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Thread: Tire Pressure Monitor

  1. #1
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    Tire Pressure Monitor

    I own a R1250gs. The tire pressure listed on the back of the owners manual states my front tire should be inflated to 36.3 psi and my rear tire to 42.1 psi. I have a pretty expensive digital tire pressure gage and I monitor my tire pressure to make sure they agree with the recommended inflation pressures. However, the tire pressure monitor/warning system on my bike consistently reads lower than my tire pressure gage. I tried a different tire pressure gage (not so expensive) and the tire pressure monitor on my bike disagrees with that one also. Should I trust the tire pressure monitor/warning system on my bike, or my tire pressure gage?

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    Registered User alegerlotz's Avatar
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    Quote Originally Posted by apbehnen View Post
    I own a R1250gs. The tire pressure listed on the back of the owners manual states my front tire should be inflated to 36.3 psi and my rear tire to 42.1 psi. I have a pretty expensive digital tire pressure gage and I monitor my tire pressure to make sure they agree with the recommended inflation pressures. However, the tire pressure monitor/warning system on my bike consistently reads lower than my tire pressure gage. I tried a different tire pressure gage (not so expensive) and the tire pressure monitor on my bike disagrees with that one also. Should I trust the tire pressure monitor/warning system on my bike, or my tire pressure gage?
    The bike does this annoying thing where it displays the pressure in the tire "corrected" to 68 degrees F. If you're checking your tire pressure when it is not 68 degrees F where you are, there will be a difference of approximately 1 PSI for every 10 degree difference in the actual temperature from 68 degrees F.
    2016 R1200RT
    2007 KTM 450 XC-W (10/17 - 5/18)
    2005 R1200RT (2/2015 - 12/2016)
    1985 Yamaha XJ 700 Maxim (7/1989 - 9/1991)

  3. #3
    Mine too...
    Tom
    2019 RT

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    Quote Originally Posted by apbehnen View Post
    I own a R1250gs. The tire pressure listed on the back of the owners manual states my front tire should be inflated to 36.3 psi and my rear tire to 42.1 psi. I have a pretty expensive digital tire pressure gage and I monitor my tire pressure to make sure they agree with the recommended inflation pressures. However, the tire pressure monitor/warning system on my bike consistently reads lower than my tire pressure gage. I tried a different tire pressure gage (not so expensive) and the tire pressure monitor on my bike disagrees with that one also. Should I trust the tire pressure monitor/warning system on my bike, or my tire pressure gage?
    Most here will tell you not to trust the TPM on your bike except as a means of assessing quickly if you have a leak. I'm in a small subset who decide to set pressure so that TPM (BMW calls it TPC) displays what we prefer to set the tires to. If you read the Rider Manual for my '16 RT they very clearly imply with TPC when adding air you need to make the adjustment with an external pressure gauge BUT you will need to do the math such that whatever you add results in TPC again displaying your preferred pressures. I find their implied trust in their TPC seems to fly in the face of potential liability in the age of deep pockets class action suits. For my RT it always takes a good few miles to warm up the tires after which TPC's displayed pressures will typically increase by 2 PSI. So when I first get on the bike cold it will read 34F 40R then a few miles later it will stabilize at 36/42. This is the way I've always done it. If I check against a presumed accurate tire gauge I'm anywhere from spot on to 2 below. I wonder if this is BMW's hedge against the clear tendency towards under-inflated tires, kind of how is done w/ mph that displays which is typically 2 mph faster than actual. Running your rear at actual 44 is certainly insignificant in terms of how the tire will perform, at least IMO. It's less than 5% difference which for a system like this is typically not significant.

    In actuality I don't use a gauge but once every few months. My tire pump has a gauge which is close enough so when I add air rather than dick around w/ trying dial in the exact amount of air to add I simple OVERFILL a bit (1-2 psi). Then after riding the bike to get the tires to road-warmed at my next stop I will simply bleed off a bit and am good enough at estimating that pressure will be back to 36/42.

    IMO it's completely trustworthy, its temperature compensation adds value for assessing for leaks in realtime, and can be relied upon for assessing the need to add air, or for realtime leak detection. It's saved me once from entering a busy freeway w/ an evolving flat, and two more times as well.

  5. #5
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    I am pretty aggressive about watching my vehicles' tire pressures (bike and cars)--some friends call me obsessive about it. All my vehicles have an on-board TPMS and I never drive beyond the first block (where the systems begin registering) without specifically looking at the gauge and either continuing the trip or returning to the garage and my air pump. But having said that, I use the on-board pressure gauge only as a guide, not an absolute rule. And I am comfortable with my tires on the higher end of my "acceptable range" of pressure; for example, I ride comfortably with the bike tires at 36-39 front and 42-45 rear.

    Good luck.
    Royce
    On the coast of Kansas
    2012 F800ST

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    Thanks Everyone

    Thanks everyone. I never thought about the temperature variant. I live in the south, and the temp in my garage is in the 80s to 90s in the summer. This is where I check my temperature. Also, as one person said, the on board tire pressure changes as the tires warm up when ridden, I think I will stick with my tire gage readings, and use the onboard tire pressure monitor as guide and safety indicator.

  7. #7
    Mehrten
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    Where's Mr. Glaves?

    Paul will swear that removing air from an overfilled and hot tire is a definite NO! NO!

    My $.25...

    Several years back I took a gage of reasonable value to a meteorologist and had it calibrated.

    I've used is since to compare readings with my other gages. What I was looking for was consistency and relative accuracy in my gages.

    I do not use the TPMS on our bikes to check or set our tire pressures. I use one of my known, relatively accurate gages.

    The TPMS on our bikes is a safety feature that will alert me if a tire is loosing pressure.

