Why do low tire pressures improve lap times?

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

  1. SPASKIS

    SPASKIS Registered

    Joined:
    Sep 7, 2011
    Messages:
    2,480
    Likes Received:
    774
    I completely overlooked at it. I was looking for a different name for it. My fault. No need to explain typical convection formula.

    According to the example values which I presume they are all in SI units. For 200 kph convection factor would be 8.4+0.6•200/3.6 = 42 W/m2K.

    This value is very low for high speed.

    The ground conductance value of 1000+0.03•P reported seems low for what I have read. Considering a load of 3000N spread in a 0.3•0.1 m2 patch would yield 100kPa. Overall conductance would be 1000 + 3000 = 4000 W/m2K.
    Reported figures in other papers suggest values way over 10000 W/m2K.

    I haven't still checked hysteresis values but I will. I am nit an expert in hysteresis so I will have to do some research.

    From my point of view the accuracy of main heating and cooling components has not been validated. They have fine tuned the numbers considering the average obtained final temperature but not in the theorerical correct way a simulation is validated.

    This yields to excessive temperature in front wheels in my opinion and too cool rear wheels.

    As an example of this. Recommended tire pressures for Radicals are basically impossible to achieve unless overinflating them and getting an uneven tire temperature distribution over the tread.
     
    Guimengo likes this.
  2. rocketjockeyr6

    rocketjockeyr6 Registered

    Joined:
    Jul 11, 2016
    Messages:
    184
    Likes Received:
    112
    Curious, what is that pressure?
    From what I looked up, according to the Radical Owners Manual, the non-HD(high downforce) models recommended hot pressure is 20-22; HD models recommended hot pressure is 28. In forums, many say they run 23-25 hot, depending on track/ambient temps.
    Is this the same for rFactor?
    Cheers
     
  3. SPASKIS

    SPASKIS Registered

    Joined:
    Sep 7, 2011
    Messages:
    2,480
    Likes Received:
    774
    Screenshot_20170908-075206.png
    https://forum.studio-397.com/index....-2017-now-available.56525/page-13#post-907540
     
    rocketjockeyr6 likes this.
  4. Andregee

    Andregee Registered

    Joined:
    Jun 23, 2012
    Messages:
    396
    Likes Received:
    47
    Where can I find the info about best tyre pressure for the different cars
     
  5. Ronnie

    Ronnie Registered

    Joined:
    Jun 15, 2012
    Messages:
    1,000
    Likes Received:
    132
    Thing is that recommended tire pressures are rarely best tire pressures.. Tire manufacturers often provide suggestions that they know is the safest option for them. So instead of looking for tire pressures online which if you find, they will most likely be the ones provided by tire manufacturer. Those actually used by performance driven teams are kept secret (maybe not secret secret but it's not sth that you can just casually find on the internet, like pricetag for toilet paper or stocks on wall street). Best way is to figure it out by yourself, trial and error, that means testing followed by testing and some testing on top of it. What I can tell you that most of the time recommended tire pressures from tire manufacturers are very high, much higher than it could and should have, but like I said their job is to keep it safe as much as possible.
     
    rocketjockeyr6 likes this.
  6. Raintyre

    Raintyre Registered

    Joined:
    Jul 28, 2013
    Messages:
    88
    Likes Received:
    30
    Rfactor 2 tires can usually be driven at 1kpa without much problem. We would like to have a RF2 tire model which is accurate on these kind of basic things, or at least we hope developers enable some multiplier to compensate wrong deviations. An RF1 styled 'grip vs pressure' multiplier would be enough.
     
    Guimengo likes this.
  7. rocketjockeyr6

    rocketjockeyr6 Registered

    Joined:
    Jul 11, 2016
    Messages:
    184
    Likes Received:
    112
    What Ronnie said. :D
     
  8. Lazza

    Lazza Registered

    Joined:
    Oct 5, 2010
    Messages:
    4,068
    Likes Received:
    541
    rF1 has an optimum pressure doesn't it? Because it doesn't do nearly everything the rF2 model does. I'm not sure it would translate well, and I'd suspect it's not the best solution to this issue. But agreed there should be something (with S397 in charge and likely not looking to rewrite the model, probably some sort of extra heat generation below a given pressure threshold).
     
  9. SPASKIS

    SPASKIS Registered

    Joined:
    Sep 7, 2011
    Messages:
    2,480
    Likes Received:
    774
    This is called hysteresis and is already implemented in the simulation. Very likely it is not correctly adjusted. Linked papers show a great contribution of this factor when testing with different tire pressures.

    In real life it is not easy to determine how much role plays in the heat and cool game each of the playing actors. There are several heating factors and cooling factors. In order to determine its relative importance, a set of controlled tests needs to be performed. Real lap telemetry is usually not a good starting point. It is however very useful for final confirmation of a previously fine tuned model.

    IMO these factors are not properly balanced in rF2. Tire sliding is greatly penalised. I wonder how much of the heat that is produced in a tire slide goes to the tire. The overall heat is easily calculated (sliding distance multiplied by friction force). However not all that heat goes to the tire. For sure part of it goes to the track itself. Especially if there is a significant rubber wear involved!

    Imagine how much of a difference would make a slight variation of this parameter.

    For me it is clear that thermal tire model is a bunch of unchecked figures thrown into simulation. A significant part of my job is validating the thermal simulation tools that we use for mould temperature prediction and believe me I do know what I am talking about.
     
