Tire pressure PSA, something I just learned

Discussion in 'Equipment' started by AZQKR, Aug 30, 2018.

  1. szurszewski

    szurszewski Long timer

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    I confess to not fully reading each post, but I do see that we're "discussing" whether or not we should need to adjust for ambient temperature, but I'm wondering why we're not also "discussing" why we should need to adjust for elevation, as also mentioned in the article.

    I suggest for every five degrees above or below 68deg F we simply move our bikes up or down, as appropriate, 1000' - this should cancel out the impact of the temperature change.
    #21
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  2. CharlieInStLouis

    CharlieInStLouis Been here awhile

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    You're kidding, right?
    I need to go tell the nitrogen people (I'm not one of them) about this thread. Hold on, I'll be right back...
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  3. AZQKR

    AZQKR Long timer

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    Is that as viable as just adjusting the pressure based on temp from the garage before we ride that day? :lol3
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  4. MarkM

    MarkM Been here awhile Supporter

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    Whose recommendation was it? My manual (see below, 2013 R1200GS without TPMS) says "with tire cold" and does not specify a temperature. The reference point for any day of the year is ambient temperature and atmospheric pressure. From a physics/engineering understanding, this makes perfect sense because what really matters is the difference in pressure between the atmosphere and the inside of the tire. Tire performance/safety does not care about 68°F.

    I understand that the TPMS adjusts the measured pressure to 68°F and if you have TPMS then they recommend you adjust inflation pressure to compensate, but that does not make 68°F the reference point for non-TPMS bikes.

    Capture.JPG
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  5. AZQKR

    AZQKR Long timer

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    BMW sets their recommendations based on 68 degrees F ambient temperature whether it's expressly written in your manual or not, it's a fact you can verify if you want to spend the time. Others have already verified that with BMW before you..
    #25
  6. szurszewski

    szurszewski Long timer

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    I run nitrogen in the rear tires of my bikes on trips heading east and in the front heading west, assuming I’ll be riding in the earlier part of the day. If I’m riding more in the evenings I’ll reverse that. It helps offset the temp related pressure differential caused by one tire being more in the shadow of the bike than the other. I have yet to figure out how to compensate for differential between left and right, so for now I avoid north/south trips.

    I do carry a small amount of helium to add to the tires if I’ve had a particularly large meal during a ride.

    OHHHH - you wanted a practical solution. I mean, if we were being practical we’d all just drive 10 year old Honda Civics, so I thought you’d want a fun solution. ;)


    Seriously: I think it’s very cool that you, all of you, care about this and put so much thought into it. I am simply not so smart, nor am I good enough at tire pressure adjusting and checking to think i can consistently set it to the exact pound of pressure. I go for the less scientific “that’s just about right” method. I also don’t have a tpms, other than my ass - which only reads serious levels of underinflation - so at least the bike can’t hound me about it while I ride.

    Do carry on please - I’m learning stuff and having fun. :)
    #26
  7. MarkM

    MarkM Been here awhile Supporter

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    BMW does not deem it relevant or important if they do not mention it in their manual.
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  8. Hammerdown77

    Hammerdown77 Long timer

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    I'm pretty sure it's in my 2016 manual. I'll check when I get home.

    Think of it this way. A manufacturer states, "Recommended tire pressure is 36/42 psi."

    Ok, fine. At what temperature? At what altitude? Because those two things affect the actual pressure exerted by the (constant) volume of air on the carcass of the tire.

    It is generally assumed, if not stated explicitly (although some manufacturers do), that these recommended pressures are given at 68F/20C and at sea level elevation.

    For most people, riding under the conditions that most people ride under, it's not going to make a big difference because it's an error of a couple of PSI if they don't compensate.
    #28
  9. MarkM

    MarkM Been here awhile Supporter

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    That's my point though: the tire pressure across the carcass of the tire is what counts and that's exactly what a tire gauge measures. No need to state a specific reference temp/pressure. Yes temperature and altitude cause the tire pressure to change and that's why you need to check it and adjust back to 36/42 at ambient conditions. Adjusting to compensate for 68°F reference is actually introducing a small amount of error in the inflation pressure for the sake of the TPMS readout, but it's so small it is deemed not an important factor. The only reason to recommend a pressure at a specific reference temperature is to get a desired readout from the TPMS--it has nothing to do with tire performance.

