Internet thread: Share the fun!

Discussion in 'General Discussion' started by John.Persson, Aug 12, 2012.

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  1. DrR1pper

    DrR1pper Registered

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  2. JJStrack

    JJStrack Registered

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    Interesting video, yes, but much of what he says on the physics-side of all this is wrong or dumbed down a lot :confused: Just as an example, his interpretation of the points on the longitudinal-force/slip-ratio/wheel-load graph is completely wrong - pointing out the peaks and saying you locked up your breaks there etc. And i wonder, why he, as a veteran of the business, doesn't even know the man is called Pacejka, not Pajeczka...
    Leaves me with very mixed fealings about this video and man :(
     
  3. DrR1pper

    DrR1pper Registered

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    When does he say that?
     
  4. JJStrack

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    25:12 and on...
    Also strange is his mentioning of "drifting" at 21:18...and poiting out, that you counter steer because you are in that region of the graph he points out...which is completely out of context.
    And still, i clench every time he says Pajeczka :p
    But I'm still not through the whole thing yet
     
  5. DrR1pper

    DrR1pper Registered

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    hmm...at what longitudinal slip angle should you have locked the brakes?
     
  6. JJStrack

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    In longitudinal case, you have slip ratio. locking your brakes stops your tires from spinning which gives you a slip ratio of 100% and way lower longitudinal forces than the peak. The peak forces are actually achieved somewhat around 15% slip-ratio, depending on your tire. For an example, traction control systems of electric race cars are programmed to regulate the exact optimum slip-ratio, to get the best acceleration performance out of your tire - you mesure your front tire (not driven) speed and regulate your rear wheels to about 115% or 120% of that speed.
     
  7. DrR1pper

    DrR1pper Registered

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    Are your sure JJ?

    I thought that longitudinal grip continually builds up, up until the moment of locking (100% slip ratio). Threshold braking is all about how close you can get to the locked state without actually locking and if this were not the peak amount of grip then drivers (and abs systems) would not be offering threshold braking at all but actually something less efficient.
     
  8. Saabjock

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  9. Esteve Rueda

    Esteve Rueda Registered

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    I'm not 100% sure, but I think JJStrack is correct and in that video he shows a simplified version or something wrong. Because I knew since I'm intereseted in sims, that optimal slip ratio is around 15% in lots of tires, not exactly that number, depends in tire. And in brake conditions you will lock your tires when the brake system is aplying more force than "ground" to the tire, in theory you will lock your tires if you do not release the pedal when you are reaching more than 15% slip ratio (example ratio, and can be lower or higher than the acceleration optimal ratio because of tire structure).

    Example, If you imagine a braking... imagine an ideal situation where brakes and tires do not change their temperatures (temperatures change grip and brake force during the whol braking zone constantly). You are going at high speed, no need to know such speed, and you gradually apply from 0% to 100% brake pedal. Gradually you are applying a braking force to the tire, even higher is that force, higher will be the slip ratio tire is suffering because od deceleration. If optimal slip ratio in braking situation is 15%, you will get the BEST deceleration and braking efficience, you are applying a braking force to the tire, the same as the tire is suffering from the ground (better slip ratio = better grip). Imagine you achieve that slip ratio when you are at 85% of your pedal brake, If you go to 90% brake, you will be applying more braking force to the tire than ground and at the same time slip ratio will be higher, that produces a lower grip (you reached optimal point and grip go down). In this point tire "collapse" and you lock, beacuse you reached the optimal point and you are applying more force than ground to the tire, this means ground has not enought strong to keep the tire spinning.

    I think is a good simplified explanation. I'm sure I did not miss the basics of the tire. It's like when you are accelerating from 0km/h, you controll the throttle to stay around 15% slip ratio, and when you reach that slip because you apply more throttle, your tires suddenly spins out of control because the balance is broken, tire go to more than 15% slip ratio, you are applying more throttle than tire can support, and this pruduces a no controlled spinning tires. At this point you have to release you thottle a lot to compensate the high slip ratio and low grip tire is suffering.

    I also have to say that this situation I exaplined It's ideal, during the brake zone the slip ratio and brake forze applied by driver changes during the whole braking zone. Tire temps, bumps, downforce, and more variables change the force is being suffering the tire from ground side, and his slip ratio changes. Also brake temps increase more or less depending in the force being applied from driver, this constantly changes the force being applied from brakes to the tire in the other side. This is why braking is so awesome, a driver, even simdrivers (is something I love in rF2) should manage all this variables in every braking, It's because of that braking som cars is so hard, or so easy when your muscles are used to his particular brakes.