    Raintyre likes this.
  10. Raintyre

    Raintyre Registered

    Joined:
    Jul 28, 2013
    Messages:
    88
    Likes Received:
    30
    >>>>>rF1 has an optimum pressure doesn't it?

    Yes, rF1 defines a optimum pressure depending on load but also the grip loss when you are off the optimum pressure for a certain instant load. ('GripTempPress' parameter)
    Just adding same parameters we could easily deviate optimum pressure from 20kpa to a more realistic range. If course it is a patch. But patches are good when they help to provide realistic results to users. I am only thinking on optimum pressure for best grip, which is the main interest of hotlappers and championship contenders.
     
  11. SPASKIS

    SPASKIS Registered

    Joined:
    Sep 7, 2011
    Messages:
    2,480
    Likes Received:
    774
    The reason for not going lower than real should be heating and wear. Precisely increasing contact patch is the basics of making wider tires when you want to increase mechanical grip. So reducing grip in lowest pressures is not realistic and should not be the way to go.
     
  12. Raintyre

    Raintyre Registered

    Joined:
    Jul 28, 2013
    Messages:
    88
    Likes Received:
    30
    The fact that surface temperature has so small effect on intermediate layers didn't seem very good, especially because both rubber and carcass were almost the same. We did an effort to achieve a faster layer conductivity, but numbers used on tgm file were very different than official cars. This is what we achieved with lots of tweaking. A bit more of corner effect on rubber, while carcass heats mainly with straight roll :
    [​IMG]
     
    TJones likes this.
  13. Raintyre

    Raintyre Registered

    Joined:
    Jul 28, 2013
    Messages:
    88
    Likes Received:
    30
    >>>>So reducing grip in lowest pressures is not realistic and should not be the way to go.

    Obviously, there is a optimum range for best grip. Try to run your real car at half of recommended pressure and tell me what happens.. And it will not be because of extra heat at all. The fact that you say that is very strange.
     
  14. SPASKIS

    SPASKIS Registered

    Joined:
    Sep 7, 2011
    Messages:
    2,480
    Likes Received:
    774
    For best grip or for best handling?
    Street car have very tall side walls. By the way not half pressure but you can certainly reduce 30% or a bit more and you will notice extra grip but very pronounced lateral wear.

    I once had a slow puncture. I used to inflate the tire once it got below a certain point. You could easily notice when it was about time to inflate it due to bad car handling. For 2.1 bar nominal cold tire pressure you would easily get as low as 1.5 before noticing laggy.

    In my opinion the problem of very low pressure is the pronounced reduction of tire lateral stiffness which affects handling making the car laggy in fast steering movements.
     
    Last edited: Sep 10, 2017
    patchedupdemon likes this.
  15. patchedupdemon

    patchedupdemon Registered

    Joined:
    Jan 3, 2017
    Messages:
    631
    Likes Received:
    352
    I always prefer to drive with higher pressures than lower due to the very sloppy handling characteristics,lower pressure give.
    Even if 1.5 psi lower is the optimal,I go 1.5 higher for better steering response,and stiffness to weight shift.
    I remember reading the reasons why race cars don't use massively wider tyres,something about once the tyres reach a certain size,the grip offered by the larger contact area is offset by other negative factors.
    I'll see if I can find the article
     
  16. David Wright

    David Wright Registered

    Joined:
    Oct 23, 2010
    Messages:
    111
    Likes Received:
    9
    Most race series specify the maximum tyre or wheel width.
     
  17. Comante

    Comante Registered

    Joined:
    Nov 20, 2013
    Messages:
    613
    Likes Received:
    261
    Spaskis, remember that aircraft tires and car tires,(as you refer to the NASA investigation on the subject) are built for very different purposes. So, You can't expect the observations made on one situation translate exactly to the other and viceversa.
    When dealing with aircraft tires, their main purpose is to slow down the aircraft during landing, even without the application of braking force. So, tire deform under the weight of the aircraft and doing so dissipate kinetic energy through heat. Car tires are really built for the exact opposite reason : to roll dissipating less energy as possible, while providing the maximun grip avaiable.
     
  18. Lazza

    Lazza Registered

    Joined:
    Oct 5, 2010
    Messages:
    4,068
    Likes Received:
    541
    If you had a parameter increasing heat below a certain pressure, the wear will automatically increase because of the existing parameters.

    But then while we're there, probably add some rolling wear... zero wear to 8 decimal places along a long straight doesn't quite seem right...

    Point is if the parameters are there the competent modders can use them, other people can copy realistic figures from the base cars or good mods, and the beginner or hack modders can break things just as well as they already can. And S397 doesn't need to spend time working on a model that, for most purposes, already provides a good enough base to work from. Some people do still complain about aspects of the handling, but it seems the general consensus is that it's pretty good. So just do some hole-filling so that there aren't any obvious omissions and let's get on with it. No doubt we'd end up with quite a few parameters, but only the modders need to worry about that.
     
  19. patchedupdemon

    patchedupdemon Registered

    Joined:
    Jan 3, 2017
    Messages:
    631
    Likes Received:
    352
    Wear only happens to any significant degree when accelerating ,braking or cornering.
    There has to be shear and slippage to tear the rubber,so only the start and finish of the straight would add to wear.
    when there's no slippage,the contact point between the ground and tyre is not moving,why would any significant wear happen in those circumstances.
    I may have Miss read the article that told me that,but oh well.
     
    Last edited: Sep 11, 2017
  20. Lazza

    Lazza Registered

    Joined:
    Oct 5, 2010
    Messages:
    4,068
    Likes Received:
    541
    After 1000000km?
     

Share This Page