    (Edited to clarify)
    #29
  10. Hammerdown77

    Hammerdown77 Long timer

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    I think you're missing a key point. The manufacturer's recommended tire pressure. That's what you base your measurement off of, correct? If so, then it absolutely matters where and under what conditions they determined that pressure, because that pressure is different depending on what elevation and at what temperature you measured it.

    The recommended tire pressure has nothing to do with whether you have a TPMS or not. It's measured tire pressure, with a gauge, at a certain temperature and elevation for a specific tire (of a given construction, size and thus volume)

    Think of it another way. If the manufacturer did all of their tire testing in Minnesota in the winter where it was -40F at the test facility, and they said the tire performed best at 25/31 psi measured with their gauge, then how do you imagine your tires would feel if you set them to those pressures with your handheld guage in, say Florida? They'd feel underinflated, because they are.

    Now, if they told you where they did the testing and at what temperature and elevation, you could compensate using the ideal gas law to determine what YOUR handheld gauge should read at YOUR location to obtain the SAME actual pressure of the fixed volume of air on the tire carcass. And, if you lived in a place where it was around 68 degrees, you'd end up with a gauge that read 36/42 psi, which would allow the tires to be inflated to the same amount as those tested by the manufacturer.

    The manufacturers specify their recommended pressure at 20C/68F, sea level elevation. That's pretty standard. If you ask an engineer from a tire manufacturer, they'll tell you the same.


    #30
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  11. AZQKR

    AZQKR Long timer

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    Is that an opinion based on speaking to the people in Germany or your own opinion?
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  12. ozmoses

    ozmoses . Supporter

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    I've been aware of this since I was a child when my grandfather taught me such things.

    What about Darksiders- which recommendations should they refer to ??

    Frankly, I think you are over thinking it- by a lot.
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  13. AZQKR

    AZQKR Long timer

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    Forget TPMS, BMW recommends tire pressures on bikes without them the same as with the sensors. TPMS discussions muddy the waters of actual facts about ambient pressures and recommended tire pressures. How the sensors work has NO bearing on their decision of tire pressure recommendations or the subject of adding or substracting +1/-1 fior every 10 degrees of temp over or below 68 degrees.
    #33
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  14. Hammerdown77

    Hammerdown77 Long timer

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    Right, you don't need to adjust them throughout the day (this is street tires we're talking about here, race tires are a whole different animal).

    If your tire pressure is set according to the recommended tire pressure by the manufacturer, at the temperature they took their readings at, then even if that pressure goes up with temperature in the heat of the afternoon, and falls during the night when it's cooler, it's still the correct pressure for your tire.

    However, if you set your tire pressure in the afternoon when it's 98 degrees, to the "manufacturer's recommended tire pressure", and their recommendation was determined at 68F, then your tires will be underinflated. Vice versa (overinflated) if you set your pressure to their recommendation when it's 10F.

    It's no different than what most of us know already about setting the pressure 2-3 psi higher if we've been riding the bike and stop at a gas station. The tires are warm/hot from riding so the measured pressure will need to be higher to be at the correct "cold" temperature after the bike is parked.

    It's all about the conditions at which you SET the pressure, and how they compare to those at which the manufacturer set their pressures when they determined the recommendation.
    #34
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  15. flei

    flei cycletherapist

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  16. canoeguy

    canoeguy Long timer

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    You are right. Even with the crazy formula none of us use the pressure would adjust accordingly based on ambient
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  17. Hammerdown77

    Hammerdown77 Long timer

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    That's really more for racing. Those guys are, rightfully so, obsessed with tire temperatures and pressure. And you'll get much bigger swings on a track, and those swings have a much greater impact on performance of a racing tire than a street tire.

    EDIT: I just noticed they said this in the ad: "Displays what pressures would be at the standard temperature (80°F)."
    Which is different than the 68F/20C you usually hear from manufacturers.
    #37
  18. ozmoses

    ozmoses . Supporter

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    Exactly.
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  19. swimmer

    swimmer armchair asshole

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    Fill your tires with hydraulic oil and worrying about tire pressure becomes a burden you'll soon forget.
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  20. canoeguy

    canoeguy Long timer

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    Helium reduces rotational weight. Expensive fills though.
    #40