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On two year old bike with new tires I am unable to get recommended Front 36 / Rear 42 PSI to show up on bike's display. it's always shows around 34-35 front and 40-41 rear.
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.

  • Is it possible that my bike's tire pressure monitor is wrong after two years?
  • Maybe 1 PSI is lost while I am disconnecting pump's hose from the wheel?
  • Or this is normal and I should just ignore?
 

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Mark – 2015 R1200R-LC Exclusive
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Just ignore it, Kramer. I don't know what the claimed accuracy is, but note that the sensor is temperature-compensated to give a reading at the standard temperature (can't recall what it is, but 24 degrees C rings a bell). Any temperature away from the standard temperature will affect what is read by a separate gauge. There's no guarantee that your electric pump's gauge, or those at service stations are any more accurate, and they won't be temperature compensated.
 
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I think the UK ones are accurate at 20 degrees (we don't often get to 24). My dealer told me to set the pressure accurately with a known good gauge and see what the tpms says and use that as a guide to pressure. You are only really interested if it starts to drop anyway.
 

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If the temperature is close to 68F when I check my tires the TPM reading matches my tire gauge.
My wife's R1200RS TPMs will display .5 to 1 PSI lower than my gauge.
I look at the TM reading within one mile of leaving the house.
 

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I don't think hardy any tire pressure system is calibrated for 100% accuracy. You pick a quality gauge and trust it to be close enough. TPMS is only supposed to flag unusually low pressure. Yeah, the display give a reading to 1/10 of a pound, but how accurate it is may vary from sensor to sensor. If your gauge said pressure was good when cold, see what the display says when it comes up. That's usually your differential.
 

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From the manual, Sensors are calibrated for 68*F, 20*C, and tire pressures should be 36.3 psi (2.5 bar) front, and 42.1 psi (2.9 bar) rear (cold tires).
 

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The actual values are supposed to be corrected to 20C. Mine is always about 2 lbs low from the gauge reading at around 20C. The actual correction for pressure is in Kelvin so a few degrees doesn’t cause a big change in pressure. My BMW mechanic says they read low by 2-3 lbs and shrugged. He’s been doing this since before the fancy electronics. I spent a lot of unnecessary angst on this but I’m now cured. Just use a decent gauge.
 

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What do you do when the battery in the TPMS dies? My front sensor stopped providing regular readings...
 

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Mark – 2015 R1200R-LC Exclusive
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What do you do when the battery in the TPMS dies? My front sensor stopped providing regular readings...
If the battery is dying, you can replace the sensor or dig out the battery and replace it (a DIY hack). The dash should indicate that the battery is low, so if the sensor hasn’t done that, it might be failing to communicate for some reason. My rear sensor has been doing thatbvery for quite a while now, and occasionally the front.

I’ve also experienced an alert occasionally where the the sensor is saying the pressure is beyond safe limits (too high - which clearly it isn’t).

The OEM sensors are very expensive and best replaced when the tyre is being replaced. They’re also available from China for a very modest price - I have a couple I’m going to try eventually.

To fit them, you need a sensor wake-up tool (about $10 on eBay) and a GS911 to register the new sensor to the bike. Don’t know if Motoscan can do this.
 

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Just ignore it,snip note that the sensor is temperature-compensated to give a reading at the standard temperature (can't recall what it is, but 24 degrees C rings a bell). Any temperature away from the standard temperature [edit: or changes in altitude] will affect what is read by a separate gauge. There's no guarantee that your electric pump's gauge, or those at service stations are any more accurate, and they won't be temperature compensated.
agree with above but STP is 1atm pressure (760mm or sea level) and 20C(~68F).
It is my understanding that the TPM shows what the tire pressure would be if you were at STP.
Tire pressure gauges can be inaccurate. If you inflate your tires and the ambient temp is around 68-70F and you are at sea level, your TPM will likely be accurate. Anything outside of that and it will not be, but will serve as a monitor of any trend up or down.
It must be a lab thing why BMW does it that way. Yes, it's stupid for the real world. Use a trusted tire gauge, and use your tpm as a trend follower
 

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Mark – 2015 R1200R-LC Exclusive
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Except, @Dr. Strangelove, there is no compensation for altitude - the sensor has no way of determining altitude, and doesn’t need to.

Tyre/bike manufacturers determine an optimum pressure for the tyre at ‘cold’ temperature, in the knowledge that the tyre pressure increases as the tyre heats up in use. In practice, this means it is sensible for Schrader to build into their sensors temperature compensation, so that at any time you can read the pressure as it would be ‘cold’ (20 degC) and hence determine whether your tyre is lower (or higher) than it should be.

Without this, the reading would go up as the tyre heated with use, then down as the tyre cooled, making it difficult to know whether it had the correct operating pressure (not specified by the bike/tyre manufacturer due to the variables just mentioned).

On my 70’s Ducati bevel SS I’d check the tyres at a local service before going out on a run, knowing the pressures would have increased from cold pressures by 2 psi during the ride there. Using my gauge (which I trusted) I’d set the pressure 2 psi higher than spec, knowing the pressure cold would be correct.
 

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Ride more. Worry less.
 

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On two year old bike with new tires I am unable to get recommended Front 36 / Rear 42 PSI to show up on bike's display. it's always shows around 34-35 front and 40-41 rear.
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.

  • Is it possible that my bike's tire pressure monitor is wrong after two years?
  • Maybe 1 PSI is lost while I am disconnecting pump's hose from the wheel?
  • Or this is normal and I should just ignore?
Keep in mind that the tire pressure monitor is temperature sensitive
 

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Does the sensor measure absolute or differential pressure?
 

