Why do low tire pressures improve lap times?

Discussion in 'General Discussion' started by davehenrie, Sep 4, 2017.

  1. burgesjl

    burgesjl Registered

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    Emery is fairly close with his assertion about the tire model. Real world tires have two highly likely failure points. First is the shoulder, where the tread of the tire meets the sidewall. This is an important part of the construction of the tire, especially in the 'green' state and before it gets put in the molds and cured. In 'green' state, the tire has very little rigidity, and can get damaged, so you have to handle them carefully. Typically the shoulder will overheat if the pressures are set too low, and you get a failure right there. The other potential place is the bead, but this usually happens in certain racing series like oval racing, where there is insufficient cooling to the bead often as a result of blazing hot brake discs. Again, its a temperature-based failure, the beads melt and the tire can't stay on the rim.

    iRacing also have the same problem in that low pressures are fastest, and there is no penalty to lower them. Despite the fact that, logically, there should be a pressure below which the tire doesn't have sufficient rigidity. The temperature effects in iRacing are incorrect, which leads to the pressures inside the tires (buildup) to also be incorrect. And they have freely admitted they have troubles modeling the shoulder, which is course is directly impacted by camber settings, which means those are all wrong too. In real life, the tire companies recommend increasing tire pressures, which also has the effect of REDUCING the temperatures in highly critical areas. But we see the exact opposite; in hot track conditions, DECREASING starting/cold tire pressures always works. Now iRacing does also have tire rigidity and compound issues that make the tires very poor during initial starting conditions; so you run the risk of the car spearing off the road until you get up to 'operating temps' - but if you start at more reasonable pressures, then the tires overheat.

    BTW, I did see some in here question the validity of the heat flow model in iRacing. It's actually correct. The tire loses more heat into the ground, than it does into the air. Sadly they don't show the infrared cameras on F1 tires any more, but it clearly used to show more blue (cold) on the contact patch with the ground, than red (hot) for the rest of the tire. And when rolling, they don't cool down that fast. The heat generation in the tire is due to micro-stretching (high frequency), the same as you get when you pull quickly on a rubber band - its not primarily caused by road surface friction. I believe the rF2 model doesn't directly model track temps (localized) in their real road model. The tires do in fact heat up the track surface locally. So neither game does a complete job of modelling the tire temps and pressures or failure modes. (I believe iRacing tires can fail, but they don't have punctures or flat spots or heating at localized lock-ups, though they just added in some code to separate that effect from other contact patch temp calcs in the last build).
     
  2. patchedupdemon

    patchedupdemon Registered

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    That last patch was more about lowering the pressures,that's why all the cars failed tech which received that update and had to have the ride height highered by a substantial amount,that code they said they introduced didn't really sepereate the heating effects from contact with the raid to the rest of the tyre,the tyre is still made up of only longitudinal bagels which have no ability to sepereate cross sections of those bagels.
    Weirdly iracing tyres only have one side wall,on the outer portion of the tyre,that's why you can never get the outer temps within 30 degs of the inner temp,I've asked why but never got an answer
     
  3. Lazza

    Lazza Registered

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    @burgesjl I could be wrong, but I'm not sure it's true that rF2 tyres don't transfer heat to the track (I should say, they don't lose heat due to contact with the track - for sure the track doesn't receive any). But there's no doubt changing track temperature doesn't affect it, so if it does happen (on the tyre side) it's to a single hardcoded track temperature.
     
  4. SPASKIS

    SPASKIS Registered

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    I would state that it is exactly as you explain. There seems to be a hardcoded track temperature.ñ for heat exchange calculations. Leaving the car stopped for several minutes should carry tire temps to the temperature that is being used in the heat exchange formula. Somebody tested this in the past and reported it. Probably easier to repeat the test than to find the post.

    I haven't checked especifically but it should be quite easy to determine which of the main real variables affecting tire cooling are being considered in the simulation:

    - contact patch area. There should be significant differences between extreme pressure values. Low surface (high pressure) should provide lower heat extraction compared

    - tire temperature: the higher the tire temperature, the higher the cooling rate.