    And I know they work - we had two flats in two days and each time the TPMS went off giving us sufficient warning to pull over safely.

    I've also learned that obsessing over tire pressures is a waste of mental energy.

    I fill our tires to either the recommended pressure in the Owners Manual or to the Maximum Pressure shown on the tire and keep a sharp eye on tire wear.

    So far so good...

  8. #8
    Registered User WWeldin's Avatar
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    If you must remove the air from a hot tire, never ride it. Only asking for trouble. But then why remove it in the first place, right? Just don't.

    Also worthy of noting is not to air up a hot tire either. Check all air when cold, meaning when not ridden for a while. First thing before a ride is probably the best time to check air presure.

    Some shops, including tire shops, may have a device to check the accuracy of an air pressure gauge, such as a calibrated air chuck (usually piped into the shop air) with a pressure read out to compare measurements.
    2000 R1200C, 2019 R1250RT

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    Registered User wbrownell9's Avatar
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    Quote Originally Posted by WWeldin View Post
    If you must remove the air from a hot tire, never ride it. Only asking for trouble. But then why remove it in the first place, right? Just don't.

    Also worthy of noting is not to air up a hot tire either. Check all air when cold, meaning when not ridden for a while. First thing before a ride is probably the best time to check air presure.

    Some shops, including tire shops, may have a device to check the accuracy of an air pressure gauge, such as a calibrated air chuck (usually piped into the shop air) with a pressure read out to compare measurements.
    When I was young and poor my car had bias-ply tires. I couldn't afford a compressor but could afford a stick gage. When my tire looked low I'd measure it cold (say it was 12 psi) which meant I needed to add 12 psi. So I'd drive several miles to a gas station (remember when they had free air?) and measure the pressure again. Now that it was hot it was 18, so I'd pump it up to 30: the 18 I already had plus the 12 I needed. When it cooled down it would be close to right, which was good enough for me.

    The key to this technique is to know what the cold pressure is, otherwise you'll have to use the Ideal Gas Law [in the form P1/T1 = P2/V2] to figure out what the hot pressure should be, and to get that right you need to know the temperature (either in degrees Rankine or Kelvin, natch) of both the outside air and the air inside the tire - which may or may not be the temperature of the tire.
    2020 R1250 GSA Low

  10. #10
    Registered User WWeldin's Avatar
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    Quote Originally Posted by wbrownell9 View Post
    When I was young and poor my car had bias-ply tires. I couldn't afford a compressor but could afford a stick gage. When my tire looked low I'd measure it cold (say it was 12 psi) which meant I needed to add 12 psi. So I'd drive several miles to a gas station (remember when they had free air?) and measure the pressure again. Now that it was hot it was 18, so I'd pump it up to 30: the 18 I already had plus the 12 I needed. When it cooled down it would be close to right, which was good enough for me.

    The key to this technique is to know what the cold pressure is, otherwise you'll have to use the Ideal Gas Law [in the form P1/T1 = P2/V2] to figure out what the hot pressure should be, and to get that right you need to know the temperature (either in degrees Rankine or Kelvin, natch) of both the outside air and the air inside the tire - which may or may not be the temperature of the tire.

    Good stuff.
    2000 R1200C, 2019 R1250RT

  11. #11
    Registered User Dann's Avatar
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    When checking my air pressure I use an excel spreadsheet. (I'm not the one who set it up)
    You enter the actual temperature and the pressure you want, and it will calculate which pressure you need to set your tire to in order to have your desired pressure at 68deg

    YMMV
    Attached Files Attached Files
    Daniel
    If you can park it, and not turn around to admire it before walking away, you bought the wrong one.
    2007 R1200RT - IBA # 56396

  12. #12
    Dress for fall & avoid it AlanColes's Avatar
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    The spreadsheet is nice for home, but this JPG file from Bendix is easy to carry on your phone and pull up. Provides a quick and dirty solution that you just need to do minor adjustments to work with your specific #'s.

    Tire Pressure and Ambient Temperature Chart - Bendix.jpg
    Regards, Alan - President BMWONS - BMWMOA/BMWRA/BMWONS/Airheads
    Current: 2019 R1250RT / '06 Ducati ST3s / '91 R100GS / '86 R80RT / '75 R90S / '73 850 & '70 750 Commando Prev: '14 R1200RT / '04 R1150RT / '81 Honda GL1100 / '77 Suzuki GS750 / '73 Norton 850 Commando

  13. #13
    Registered User Dann's Avatar
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    Quote Originally Posted by AlanColes View Post
    The spreadsheet is nice for home, but this JPG file from Bendix is easy to carry on your phone and pull up. Provides a quick and dirty solution that you just need to do minor adjustments to work with your specific #'s.
    I always use the spreadsheet on my phone
    Daniel
    If you can park it, and not turn around to admire it before walking away, you bought the wrong one.
    2007 R1200RT - IBA # 56396

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    Temp/Pressure Chart

    The charts are awesome. Thanks.

    Dan's Chart does not match with Alan's Chart. Dan's chart indicates you should under inflate and Alan's chart indicates you should over inflate for temperatures above 68 degrees to get to the recommended cold inflation pressure, unless I am reading them wrong.
    Last edited by apbehnen; 08-22-2019 at 01:02 AM.

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    Registered User skibumwi's Avatar
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    Quote Originally Posted by alegerlotz View Post
    The bike does this annoying thing where it displays the pressure in the tire "corrected" to 68 degrees F. If you're checking your tire pressure when it is not 68 degrees F where you are, there will be a difference of approximately 1 PSI for every 10 degree difference in the actual temperature from 68 degrees F.
    Thanks for that fantastic explanation! That's why I come here

    Ski

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