Tire Over-Pressure and TPM - K-Bikes.com - Excellence in Motion
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post #1 of 4 (permalink) Old Oct 15th, 2007, 9:58 am Thread Starter
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Location: Anaheim, CA, USA
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Tire Over-Pressure and TPM

The TPM on my K1200GT works well (so far). In cold tire comparisons with a pressure gauge, it is accurate to within +/- 1 PSI. Despite other forum threads to the contrary, I rely on it to know when to refill my tires. Of course, when I refill the tires, I use a pressure gauge, not the TPM. I believe that I would notice it quickly if the TPM were to start giving erroneous readings.

All tires lose pressure slowly over time. If I inflate them to exactly the specified pressure, then in a relatively short period of time they will be at least 1 PSI lower than spec. If I fill them to a slightly higher pressure, then they will pass through the specified pressure, and I won't have to fill them so often. (I have looked into nitrogen. While promising, I don't think it is worth the cost.)

How much "over pressure" do you consider to be acceptable and safe when you fill your tires, and how much "under pressure" would you allow before the tires should be refilled?
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post #2 of 4 (permalink) Old Oct 15th, 2007, 11:57 am
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The trouble with trying to accurately measure something is it rarely works at a price they make these things for (+- 1psi) and everybode forgets the physics. Inflation pressures change significantly with the temperature of air in the tire and with altitude.

These things only serve as a guide. E.g if you were losing 3-5 psi a week I'd be looking for a rim seal leak, if they show over pressure after a fast ride at high speed that's what you expect. On a freezing cold day you can't expect anything this cheap without temperature and altitude compensation to have no errors.

If you check and top up your tires regularly, you'll not be relying on a gizmo 'ride by wire' and just might look at your tires and find a tread problem or a nail stuck in it - which the monitor may not show and is more likely to kill you!

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post #3 of 4 (permalink) Old Oct 15th, 2007, 1:51 pm
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TPM is temperature adjusted

FWIW, the TPM in the GT is supposed to be temperature adjusted. That is, the TPM readout on the dash gives the "cold" tire pressure numbers, even when the actual tire pressure increases due to temperature.

I've had this cause the TPM warning light to come on when I made a stopover on a cool day. I think the air in the tires cooled off faster than the TPM's temperature sensor, which caused to to report a lower pressure than actual. Once I started riding again and the tire warmed up, the (adjusted) pressure reading went back up, but the TPM warning stayed on -- annoying.

This will also make a difference if you're inflating a warm tire. How much depends on how warm the tire is, I think the rule of thumb is something like 1 PSI for every 10 degrees.

That's why it's better to check and inflate your tires before you ride. The TPM is less valuable here, because who wants to ride around the block then come back to pump up a tire?
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post #4 of 4 (permalink) Old Oct 15th, 2007, 2:39 pm Thread Starter
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The TPM is temperature compensated. It isn't perfect, but it is good enough. I have seen minor variations in the TPM readout for a cold tire at a known pressure. The variations are small. They are nearly always +/- 1 PSI, and very rarely +/- 2 PSI. You can assume that the rims are straight, the tire is not worn or damaged, it is mounted properly, the seals are good, the valve is good, etc.

It is reasonable to assume that variations in pressure measurements, whether temperature compensated or not, can be caused by:
* Rounding error (nearest PSI)
* TPM temperature compensation isn't perfect (see bonafidebob's post above)
* Ambient air pressure differences due to weather or altitude changes
* Usage patterns that heat the tire differently (speed, cornering, acceleration...)
* Other environmental factors (humidity inside and outside the tire)
* Etc.

--> The real question is how much over and under the nominal pressure (36 PSI front, 42 PSI rear) are reasonable and safe? For example, would you feel okay at 39 PSI (3 over) for the front tire? What about 33 PSI (3 under)?
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