a "ridden" tire under any outside temperature.