    - speed: the higher the speed, the higher the heat removal. Especially from the air. For ground contact it should only increase from 0 up to certain speed above which it would estabilize. For testing straight line testing is required and this it is necessary to eliminate toe in from the tested wheels in order to minimize heat build up due to slip angle. There is some heat contribution due to hysteresis in the tire rubber that can only be minimised by increasing pressure but this will affect

    - track temperature: already commented. It seems to be fixed for calculations. Stopped car testing will take temperatures to track temperature. This considers air cooling effect being 0 when the car is idle.

    - air temperature. It can be checked whether modifying it makes any difference or not. In order to account for its accurate effect the rest of the cooling/heating parameters should be under control.
     
  5. patchedupdemon

    patchedupdemon Registered

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    We had a 13 page discussion about airs ability to cool the tyres,and most people were saying it's negligible because David kaemmer said so lol.
    I don't agree that it's negligible at all,it has to have an effect especially on open wheel cars
     
  6. SPASKIS

    SPASKIS Registered

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    I remember that discussion. I myself expected a higher value for this. However after documenting myself I found a very good paper from NASA regarding air planes landing. It was clearly stated that ground effect was clearly the main factor.

    If I can find it again I will post the link.
     
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  7. Comante

    Comante Registered

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    I disagree with the notion that a still tire on the ground should receive cooling from the air. Tarmac is not a heat source, his temperature is the result of environmental factors. A tire, still on the tarmac is subject to the exact same environment factor. So yes, they will be at the same temperature at some point.
     
  8. Comante

    Comante Registered

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    Air cooling is considered negligible for a moving tire, because even a tire without camber and toe angle generate heat rolling, it is quite evident that heating is more than cooling, so, in math, you can consider the final value and call it a day.
     
  9. TJones

    TJones Registered

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    Air cooling and sliding friction are the major factors of tyre surface temperature in rF2. While tyre flexing generates heat through the whole tyre.

    This is a chart of a stint over 14 laps at Silverstone int. with the McLaren 650S GT3. The four rows are the tyres in order: FL-FR-RL-RR.

    Tyre.JPG

    Easy to see that the surface temps (I,M,O) are fluctuating a lot while rubber temps (I,M,O) and carcass temps (green) are pretty steady, and rise only slowly over a couple of laps.
    In rF2 there is an influence of air and road surface on tyre surface temp. But air and road temps are static ATM, so changes in the session menu doesnt have any effect on simulation.

    I think tyre readings are a lot better with the latest cars, in comparison to older ones, the Corvette c6r for example. Tyres feels more grippy, more "rubber like".
    Also the influence on handling with different tyre pressure are very good, the Brabham BT44 with his huge bias-ply tyres are a good example here.
    Flat spotting and the effect on FFB are spot on IMO.

    Where i still see some room for improvement is the grip in relationship to tyre surface temp, i think there should be less grip, while the temps are outside the normal working zone, so lets say lower than 60°C and higher than 110°C.
     
    Last edited: Sep 6, 2017
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  10. patchedupdemon

    patchedupdemon Registered

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    A rolling tyre at a constant speed creates no heat due to no slippage or sheer.
    Obviously we are talking about this when air isn't involved because during acceleration braking and cornering too much heat is being produced.
    So basically on the long straights people were questioning airs ability to cool the tyre.
     
    Last edited: Sep 6, 2017
  11. TJones

    TJones Registered

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    It still generates (inner) heat, because of rubber/carcass flexing.
     
  12. rocketjockeyr6

    rocketjockeyr6 Registered

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    Which?
    Formula cars, DTM, V8 SuperCars, these are most definitely bias-ply.
    Im not sure about WTCC, dont really follow that, but most GT's are on bias-ply slicks.
    Most slicks, outside of DragRacing, will be bias-ply tyres.
    Karts... not sure anyone makes a radial tyre that small. :D
     
  13. David Wright

    David Wright Registered

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    Even at constant speed there is some slippage. And of course there is also heating due to the tyre flexing.
     
