TPM Accuracy

[duegatti]

And yet, I'm confident that the tire pressure specifications that we are offered today assume inflation with air, and the temperature-induced pressure increases that go with it. If we change the gas, and change the temperature dependence of the pressure, we will not achieve the intended operating conditions of the specification.

[realshelby]

My air hose for filling my tires comes out of a rather expensive air drying/filtering system I use for high quality painting projects. So, I doubt there is much humidity left in it. Filling with Nitrogen over a typical air hose will net less water vapor and that is the ONLY reason it is better in any real way.

 

As for the TPM (before Nitrogen vapor locks this thread...) I wish it just read the pressure in the tire with NO compensation. I can compensate in my head enough to understand raised values after an hour of high speed riding.....or that the tires cold pressure is lower after a cold front blows through......

[dirtrider]

I wish it just read the pressure in the tire with NO compensation. I can compensate in my head enough to understand raised values after an hour of high speed riding.....or that the tires cold pressure is lower after a cold front blows through......

 

Afternoon realshelby

 

The problem is IF the system isn't temperature compensated then you lose part of the low tire pressure warning (or can get an errant low pressure warning even if within specs under certain temperature conditions).

 

Due to new (since about 2005 or so) FMVSS requirements it usually takes some sort of temperature compensation on lower volume tires to meet the FMVSS requirements. (I don't know if this also holds true on motorcycles) but knowing BMW they like to stay ahead of the requirement curve.

 

[realshelby]

Thanks DR, at least it makes more sense why it does what it does!

 

About the only thing I use it for is to check pressures on a trip ( don't carry a gauge now ) to confirm I don't have a leak. I'll now remember to check it right away in the morning before it "temperature compensates" to get a more consistent idea of pressure.

[NoelCP]

The spec is 42/36, the way we operated for a hundred years before TPMS, set on an equilibrated system prior to riding ("cold", if you will) for the ambient conditions of the ride. 42/36 at 38F; 42/36 at 68F; 42/36 at 98F.

 

I don't get the logic behind this and here's why:

 

Let's say it's exactly 68F outside and your tire is cold and out of sunlight. I set the tire pressure to 42psi.

 

I now put the tire into a refrigerator at 38F and leave it there until the tire and its air etc cool down to 38F. Everything I read now says if I check its pressure, it will be ~42psi - ((68-38)/10) = 39psi. If this is true, then it stands to reason that if I now take a tire that is 38F and set it manually to 42psi, when that tire is in a cold state AND ambient is 68F, would not the tire then test out to 45psi? Forget the TPM issue I'm still wondering why one always sets cold inflation pressure to X no matter what ambient is.

[duegatti]

If this is true, then it stands to reason that if I now take a tire that is 38F and set it manually to 42psi, when that tire is in a cold state AND ambient is 68F, would not the tire then test out to 45psi? Forget the TPM issue I'm still wondering why one always sets cold inflation pressure to X no matter what ambient is.

 

You set for the ambient conditions of the ride. If it is 38F, but you are going to be riding at 68F, you set for 42 lb at 68F, which would be 39 lb at 38F.

 

The implication is of course that our pressures will not be at the recommended level throughout a typical day, which will see temperature extremes of some 20F. So, assuming we don't want to adjust pressure throughout the day, we chose some temperature which we feel will be a useful, safe compromise (such as a mid point), declare that our ambient temperature for the day, and set pressure for that ambient temperature, making corrections for what the temperature of the air in the tire is at the time of filling.

 

With any remaining energy, we now get to calculate what the pressure we just set at current temperature X, for a ride at ambient temperature Y, ought to be at 68F, and compare that to our TPMS.

Edited September 26, 2016 by duegatti

[NoelCP]

If this is true, then it stands to reason that if I now take a tire that is 38F and set it manually to 42psi, when that tire is in a cold state AND ambient is 68F, would not the tire then test out to 45psi? Forget the TPM issue I'm still wondering why one always sets cold inflation pressure to X no matter what ambient is.

 

You set for the ambient conditions of the ride. If it is 38F, but you are going to be riding at 68F, you set for 42 lb at 68F, which would be 39 lb at 38F..

 

OK well that's what I've been doing after reading about this years ago.

 

Thanks and Cheers

[narcosis]

Had to convince myself that 1 psi for every 10 degrees was close enough...and it is. Not having anything better to do with my valueless time, played with the ideal gas law and

 

Temp___Front___Rear

18_____32.86___38.11

23_____33.20___38.51

28_____33.54___38.91

33_____33.89___39.31

38_____34.23___39.71

43_____34.58___40.11

48_____34.92___40.51

53_____35.26___40.91

58_____35.61___41.30

63_____35.95___41.70

68_____36.30___42.10

73_____36.64___42.50

78_____36.98___42.90

83_____37.33___43.30

88_____37.67___43.70

93_____38.02___44.10

98_____38.36___44.50

103____38.70___44.90

108____39.05___45.29

113____39.39___45.69

 

When I took a look at this, I saw pressures don't change radically over a pretty large temperature range. Helped to see it.

Edited September 26, 2016 by narcosis

[Mike]

The best way to get around all the calculations is to use nitrogen in the tire. It is expensive to get the setup, but it removes all the problems. It is used in racing tires to reduce the number of calculations to get the "wedge" in a round track car.

 

If you are going to spend that amount of time on tire pressure, you should already be using nitrogen.

 

Yes, if you need to add air on the road, that will create a problem, but back at home, you can rectify that situation.

 

If not using nitrogen, the exercise is quite moot. I think a gauge manufactured for your location and pressure checked before riding will suffice. But this is my opinion.

 

I know many of you are extremely anal like myself, but I find I dig way deeper than really needed.

 

Morning James

 

What am I missing here? Are you saying that nitrogen doesn't adhere to the same ideal gas law as air does?

 

Exactly WHAT problems does nitrogen remove other then take some weight out of the rider wallet?

 

First, nitrogen escapes a tire more slowly. then the gas is purely humidity free.

 

If you are going to check your tires with the uber small pressure head digital gauge that cost $90 bucks because it is a BMW item and uses/loses virtually no air, I think nitrogen would be the way to go. If a person is going to get that anal, nitrogen would help.

 

This is about precision, maybe it doesn't solve all problems(mis-statement on my part), but will get closer to the focal point of extremely anal people.

 

James L

 

Click here to see the definitive analysis of using nitrogen in motorcycle tires.

[TJRL]

but will get closer to the focal point of extremely anal people.

 

James L

 

Not sure I would want to!