Tire pressure PSA, something I just learned

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

  1. AZQKR

    AZQKR Long timer

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    Santa Cruz's average temp is 58.7 degrees F. It's high is 69.8 F. In other words, you don't have to worry about wild swings in temps like many parts of the country do. I move from 35 to 115 degrees within 6 months time.
  2. JTerryM

    JTerryM -/\~

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    We do have it rough here, eh? But I'm not a native, so familiar with adjusting pressures for season change. Just always did it for current conditions (ambient).

    I hope you report back if you see any benefits, like improved tire life.
    AZQKR likes this.
  3. Johnny4x4

    Johnny4x4 Hope 2 B Cool Sumday

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    Oh Man, I NOW realize have been driving/riding/towing on incorrect tire pressure my entire life!

    From my bicycles, to motorcycles, to vehicles, to trailers, and all those years in the military setting every tire to the recommended PSI before driving/riding/towing. This obviously is the reason for any/all of my problems in life.

    bad grades at school = maybe 2 psi off on my bicycle.

    got a cold/flu = 1.5 psi under

    got divorced = 4 psi over

    Gallbladder had to be removed = 5 psi over, clearly over inflation of my truck tires causes them to be too hard resulting in harsher impact to my body when going thru the gutter at the end of my driveway. this forced the impurities in my gallbladder to the bottom, where they collected and made stones.

    I think I have a headache, it must be because the current pressures in the tires of both my motorcycles are .5 psi different than each other.
  4. AZQKR

    AZQKR Long timer

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    It would be hard to determine about improved tire life as there as too many variables in the life of a tire. What can be said is they won't be under inflated cold based on the recommended pressure at 68 ambient
  5. AZQKR

    AZQKR Long timer

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    Okay, so I asked some very long time owners of BMW's who have ridden an aggregate of over 300 years, and they all stated they've just set the tires at ambient before riding at recommended pressures by the makers [ bmw in this case ] and never given it a second thought. They normally check theirs if not ridden in a week or two, but if riding daily, maybe every other day or more between pressure checks.

    I'm going back to the way I used to air them which is ambient that morning at recommended myself. That +1/-1 formula being discussed just confused the crap out of me, never having seen it before. The long time riders of beemers haven't paid attention to it being relevant enough to matter on their bikes/.
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  6. bmw_rider

    bmw_rider Adventurer

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    The BMW TPMS readout is based on an algorithm that converts the pressure reading to 20C. 20C is the Normal Temperature and Pressure Standard.
    What this means is the pressure that the tyre would be when effectively the temp the tyre is and within is 20C

    Above or below 20C ambient air temp (if the tyre has not been used yet) or 20C tyre temp (if it has been used), a manual tyre gauge read out will be above or below the desired 'cold' tyre pressure you want to set.

    For example, if the tyre temp is at 30C then the tyre pressure gauge should show circa 43.6 psi for a 'cold' tyre setting of 42 psi @ 20C.
    Prior to riding, if the ambient temp is 30C and you want to run 'cold' 42 psi in rear tyre, use your tyre gauge to set 43.6 psi, before you ride.

    Based on a fair amount of internet searching and some variation i found, i have chosen 0.16 psi increase per 1 degree Celsius as to be expected.
    I then used this value in a spreadsheet to arrive at 'cold' tyre pressures based on ambient air/tyre temps (where the bike is stored before riding) to arrive at the pressure the tyre gauge should read, before riding.
    Theoretically, below 20C (where the tyre gauge readout will be lower than the desired 'cold' tyre pressure), the tyre will flex more and get heat into it quicker (and grip) and as the tyre reaches 20C it will be operating at the desired 'cold' pressure and continue to have a pressure increase up till the tyre temp stabilises and be its operating pressure.
    Conversely, above 20C (where the tyre gauge readout will be higher than the desired 'cold' tyre pressure) the tyre flex will be less and tyre not be too hot once at operating pressure. Tyre wear should be less and last longer.
    Mindful of the varying pressure changes i found for 1 degree Celsius increase or decrease and the (maybe) slight variation with the tyres temp resting on a cold concrete floor, i provide the following:

    upload_2019-6-15_17-11-32.png
    please let me know if this is mathematically in error and i will try again :fpalm
  7. AZQKR

    AZQKR Long timer

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    Can you convert the celcius chart to degrees for us yanks?
  8. NCD

    NCD Dirty Hairy Supporter

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    I think the root cause for all of the trouble is inflating tires on a metric/euro bike to pounds per square inch. Is there a suitable calculation to convert it to metric? Could help ease the confusion and debate.
  9. brianbrannon

    brianbrannon They'll ride up with wear

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    There's actually a name for saying things aren't true because you can't understand them
  10. Plawa

