Tire pressure fluctuation from heat

Discussion in 'General Discussion' started by rocketjockeyr6, Aug 30, 2017.

  1. rocketjockeyr6

    rocketjockeyr6 Registered

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    Curious,
    I ran a 20 min race a Zandvoort today, and my LF tyre went from 20psi up to 35!
    I have an education in automotive technology, and Ive served a full tour of duty in a shop; but I dont have much experience with race cars themselves.
    In my time, Ive learned that normal pressure fluctuation is 3-5psi(2-3 with Nitrogen, and dont most race cars run nitrogen now?), a really hot tire Ive seen up to 8 pounds over the cold pressure... but 15 psi seems a tad excessive to me.
    Can anyone with real race car experience(yours, a friends, whatever) confirm or deny that this is feasible?
    Many thanks.
    rocket
     
  2. Paul Stante

    Paul Stante Registered

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    Basic Physics.
    Look up 'pressure temperature relationship'
    Basically the hotter the tyre gets the higher the pressure.

    The pressure law states -
    "For a fixed mass of gas, at a constant volume, the pressure (p) is directly proportional to the absolute temperature (T)."

    I think the key word in this law is 'proportional'
    This law is what makes refrigeration possible.
     
  3. rocketjockeyr6

    rocketjockeyr6 Registered

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    Yeah, I know the basic principle is ~1psi per 10 degrees F, Ive just never heard of tyres over-inflating that much.
    Was curious if anyone had ever seen that kind of increase.
    Also, as I stated, I know a lot of race teams use nitrogen, as there is less moisture in it, and its the expanding moisture that causes tyre pressure to increase.
    I.E. if you inflate your tyres on a humid day, they will be more susceptible to pressure increase, as compared to a non-humid day or a less humid gas(like nitrogen).
    Ive spent a fair amount of time googling, but as you can probably understand, most teams/drivers/crew chiefs dont publish their tyre data.

    Basically, Im hoping that someone with experience in the pits can confirm or deny whether or not a 15 pound increase in 20 minutes of driving is even possible.
    We are about to have a championship with this mod and it will be 1hr races. I wonder what the pressures will be like by the end... :rolleyes:
    Cheers all
    rocket
     
  4. Comante

    Comante Registered

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    Differently from road cars, race cars heat the tires up to 100 degrees C, maybe even more. In real life even brake heat contribute to this as heat go from the brake to the drum then the gas inside the tire. Maybe the tire overinflated because the car was overdriven, and that tire suffered too much.
     
  5. patchedupdemon

    patchedupdemon Registered

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    Dare I say it,over driving can cause huge temp spikes,were you locking that tyre much,there's some real fast right Hand turns that really scrub the front left at zandy.
    Weather or not that's enough to cause the tyre to increase by that much pressure I don't know
     
  6. rocketjockeyr6

    rocketjockeyr6 Registered

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    Oh, it was definitely over driven. :D
    Not brake lock, but Im still not used to FWD cars, so theres tons of understeer going on.
    Next time I do a drift session with the Supra, Ill see what the temps do.
    I know about brake heat and thermal dynamics, and Im pretty sure rF implements that as well, and theres tons of hard braking at Zandvoort.

    I guess this really means that Ill have to set my cold pressure to ~16-17 psi up front...
    thanks guys
     
  7. Lazza

    Lazza Registered

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    No, brake heat doesn't affect tyre temps. There's no transfer there. (in rF2)

    I've seen 10psi increases mentioned from real racing (7-8 even for street legal), so while 15 seems high I suppose it's not out of the question.

    What I'm a little more concerned about, and bear in mind I might have missed something here, is that the tyres in rF2 don't seem to define the internal 'gas' expansion rate at all. I suppose this means it assumes 0% moisture in the tyre at all times. (oxygen or nitrogen doesn't really matter, it's about moisture expansion). That makes a large pressure change even more extreme in terms of realism, at least compared to tyres filled with normal (humid) air.
     
  8. SPASKIS

    SPASKIS Registered

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    The only expansion allowed is the deformation of the tire itself. The increase in temperature is therefore translated into pressure. In a case that could be considered almost a constant volume transformation. Gases do not expand if they are constrained.

    Humidity shouldn't make any difference unless condensation occurs at the moment of initial inflation of a tire. This can only happen if the used air has a relative humidity close to saturation. Vapor saturation is reduced as pressure increases. Therefore a 100% humid air being introduced with a manual pump (the one many people have in the trunk) would certainly show some condensation when being pressurized. When the temperature of the tire raises, so does the saturation limit. In that case liquid water would vaporize again and increase the pressure of the tire more than if condensation hadn't taken place during the initial cold inflation.

    However typical teams would have systems where compressed air is at considerably higher pressure compared to tire values. Therefore, condensation in the inflation of a tire is impossible in that case since the gas undergoes an expansion.

    So in my opinion the only doubt is if the tire volume deformation is considered when determining the equilibrium pressure of the tire for a given working temperature. This can be addressed with telemetry study after performing several tests.
     

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