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Thread: Anything wrong w/ this logic? Setting tire pressure using a warm tire with TPMS

  1. #1

    Anything wrong w/ this logic? Setting tire pressure using a warm tire with TPMS

    I know the conventional wisdom including in the rider manual you are to set tire pressure w/ a gauge on a cold tire. And also I get the sense people trust a $30 digital over TPM's readout but I'm not sure why considering the tire pressure gauge presumably isn't temperature-compensated. You can ride in ambient temps from 40F to 90F and TPM will display the pressure as unchanged because of the temperature-compensation logic that TPM utilizes.

    I can't see why you couldn't get accurate pressure adjustment by adding or removing air using the TPM readout on a road-warmed tire. So let's say your road-warmed front tire pressure reads 36psi, and you'd like it to be 38. Simply stop, leave the ignition on and add air until TPM displays 38psi. Why would this not be the most accurate way to set tire pressure? I stopped the bike once and took a little air out once on a warm tire and noticed TPM did drop down in real time w/o the wheels turning as I left the ignition turned on.

    The excerpt below from the rider manual seems to be saying they consider TPM to be the benchmark and adjusting pressure using a filling station pressure gauge requires you to take into account the temperature compensation, no?

    Screen Shot 2017-07-27 at 6.19.17 AM.jpg

  2. #2
    I have been through this subject over on AdvRider for several months. Too much of the advice from the elves in the Black Forest makes sense to engineers but really isn't practical.
    #1 - You can's set your tires cold and have the TPMS reading come out right.
    Why? Because you have to ride the bike to 19mph to get the TPMS to display. It really doesn't settle down for maybe 15 minutes too.
    What most people really want to know is what the pressure is in the tire. That isn't what you get in a compensated reading.
    How do I know? I attached my GS911 and connected my laptop to get the tire temp and pressure from the sensors. It isn't what is on the display because the display compensates.
    What I recommend is set your pressure to an amount cold. If it is 68 degrees then 36/42 like the book says. Then go for a ride for maybe half an hour. Depending on ambient air temp take the numbers off the display, write them down or remember them. Then measure the temp at the tire. If you want corrected pressure, add or remove air to the tire using the difference between actual tire pressure and displayed corrected pressure to adjust the actual tire pressue to give 36/42 on the display. OR if you like what the gauge says either use it or correct it to what you want as an actual pressure IN THE TIRE without any corrections to 68 degrees.

    IMO, the manual and the engineers are making this subject WAY TOO COMPLICATED. For the last 50 years I've been riding I've used the 10% rule and so have the majority of riders. I find it totally counter intuitive to make the actual pressure in the tire read at 68 degree temp levels whether it is 10 below zero or 120 degrees.

    Please don't bring up PV=nRT. That is an IDEAL gas law.
    Old But Not Dead
    Semper Fi

  3. #3
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    If everything works as presented in the manual then I agree.

    A lot of what I have been reading on the system also seems to support your position.

    This is how I plan on setting tire pressure going forward.
    Don
    '16 R1200 GS Adventure, '05 DRZ 400, '08 Royal Enfield Bullet
    http://www.youtube.com/user/SlooDon#p/u

  4. #4
    Registered User greenwald's Avatar
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    Thumbs down

    I find the roller coaster reading of the TPMS a constant annoyance.

    If it had been an option on my bike, I'd of opted out of having it.

    KISS - just check your tire pressures with the tires cold, set to your preferred psi and go riding.
    Kevin Greenwald - MSF Lead RiderCoach # 121656 (BRC,SBRC,IS,IME,SMARTrainer)
    Nationally Certified Law Enforcement Motor Officer (Ret.) / IBA Member #34281
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  5. #5
    FUKENGRUVEN SURVIVOR akbeemer's Avatar
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    The idea of using the TPMS to set tire pressure presupposes that they accurately measure the pressure; I do not think they are particularly accurate. Their intended function is to alert a rider to a deflation and from what I hear/read they do that well.
    Kevin Huddy
    Tm Pterodactyl MT Outpost

  6. #6
    Debbie's Servant Lee's Avatar
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    Quote Originally Posted by greenwald View Post
    I find the roller coaster reading of the TPMS a constant annoyance.
    With the 4 BMWs we have owned with TPM the readings do not jump up and down while riding.
    Lee
    2016 R1200RS
    MOA # 30878
    Past BMW Bikes: 2011 K1300S, 2003 K1200RS, 1991 K75S, 1987 K75T, 1984 R100RT

  7. #7
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    I agree with the post that tpms is there for warning of low pressure - and it has saved me.

    My pressure will increase 2psi after the tires get warm - its pretty consistent with that. Since I plugged the nail hole I am checking constantly.

    I've always used them as a guide and not accurate PSI. My digital hand held gauge is my guide.

  8. #8
    Quote Originally Posted by greenwald View Post
    I find the roller coaster reading of the TPMS a constant annoyance.

    If it had been an option on my bike, I'd of opted out of having it.