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Mark – 2015 R1200R-LC Exclusive
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Except, @Dr. Strangelove, there is no compensation for altitude - the sensor has no way of determining altitude, and doesn’t need to.

Tyre/bike manufacturers determine an optimum pressure for the tyre at ‘cold’ temperature, in the knowledge that the tyre pressure increases as the tyre heats up in use. In practice, this means it is sensible for Schrader to build into their sensors temperature compensation, so that at any time you can read the pressure as it would be ‘cold’ (20 degC) and hence determine whether your tyre is lower (or higher) than it should be.

Without this, the reading would go up as the tyre heated with use, then down as the tyre cooled, making it difficult to know whether it had the correct operating pressure (not specified by the bike/tyre manufacturer due to the variables just mentioned).


snip
Hey, Mark
I was only commenting what STP was. Although there may be no altitude compensation in the sensor, the tires and their pressure certainly react to altitude.
I found the bolded comment interesting and informative---Discussions on this issue have been frequent here and in the GS forum for a long while. Your explanation is what I assumed, but I thought it was a lab thing to study/develop the sensor. I've never seen the explanation you offer, but it makes sense AND offers a real world rationale for the behavior of the sensor.
However, most people, esp those who notice the discrepancy and post the above questions, do not like the explanation you offer, scientifically accurate as it is, and wind up saying they "trust" their "reliable" "XYZ gauge."
Questions for you though.
1)If it's cold outside, say ambient temp 4-6C or around 40F, and you're going for a ride, the sensor is going to give you a pressure at 68F/20C. But the pressure in your tires will be lower than that in the real world environment you're riding. So, do you top up your tires to 36/42 with a hand held gauge, knowing that, according to the sensor reading, it will compute and display a different than "optimal" reading? Or do you run your tires with the ambient lower pressure counting on the heat buildup to bring it up to the correct value and not worry that your hand held gives a reading of 32/37, and trusting the sensor is smarter than you? Same thing for temps higher than 20/68.
2) It seems to me that the displayed reading changes somewhat as I ride, ie it goes up a bit (and I could be worng, I need to check, but it seems that way). If there is some sort of algorithm within the sensor to compute the ambient sensed pressure to what it would be at 20/68, why does that reading change as we ride. If the tires are heating, that should not make a difference in what the smarter-than-a-handheld-tire-gauge sensor reports, unless there is a leak.

I think that is the crux of the questions posed. What cha think?
 

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1)If it's cold outside, say ambient temp 4-6C or around 40F, and you're going for a ride, the sensor is going to give you a pressure at 68F/20C. But the pressure in your tires will be lower than that in the real world environment you're riding. So, do you top up your tires to 36/42 with a hand held gauge, knowing that, according to the sensor reading, it will compute and display a different than "optimal" reading? Or do you run your tires with the ambient lower pressure counting on the heat buildup to bring it up to the correct value and not worry that your hand held gives a reading of 32/37, and trusting the sensor is smarter than you? Same thing for temps higher than 20/68.
2) It seems to me that the displayed reading changes somewhat as I ride, ie it goes up a bit (and I could be worng, I need to check, but it seems that way). If there is some sort of algorithm within the sensor to compute the ambient sensed pressure to what it would be at 20/68, why does that reading change as we ride. If the tires are heating, that should not make a difference in what the smarter-than-a-handheld-tire-gauge sensor reports, unless there is a leak.

I think that is the crux of the questions posed. What cha think?
1) If it's that cold, you WANT a slightly lower pressure - so that the carcass can move a little over the cold ground, and to allow the tires to warm up and still have the correct pressure. If the tires are supposed to have 36/42 at 60*F, and it is 40*F, then the ambient pressure should be scaled down - the TPMs allow for that, and setting your tire pressure with a gauge that is not calibrated/temp adjusted is just putting too much air in your tires.
2) I'd be interested to know if you are actually seeing changes in the TPM reported pressure shifting upwards during a ride... The sensor reads the air temp inside the tire, the pressure inside the tire, and then reports the converted value to the display.

This patent actually has a really good description.
 

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Mark – 2015 R1200R-LC Exclusive
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What @r0ckrat said, John.

On your other point, I too have noticed my readings vary a little and put it down to the fact that the sensor is screwed to the wheel, rather than suspended by the valve stem as in some car systems. I figure the tyre warms up quicker than the rim/sensor, so the pressure reading will increase a bit as the tyre warms, then settle back once the sensor has warmed. It’s one of the reasons I switched the readout to bar from psi (yes, I know, for all my preaching about metrication, I should have always been there). psi is a little more granular than bar, which is why I had originally selected it, along with its familiarity and I think one of my gauges is psi only, so the readout varies less. But I realise now I was chasing poofteenths anyway.
 

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This subject bugs me. First, the tire volume is assumed to be constant. Maybe it isn’t but by how how? The manual specifies recommended pressure when “cold”, not at 20C. The tires themselves don’t say “20C”. I don’t know that tire manufacturers’ recommendations are at any specific temperature other than “cold”. A local tire store says the same. The difference between summer and winter morning temperatures can be 20-30C where I live. I haven’t noticed the actual tps temperature vary by more than 0.1 psi. The difference in pressure for a tire at 36 psi at 10C is 1.27 psi when the temperature is raised to 20C.

The ambient temp is now 17-18C. I have 2 electronic gauges. Just now, the tps was 2 psi lower than one gauge and 3 psi lower than the other, front and rear. The significance of this? None.
 
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