  14. Jokeri

    Jokeri Registered

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    never mind, I read you text wrong
     
    Last edited: Sep 6, 2017
  15. rocketjockeyr6

    rocketjockeyr6 Registered

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    https://www.isma-isaac.be/past/conf/isma2010/proceedings/papers/isma2010_0264.pdf

    Standing waves my friend. At least in F1's Pirelli issues. ;)

    https://www.motorsport.com/f1/news/pirelli-fears-standing-wave-issues-on-baku-s-straights-782624/
     
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  16. SPASKIS

    SPASKIS Registered

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    Here is the article I mentioned.
    https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760007024.pdf

    - The effect of hysteresis seems to play a very important role according to several papers where there is a significant temperature increase at low pressures.
    http://www.dtic.mil/dtic/tr/fulltext/u2/a566809.pdf
    https://pdfs.semanticscholar.org/4323/b8af2fdc080fef3fa2dc8b279f0593f4edbe.pdf
    - air cooling should not be neglected directly. The way NASA approaches is the correct one. They cross simulation results with real test results and check for correlation. The big error from the other papers is that they are assuming values for ground conductance and air conductance coefficients and throw them in the simulation. They have no evidence for those values that believe me are difficult to obtain. Convective and conductive coefficients are very uncertain in real world and require testing and usually a correlation with a simulation to be obtained. They are actually deduced more than measured. Open wheelers have huge relative speed between tire and air. I would never neglect its effect.

    - from a recent thread you can also see what parameters are accounted in the tgm file for tire heating/cooling. If what is written there is complete, it seems that tire radiation cooling is being considered. Any thermal expert knows that radiation is really negligible for typical tire temperatures. Forced convection with air is several orders of magnitude higher at those temperatures. However there seems to be no parameter for that. Please correct me if there is one since I am basing in this thread.
    https://forum.studio-397.com/index.php?threads/sliding-and-tire-heat.57642/#post-911616

    IMO rF2 devs did the same as in the last two papers. They just put some values that probably were not obtained with a NASA approach and granted them for good. It should be noted that tire builders do not release publicly this type of information as Gjon himself stated in an interview and even saying that in some cases they had themselves to buy some tires to do testing an correlation with simulation. But up to what extent? With how much resources considering the huge amount of money required to do proper testings? Let me have my reservations about this.
     
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  17. patchedupdemon

    patchedupdemon Registered

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    I read a PDF on tyres I think you link for me a few months ago,and it cleared up a misconception I had about a tyre generating heat on the straights.
    I thought the tyre would be generating too much heat for the road to cool the tyre more than air,but that is clearly not the case.

    Thanks for the links,this will be great reading.
     
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  18. TJones

    TJones Registered

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    Just learned there are indeed still bias-ply (besides radial) tyres in use for GT-cars, wasn't aware of that.
    Formula one cars are using radials since late 70's or early 80's, sorry dont know for sure (Michael Borda wrote about in his blog).

    Sorry for confusion. :)
     
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  19. rocketjockeyr6

    rocketjockeyr6 Registered

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    No worries at all.
    I do believe that the Bridgestones, Firestones, and Michelins were indeed radial, but the current Pirelli's are definitely bias. Im not sure if a radial tyre could experience standing waves, but I think that is a primary benefit of the steel belt.
    I know a lot of sportsman level drag racers use steel belted radial slicks, as they keep the car more stable at high speeds with such low pressure(usually 3-7psi in the rear, depending on tyre size and vehicle weight).
    What GT class uses radials? Apart from the wet weather tyres. Here in America, we have the Pirelli World Challenge, which uses Pirelli PZero Racing Slicks, which are bias as well.
     
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  20. Led566

    Led566 Registered

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    Well, as far as I can understand, the
    "ExternalGasHeatTransfer=(8,4,0.6) // (base, mult, power) - heat transfer coefficients to external air = base+(mult*(vel^power)), where vel is linear velocity of tire"
    in the Jokeri post is exactly what you are looking for: convective cooling of tire to external air.

    Then the equation in typical engineering form (cfr. Bird, Lightfoot - Transport phenomena) should go like this :
    Q=U*A*DeltaT
    where Q=Heat lost to external air, U=overall heat transfer coefficient to external air, A=surface of tire cooled by external air, DeltaT= delta Temp between tire surface and external air.
     
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