    KISS - just check your tire pressures with the tires cold, set to your preferred psi and go riding.
    Neither my '13 F800GT nor '16 RT demonstrates any roller coaster readings whatsoever. There is alway a road-warmed jump of 2psi, then it's rock solid no matter what the ambient temp. The clearest evidence of that in my experience was leaving Bishop CA at 5:30am at 52F and then riding up to St George where the last 3 hours was 113F! Pressure stayed exactly the same--an acid test for temperature compensation. If I looked at actual rear tire pressure it was probably approaching 50psi.

    TPM is fabulous--I'd never want a bike w/o it. Saved me on a freeway onramp as I was accelerating to enter traffic when the flashing commenced, I pulled over and plugged the tire. Second instance was after leaving for work so didn't go far from home only to notice a 5 psi drop from normal for me, so turned around and repaired the flat in the garage.
    Last edited by ncpbmw1953; 07-28-2017 at 03:10 PM.

  9. #9
    Quote Originally Posted by chewbacca View Post
    What I recommend is set your pressure to an amount cold. If it is 68 degrees then 36/42 like the book says. Then go for a ride for maybe half an hour. Depending on ambient air temp take the numbers off the display, write them down or remember them. Then measure the temp at the tire. If you want corrected pressure, add or remove air to the tire using the difference between actual tire pressure and displayed corrected pressure to adjust the actual tire pressue to give 36/42 on the display. OR if you like what the gauge says either use it or correct it to what you want as an actual pressure IN THE TIRE without any corrections to 68 degrees.

    IMO, the manual and the engineers are making this subject WAY TOO COMPLICATED.
    Gee, the manual and engineers made it way too complicated?

    I'm afraid nothing said here does anything to debunk the theory I put forth. The very reason you and others and myself follow the 10% rule is because your pressure gauge isn't temperature compensated! Following the 10% rule makes sense when you are checking cold tires to get you in the ballpark, but in the end TPM takes this into account.

    This appears to be more of the same ingrained assumption that a $30 gauge and 10% rule somehow trumps TPM--and I have yet to hear any evidence that assumption is at all valid.

  10. #10
    MOA #24991 Pauls1150's Avatar
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    I'd instantly be suspicious of anything that says to set the pressure to 36.3 PSI. Not just too low, but nobody's going to make the effort to hit the decimal.

    Thanks, but I'll stick with "set it cold and manually".

  11. #11
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    Quote Originally Posted by Lee View Post
    With the 4 BMWs we have owned with TPM the readings do not jump up and down while riding.
    I agree with the consistent pressure readings on my system. The pressure increases 1-3 psi after the tires are warmed up. That assumes I am using 40/42 cold pressure. Anything lower and the rise from heating is 3-5 psi.


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  12. #12
    Registered User CABNFVR's Avatar
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    Complicated?

    I see simplicity, not over complication. I set my tires at 38/44 and the TPS says 36/42. OK, now I know. If the TPS is 36/42 I'm good because I'm really at 38/44. Simple.
    "Have BMW. Will Travel"

  13. #13
    Quote Originally Posted by Pauls1150 View Post
    I'd instantly be suspicious of anything that says to set the pressure to 36.3 PSI. Not just too low, but nobody's going to make the effort to hit the decimal.

    Thanks, but I'll stick with "set it cold and manually".
    I know that's interesting. I think it must have come out of physics/engineering computer modeling which determined the optimal pressure for some such set of criteria, and they published it out to one decimal place.

    The set it cold and manually is ultimately in some ways easier to do since you don't need sync it with stopping a warmed bike tire, connect a compressor, and make your adjustment while TPM continues to display and update. But still, needing to do the math to apply the 10% rule is eliminated, and I'll venture a bet quite a few people either don't know about the 10% rule, or don't bother to apply it. You have to do it when you use a manual tire pressure gauge, whereas you don't if you use built-in TPM.

  14. #14
    Registered User marcopolo's Avatar
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    Quote Originally Posted by Pauls1150 View Post
    I'd instantly be suspicious of anything that says to set the pressure to 36.3 PSI. Not just too low, but nobody's going to make the effort to hit the decimal.

    Thanks, but I'll stick with "set it cold and manually".
    I think most people would know immediately that 36.3 is simply a conversion from Bar to PSI, and to round it off to 36. And why would you say 36 is too low? That's what BMW recommends for my GSA, and that's what I set it to. Works for me.

    As for TPMS, my bike has it, and I think it's very easy to understand, and use.
    Mark
    2015 R 1200 GS Adventure

  15. #15
    Addicted to windshields Realshelby's Avatar
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    I'll say this about temperature compensated tpm's. They are good to let me know if there is a drastic change in pressure like when you have a leak. Which is great and I want that information. I even check mine going down the road to reassure myself a "funny feeling" isn't a tire going down.

    Here is my beef with them. They don't accurately tell me tire pressure. If I set the pressure at 36 front and 42 rear ( with a Longacre gauge used on race car tires where 1/2 pound makes a difference and the gauge is calibrated yearly ) the tpm readout won't show that amount. It usually shows 2-3 pounds less in warmer temps. So, If I were to "adjust" the pressure in the tire to make the tpm readout more or less correct, I would be over inflating the rear tire. Current Dunlop has "42 psi maximum cold pressure" on the sidewall.

    Just give me the tire pressure like a tire gauge would see it. I have plenty of experience with changes in temperature, speed, and load to understand how pressures act.

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