Anything wrong w/ this logic? Setting tire pressure using a warm tire with TPMS

If it was 88 the next day, instead of 58, and you checked your tire pressure expecting to see 44/38, would that still be right where you wanted it? Would you go ride with your tire pressures 2 pounds above the manufacturers recommendation, and the TPMS displaying 42/36, for a temperature 20 degrees below your current ambient?

If the treatise is sloppily written and admits to several different interpretations (including the correct one) then it also engenders sloppy application and invalid conclusions. If the author wants to change our expectation of what the TPMS will display, his examples go in the wrong direction. Implicit in the notion that the TPMS ought always show 42/36 is that the mass of air, or number of air molecules, be held constant across the ambient temperature range. Do we really want to believe that? That tire pressure should be independent of ambient air temperature, as long as a particular value is achieved at a particular temperature?

This is really a giant cluster.

duegatti said:

If it was 88 the next day, instead of 58, and you checked your tire pressure expecting to see 44/38, would that still be right where you wanted it? Would you go ride with your tire pressures 2 pounds above the manufacturers recommendation, and the TPMS displaying 42/36, for a temperature 20 degrees below your current ambient?

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YES, because the TPMS display described is temperature corrected and your guage is not. If that correction were perfect and you stayed at the same altitude and the ambient barometric pressure stayed the same then as the tire heated up or cooled down the display would stay the same. Since these constraints are pretty rigid the displays often vary a pound or so.

I tried.

I was totally shocked that IUPAC changed the definition of STP in 1982. Seems I was in the Marines at Ft.Meade and didn't get the memo. They didn't send me the memo to disregard STP and use 20 degrees C for the standard to set tire pressures. Funny I don't even know who "they" are and who put them in charge. Do you know? That means for all these years I have been riding motorcycles and driving cars with non-standard air pressures. How did I survive???


Just an FYI, ambient temp and tire temp are NOT the same. The TPMS uses tire temp to make corrections. The only time it is even close is when the tire is measured cold.

duegatti said:

If it was 88 the next day, instead of 58, and you checked your tire pressure expecting to see 44/38, would that still be right where you wanted it? Would you go ride with your tire pressures 2 pounds above the manufacturers recommendation, and the TPMS displaying 42/36, for a temperature 20 degrees below your current ambient?

If the treatise is sloppily written and admits to several different interpretations (including the correct one) then it also engenders sloppy application and invalid conclusions. If the author wants to change our expectation of what the TPMS will display, his examples go in the wrong direction. Implicit in the notion that the TPMS ought always show 42/36 is that the mass of air, or number of air molecules, be held constant across the ambient temperature range. Do we really want to believe that? That tire pressure should be independent of ambient air temperature, as long as a particular value is achieved at a particular temperature?

This is really a giant cluster.

Click to expand...

I think the MM article was very well written, but it's a tricky subject ultimately, so it's hard to follow in parts. I think a whole lot of people don't understand it. The good news is most of the time it's not going to matter a whole lot. You've made it this far w/o doing the temperature compensation when you adjust pressures, right?

Would you go ride with your tire pressures 2 pounds above the manufacturers recommendation?...
The question ignores what 'the manufacturer's recommendation' really is: it's 36/42 ONLY relative to a tire air temperature of 68F. Since your tire gauge does not appreciate the temperature/pressure relationship it's up to you to compensate to the tune of 1psi/10F change if you want to get it in the ballpark. They take it one step further in the Rider Manual when they tell you when you make your temperature compensation w/ a manual pressure gauge to use the TPM value as the reference point to adjust to! They apparently have way more faith in their TPM than they do our ability to set tire pressure 'correctly'!

Implicit in the notion that the TPMS ought always show 42/36 is that the mass of air, or number of air molecules, be held constant across the ambient temperature range. Do we really want to believe that? That tire pressure should be independent of ambient air temperature, as long as a particular value is achieved at a particular temperature?

Absolutely, the mass of air or number of molecules stays constant as long as none is added in or leaks out of your tire. In a confined non-expanding volume pressure increases w/ increased temperature and decreases with decreased temperature. Your TPM sensors send their raw pressure value to a logic circuit that does the temperature adjustment for you, thereby keeping displayed pressure essentially unchanged in your TPM display if it's working as it was intended. It's so good at it that when I left Bishop CA at 59F w/ a TPM displayed pressure of 36/42, many hours later at speeds of 75mph+ on red hot pavement thru ambient temperatures over 112F TPM still displayed 36/42. That's what it was more or less supposed to do, but wow, to be spot on was more than I would have expected. There was a small effect from elevation change but I have to think now it was offset by tire carcass expansion--no, don't go there!

From the rider manual:

chewbacca said:

That means for all these years I have been riding motorcycles and driving cars with non-standard air pressures. How did I survive???

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Probably because having a precise manufacturer prescribed tire pressure is not as critical as this discussion would lead us all to believe. For example - for my K75 BMW specifies pressures for Solo, Solo with luggage, and Two-Up. So is solo me at 160 or Bubba at 320? Is with luggage a computer and jacket in the bags on a commute or 100 pounds of camping gear on a trip? Is two-up with a svelt young friend or a Bubba's heavy weight wife? And BMW doesn't mention two-up with luggage.

Thus have evolved a few rules of thumb for more precisely adjusting tire pressures but I won't even go there. This thread is enough.

Just kick the tires and light the fires...

PGlaves said:

...Is two-up with a svelt young friend?

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As an old married guy whose wife can't ride anymore, I have strict orders from the Director of Finances and Recreation not to engage in that practice. The implications of incorrect tire pressure would be the least of my problems.

This not a discussion about new technology but more of a discussion about" I have always done it that way for 50 years and don't want to change"

Set the pressures anyway you like. Cold with a gauge, hot with TPMS" just ensure you set them.
The science of BMW TPMS ambient tyre pressure calculations are not arguable.

If you don't like technology, don't use it, but don't bag it because it is different to the way you do it.

pch123 said:

This not a discussion about new technology but more of a discussion about" I have always done it that way for 50 years and don't want to change"

Set the pressures anyway you like. Cold with a gauge, hot with TPMS" just ensure you set them.
The science of BMW TPMS ambient tyre pressure calculations are not arguable.

If you don't like technology, don't use it, but don't bag it because it is different to the way you do it.

Click to expand...

You are correct, the science is not at issue, however, the execution is undoubtedly in question.

akbeemer said:

You are correct, the science is not at issue, however, the execution is undoubtedly in question.

Click to expand...

Cant see why
Perfectly straight forward to me