Tire Pressure - What's "cold"?? | FerrariChat
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Tire Pressure - What's "cold"??

Discussion in 'Technical Q&A' started by chrisbinsb, Dec 12, 2014.

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  1. chrisbinsb

    chrisbinsb F1 Rookie

    Oct 20, 2011
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    Chris B
    #1 chrisbinsb, Dec 12, 2014
    I recently bought a cool bluetooth tire pressure monitoring system that sends realtime pressure info to a phone app and alerts me if pressure gets too high or low.

    My usual routine has been to check tire pressure periodically, almost always during the day and after I'd driven at least a short distance. Not because I thought this was best but just because I'd typically do it when filling up with gas.

    Since I now get low pressure alerts every morning, I starting looking into how to do this right and what exactly "cold" is. The best answer I get is that it's when the inside tire temp and exterior ambient temp match. One site advised that waiting 3 hours would give you this temp. But obviously, waiting three hours during the day, even in the shade, is going to give a different temp and thus pressure than first thing in the morning. Particularly in places with wide variation between night and day temps, this would seem to make a big difference.

    So I'm curious what's considered the best practice - check pressure first thing in the more when it's coldest? Check it during the day at typical daytime ambient?

    I realize I may acting a little neurotic here, but it's something I never gave much thought to before.
     
  2. PFSEX

    PFSEX Formula Junior

    Jun 30, 2006
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    John Ratto
    #2 PFSEX, Dec 12, 2014
    Check pressure at about 60-70F ambient and when the car has NOT been driven in the past many hours. If you check at a significantly higher ambient temp, then pressure should be a bit more than spec. At significantly lower temp - pressure a bit less. The tire warms up as it rolls and flexes, and its pressure will increase.

    I have no idea if this is the PROPER APPROVED way, but it seems to work.
     
  3. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,636
    #3 Mitch Alsup, Dec 12, 2014
    A tire is cold after the car has not been driven for 4 hours.
    The best cold is in the morning after the car has rested all night.
     
  4. EP328

    EP328 Formula Junior
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    Ed
    #4 EP328, Dec 12, 2014
  5. chrisbinsb

    chrisbinsb F1 Rookie

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    #5 chrisbinsb, Dec 13, 2014
    Thanks all. Guess the bottom line is that you fill the tirs first thing in the morning and that ambient temperature increases are expected. This would suggest that those in areas of minor temp changes will drive around at lower pressure than those with larger temp swings but I guess that's just how it is. Seems simultaneously very exacting and very loose.
     
  6. bill308

    bill308 Formula 3
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    Bill Sebestyen
    #6 bill308, Dec 13, 2014
    Chrisbinsb,

    I think you got it right first time, when the inside temperature equals the ambient temperature, there has been no heat build up due to tire flex and friction. This is the time adjustments should be made.

    Bill
     
  7. chrisbinsb

    chrisbinsb F1 Rookie

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    #7 chrisbinsb, Dec 14, 2014

    I guess what confuses me is that ambient temperature is different at different times of day. Could be 55 first thing in the morning and 75 in the afternoon. In that case and assuming the car hasn't been driven and is not sitting in the sun, which is the "right" ambient temperature. I think I may be overthinking this but it makes me nuts for some reason.
     
  8. wise3

    wise3 Formula Junior

    Oct 10, 2004
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    Ward Orndoff
    #8 wise3, Dec 15, 2014
    Setting them at 55 vs 75 degrees won't make much difference.
    But if you want to be precise, the goal would be to set the pressure for the ambient conditions that exist when you drive the car. If you do most of your driving at mid-day, setting the cold pressures at mid-day would be optimum (making sure the car has not been setting in the sun, of course). Of course, chances are you will be driving it in a variety of conditions anyway. Your daily driver probably goes to work in the morning when it's cool and comes home when it's warmer. So optimally you'd pick an average between those two conditions to set your pressures. Not that I've ever bothered to do this myself. ;)

    If you drive in the winter, and live in an area that gets cold, you want to set the pressures at that colder type of condition.

    Now if you really want to get picky, the optimum pressure for a given tire varies some with ambient temperature as well. How much? That depends on the tire. High performance "summer" tires in particular are less sticky if they don't get warm enough during operation, so you don't want them over-inflated in the winter. But we're only talking a Very few psi difference here, I'd say. (Then again, what's your definition of optimum? More sticky at the expense of wear?) For the average tire, I'd ignore this effect myself.

    Just my opinion; presumably Tire Rack is much more knowledgeable on the subject.
     
  9. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    #9 finnerty, Dec 15, 2014
    Make sure the tires have not been exposed to direct sunlight for any significant length of time before you set your *cold* pressures ---- even if the air temp is where you want it while checking, the radiant heat absorption into the tire will raise its internal temp substantially over *cold* as compared to the ambient air temp. ;)
     

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