    Plawa ¿ʞO ǝʞᴉq ʎɯ sI

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    There's nothing wrong with not knowing the formula from the top of your head, it's the fact that you expect someone else to do the conversion for you that's maybe more telling of what keeps you from learning...
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  11. Coastie3202

    Coastie3202 King of MY Mountain

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    So complex....uh, NO. Me, simple, 32 Front/32 Rear cold. Works best for me. And......DONE.
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  12. bmw_rider

    bmw_rider Adventurer

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    AZQKR Here you go.

    You yanks need to get on board with the Metric system. So much easier and simpler. The transition from Imperial to Metric measurements for us Australians in 1971 was painless.

    Again, if the conversion is wrong, let me know.
    cheers

    upload_2019-6-17_16-17-32.png
    AZQKR likes this.
  13. bmw_rider

    bmw_rider Adventurer

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    While there are not huge differences say between 10C & 30C (either side of 20C Normal Temperature Standard), the ONLY time you will have 32/32 in your tyres is when you measured the pressure when the tyres were at 20C
    If you measure 32 psi in your tyres at ambient/tyre temp of 30C you actually are 1.6 psi low and the under inflation may heat the flexing tyre up more once riding and increase tyre wear.
    End of the day, whatever works for you.

    The benefit of the BMW TPMS system is it shows you what the actual, operating pressure is (related to 20C) and if say it showed 35 and you want to run 32, you can read the tyres pressure with a tyre gauge and simply let out 3 psi off of whatever the gauge reading is. Or add 3 psi in the converse.
    I generally just let a bit out and ride some more and read the TPMS and let more out as required (if i can be bothered).
    An added benefit of the TPMS is warning you when you have a puncture and the air is lowering to a critical level. I have got 2 rear tyre punctures recently from stone shards and the TPMS warned me of the puncture long before i could feel the tyre going down.
  14. petertakov

    petertakov Been here awhile

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    Here is what Michelin have to say about ambient temperature: "When using tire warmers, the pressure set at ambient temperature before the first ride should be the same as without tire warmers."

    Considering that warmers basically create higher "ambient" temperature around the tyres, why do Michelin specifically advice against adjusting recommended ambient tyre pressures?
  15. bmw_rider

    bmw_rider Adventurer

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    My interpretation of this is, whether tyre warmers are used or not, tyres are initially set to a pressure relevant to ambient temp, before the tyre warmers are turned on/used.
    As tyres get ridden on they flex and heat up increasing the internal pressure of the tyre. Use of tyre warmers raised the temps and pressures to in use values so when you hit the track the tyres temps & pressures are good to go. The lower ambient temps & pressures of tyres without warmers allow the tyre to flex for a while, generating heat and 'pumping' the tyres up, eventually getting to temps & pressures the heated tyres hit the track at.

    Using something like the chart, if you accept 20C as being the Normal Temperature Standard, if temp is below 20C you could inflate less at ambient which will then allow the tyre to flex more, heat up more and quicker & pump itself up to operating temps & pressure. That's my take on it anyway.

    Addendum

    Looking more at the info on the Michelin page, i could also interpret it as, no matter what the ambient temp, 10C or 40C, pump the tyres up till they match what Michelin specify and then put the warmers on or go ride.
    Based on this, for us road riders, before we ride, whatever the ambient temp, get your tyres to the pressures you want to run or say BMW's specs of 36/42, and go ride. :hmmmmm
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  16. petertakov

    petertakov Been here awhile

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  17. bmw_rider

    bmw_rider Adventurer

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    FWIW and interest.
    Last night, before i went for a ride today i set the tyre pressures. My target pressures were for the TPMS of my BMW 1250GS to read 34 Front & 40 Rear in use, relative to 20C Normal Temperature Standard.
    The temp in my shed last night was 12C. Using the chart above, i used my tyre gauge setting the front at 32.5C and the rear at 38.5 psi (my gauge reads in 0.5 psi increments).
    This morning when i started riding it was 9C and once the TPMS started transmitting after about 20 seconds of wheel rotation, the pressures displayed on the TFT screen were 34.1 psi Front & 39.7 psi Rear.
    I am ignoring any variation & discrepancy with tyre gauges & TPMS accuracy but in the ball-park imo.
    Temps during the ride went down to 6C and up to 11C and over 3+ hours of riding, the TFT TPMS readout remained the same, as it should.