    PS: I just saw that point in the video... and Braian is not saying at 100% slip tires will lock, he is actually saying that tire will lock when you reach optimal slip ratio under braking, loock at the graph. I think the main difference between "sims", is some of them seems like are just applying Pazejka curves, and grip is not being altered by thermodynamics or temps are not being simulated in a complete way. If something rF2 is doing great with his tire model is just this point, tires work as actual tires, and changes temps in a natural way, this is what affects grip in every stage of the tire, longitudinal and lateral forces. Seems like Its simulating different layers of the tire, outside (affects directly the grip), and inside the tire, different layers compensate themselves creating the temp behaviour we see in the external layer but this is something more complex to explain here. Other sims just use a simplified temp layer, and because of that temps are simplified and change not in the same way they should, this afect directly to grip, and if you are in critical grip point and tire It's not increasing his temp realisticaly, tire will not behave reallisticaly in the limit because will have a more stable grip, will have the "dumbed down physics" effect we see in other sims.
     
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  10. Jamie Shorting

    Jamie Shorting Registered

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    I believe it's less than that. I know during braking a slip% over 10% would be considered a lockup. Ballpark numbers though I guess. Tires are all quite different and very complex.
     
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  11. Esteve Rueda

    Esteve Rueda Registered

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    Well, is just a example slip ratio and I can be wrong in the number, every tire has his optimal slip ratio and slip angle. And yes, when you reach the optimal slip ratio you will lock your tires if you do not decrease brake force as I explained before, the tire will stop spinning because of applied forces (considering lock = no spin).

    All these numbers are orientation numbers, can be little mismatches because of infinite and hard to control variables in real life, but the principle does not change, just the variables.
     
  12. Hectari

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  13. JJStrack

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    The numbers really depend on the tire. Last tire i looked at, it is 10% if you drive straight on (slip angle = 0°)
    View attachment 11725
    In the screenshot you can see graphs i made, using tire-testing data. The data is a bit "noisy", if you want to call it like that in this case, but you can see the point of it.
    You can indeed see, that peak-force is reached at 0.1 or 10% slip ratio at acceleration (positive slip ratio) and a bit more under deceleration (negative slip ratio).
    In 3D you can see, that this is the kind of graph that was shown in the video, but from test-data, not from an equation and in a different coordinate-system:
    View attachment 11726
    However, if we now look at the mixed forces, the position of the peak changes:
    View attachment 11727
    What you see here, is the Slip-Ratio/Longitudinal-Force graph, shown at three different slip angles. As you can see, at higher slip angles, the graph gets more flat and peak forces are reached at about 15% positive slip and again a bit more at negative slip.

    As is said, it all depends on the tire. Thats why you need such tire-testing data.

    PS: esteve very nicely explained, why you can easily lock up your brakes beyond that peak point, if you don't have assistence.

    PSS: i should add, that the 3rd graph was taken at an inclination angle of 2°. Thats why there is such a big difference between negative and positive slip at slip angles not equal 0°.
     
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  14. DrR1pper

    DrR1pper Registered

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    The issue you highlighted in brian's video JJ doesn't actually make sense in of itself since the bottom of the longitudinal response graph appears to be in slip angles. What is a longitudinal slip angle? Is there even such a thing?

    I'm not saying this is the case but if you imagine that the values along the bottom were slip ratio and more some reason scale factored by 100x (perhaps to represent as a solid percentage?) then the peak and trough values occur just before 10/-10 (or if scaled for some bizarre reason, 0.1/-0.1)
     
  15. ZeosPantera

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    When a sim can handle this.. I will support it fully.

     
  16. MikeeCZ

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    Guys, he was speaking simply so normal people can understand what is he talking about

    Threshold breaking (the peak) is .. .relatively long before actuall lock up, long for a racing driver who is very skilled, but just a half a mm on the pedal difference for a common driver. therefore for a common person it is simpler to say "when you lock up your wheels"

    The same is applied when he mentions the "slip correcting by countersteering the wheel". because in essence he said it right, only in case of countersteering the problem is a lot more complex because you steer with wheels that arent slipping in order to get the wheels that are back onto or below the peak.

    So the only thing he got wrong was the actuall name Pacejka, which COULD be easily caused by something so simple as for example he could learn it wrong the first time he has heard it in his life and simply memorized it that way making it very difficult to re-memorize it, im sure all of us had the same with some other case

    I think it is very good video and the lad really loves what hes doing
     
  17. Nimugp

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    Not the most exiting video to see, BUT THE SOUND!!!!! [​IMG]

     
  18. DJCruicky

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  19. Jamie Shorting

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    Some rich Swedish guy I guess.

     
  20. Jamie Shorting

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