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Mark – Moderator 2015 R1200R-LC Exclusive
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My observations thus far have been confined to temperature change as the tyre warms up with riding. This is where I find the TPMS useful.
Substantial altitude changes though, like that you have described @Dougl, can be significant if you wish to maintain optimum contact patch, tyre warm-up, handling, ride and wear.
As you say, the sensors are in a confined environment with no vent to atmosphere, so until the vehicle manufacturer incorporates a separate barometer into the vehicle that would inform ambient pressure compensation via the TPMS processor, the readout will be referenced to the sensor’s calibration pressure (presumably sea level).
If at high altitude, set the cold tyre pressure with a good gauge, and see what the TPMS reads. That, then, is your TPMS readout reference value, which should remain relatively steady during a ride. I say ‘relatively’ as I have observed occasions where the tyre has heated up more quickly than the very cold wheel rim (and hence sensor) so it will read a bit higher until the two stabilise.
 

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Questions for the scientists:
(1) If an airline takes off from sea level and seals up for pressure before leaving the runway, what pressure will your tire gauge read when the plane reaches 30,000 feet?
(2) If your gauge is sealed into a thick, clear plexiglass box at sea level, what will it read at 10,000 feet (a little short of the top of Pike's Peak?)
(3) Do you believe that your tires are less able to contain 40 sea level psi at 5 to 10,000 feet than either of the previous examples?
Last questions - (4) what is the psi within the ISS when the outside pressure is absolutely non-existent? (5) Is that inside pressure affected by the outside ambient pressure?

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Mark – Moderator 2015 R1200R-LC Exclusive
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I think you are toying with the scientist(s) among us, @r0ckrat. 😄
I suspect the answer to #3 is the point you are making - I’ll leave the scientist(s) to debate it.
 
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I think someone might be going a bit troppo...




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Assume you fill the tire to 30 psi gauge at sea level (14.7 psi) and that the temperature is the same as at sea level wherever you take the tire. Airliners are pressurized to roughly 7000 ft (11.35 psi according to the table, above), so the tire gauge will indicate 33.35 psi regardless of the plane’s altitude, including in outer space. If you, the tire and the gauge are sealed in a leak proof chamber at sea level, the pressure in the chamber will be 14.7 psi and the gauge will read 30 psi on top of Pike’s Peak or Mt. Everest, assuming the pressure in the chamber remains at 14.7 psi. It may not, because you will eventually die from lack of oxygen, rot, and release gases. If the seal on your tires is as good as the seals on airliners, they will not be more prone to leakage at altitude. If not, then they probably will be. According to Google, the pressure inside the ISS is kept at 14.7 psi, the same as sea level. #5 No. How about if you put all of this kit into a weather balloon at sea level. What would the gauge read at 100,000 ft? (Unless you were in a pressurized suit, you’d be too dead to take a reading).
 

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I actually got a call from NASA once regarding pressure. I worked at Merck and they were going to send one of our metered dose asthma inhalers to the station and wanted to know everything about it's burst strength. I had to send them materials data and hoop strength calculations, and they do indeed keep the pressure at 14.7. rOckrat missed one remaining important question: 2 scenarios, one at the beach and one in the mountains at 5000 feet. If you shake up a can of beer and ask your wife to open it, which one will make her madder?
 

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I rOckrat missed one remaining important question: 2 scenarios, one at the beach and one in the mountains at 5000 feet. If you shake up a can of beer and ask your wife to open it, which one will make her madder?
Best answer ever!

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Mark – Moderator 2015 R1200R-LC Exclusive
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I actually got a call from NASA once regarding pressure. I worked at Merck and they were going to send one of our metered dose asthma inhalers to the station and wanted to know everything about it's burst strength. I had to send them materials data and hoop strength calculations, and they do indeed keep the pressure at 14.7. rOckrat missed one remaining important question: 2 scenarios, one at the beach and one in the mountains at 5000 feet. If you shake up a can of beer and ask your wife to open it, which one will make her madder?
Which illustrates the point that there’s a greater pressure differential between inside the container and outside at higher altitudes than sea level, whether that be a beer can or tyre.
Applying this to the tyre at higher altitudes, the carcass will feel harder to compress, making the contact patch smaller, and less heat generated while running, leading to less grip and different handling and wear characteristics.
 

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So in conclusion (I hope), use a gauge and correct the pressure for your altitude with the chart I provided if the gauge doesn't reference your local pressure. Most don't. BTW, my $100 gauge reads within 0.2 PSI of my $2000 gauge when corrected for my altitude with the chart. Just FYI my tire pressure also goes up by that much when I sit my 162 pounds on the bike. A very large part of the force pushing back on the air pressure is the carcass of your tire. This varies quite a bit with different tires. I do very respectfully find your last sentence to be a bit extreme/exaggerated if you mean the realm of a 2.5 psi difference. At a greater pressure difference, yes. If you could feel this difference with a 2.5 psi delta under normal riding then your bum sensors are far better than mine :). FWIW there's a similar TPMS thread on a Porsche forum and the Porsche techs say TPMS should only be used to diagnose a flat tire or significant low pressure. They also recommend using a gauge.
 

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Mark – Moderator 2015 R1200R-LC Exclusive
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So in conclusion (I hope), use a gauge and correct the pressure for your altitude with the chart I provided if the gauge doesn't reference your local pressure.
To clarify, use a gauge (that you know is sufficiently accurate).
The desired outcome is gauge pressure, which is pressure above ambient. The tyre will then have the running characteristics intended by the tyre and bike manufacturers.
Gauge pressure reading at altitude will be higher than an accurate TPMS reading, given that the TPMS is referencing sea level, or thereabouts.

Now, should I reduce my tyre pressures when riding through an atmospheric low? 🤪🤣🤣🤣
 
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