Motion friction vs. static friction and where are the coefficient values from?

Discussion in 'Car Modding' started by vilivili, May 3, 2012.

  1. vilivili

    vilivili Registered

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    In Renault Megane's tyre file there's static friction coefficient 1.0 and motion friction coefficient 0.64.
    Where is this 0.64 coming? Is it tested to be a correct value for some kind of a slick tyre?

    edit. actual numbers are 3.00 and 1.92 respectively if someone's confused


    I'm just curious how has ISI ended to this 1.92 'cos it means that when the tyre is sliding there's only 64% (1.92/3) grip left and when the slide ends there's suddenly 56% (3/1.92) more grip.

    I think this is why it "feels like driving on ice" for some drivers.
    I think motion friction coefficient should be more than only 64% of the static friction coefficient
     
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  2. vilivili

    vilivili Registered

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    and Nissan GTR tyres have numbers:

    StaticBaseCoefficient=3.075
    SlidingBaseCoefficient=1.968

    meaning motion friction coefficient --> 1.968/3.075=0.64 !!!!!

    ISI how did you come up with these numbers?
     
  3. jtbo

    jtbo Registered

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    You are still missing some parameters.

    These are very important ones, you can find explanations from skip barber tires for those curves, worth to check because there are other new comments too.
    StaticCurve=(173, 0.61, 373, 1.176, 673, 0.61) // at -100C there's 52% of maximum static grip, at 100C it's maximum, at 400C it's back down to 52% of max static grip
    SlidingAdhesionCurve=(-9.2, 0.4, -5.2, 1.7, -1.2, 0.2)
    SlidingMicroDeformationCurve=(-5.2, 0.3, -1.2, 1.8, +1.8, 0.3)
    SlidingMacroDeformationCurve=(-1.2, 0.2, +2.3, 2.0, +5.8, 0.4)
    RubberPressureSensitivityPower=(-0.087,5000,500000,1) // power,offset,nominal_max,normalize

    There is probably even larger change in grip when sliding because of other parameters, all those together + track's grip makes total grip.

    For example normal street tire is said to have friction coefficient of 0.7 to 0.8 on tarmac, that is sliding, afaik. Static is over 1, maybe close to 2?

    Toyo AR1 semi slick racing tire had bit under 1.5 from my memory, that too is sliding, imo. Static friction of rubber on tarmac I have seen being around 2, but rubber is not a tire.

    Tire is bit different from rubber and very difficult to find any proper numbers for that.

    Without touching staticbase and sliding base, you can get lot more slide friendlier handling by playing those curve parameters, car's inertia and sensitivity power, I'm not all too sure that it is really required for sliding coeff and static coeff to be closer to each other as there are those other parameters that change pretty much everything for tire's grip.

    As usual, ISI probably has had access to data that they can't share because stuff is secret, but I'm not sure if it is experience of their's or data they have received/sourced that values are based from.
     
  4. DrR1pper

    DrR1pper Registered

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    Did you mean 46%? :p

    I think these numbers come from empirical testing of these tyres (by ISI if data has not been shared directly from manufacturer or racing teams).

    "Truth is stranger than fiction, but it is because Fiction is obliged to stick to possibilities; Truth isn't." - Mark Twain
     
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  5. CdnRacer

    CdnRacer Banned

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    You'll never please everyone. Megane driving on ice? :eek: I don't get that feeling at all with it.
     
  6. Lazza

    Lazza Registered

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    Well, remember it's not 'sudden' - the tyre slowly loses static contact so will move towards the lower grip value, getting there when it's completely 'sliding'. And I think the difference between the two is quite normal - if you look up static/sliding friction tables some materials have a much bigger dropoff.

    If you've ever tried to push/drag something heavy across a floor you can probably appreciate that once you get it moving it's a lot easier to keep it going than it was to budge it in the first place - and that's not all about inertia.

    No, 3/1.92 = 1.56, so a 56% increase of sliding friction to get back to static friction. Nice Twain quote though ;)
     
  7. DrR1pper

    DrR1pper Registered

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    Christ, i even got my 46% wrong. I meant 36%. (100% - 64% = 36%) - incidentally sounds about right, i remember Martin Brundle once commenting that when you slide a tyre it looses approximately a 3rd of its maximum grip.

    If sliding causes tyre CF to drop from 3 to 1.92. Then grip level is reduced to 1.92/3 = 0.64 = 64% of max grip. Once the tyre has stopped sliding, it regains the percentage of CF lost (3/3 - 1.92/3) = (3-1.92)/3 = 1.08/3 = 36%.

    And thanks, i love that Twain quote. :p
     
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  8. Lazza

    Lazza Registered

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    If I lose 50% of 100, I end up with 50. Getting back to 100 requires getting back 50% of what I started with, or 100% of what I ended up with.

    So you're adding a percentage of 3, I'm adding a percentage of 1.92. I'm saying a 56% increase of sliding friction, you're saying a 36% increase of static friction (to get to static friction). So we're saying the same thing, but in different ways :)
     
  9. DrR1pper

    DrR1pper Registered

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    Oh, son of a gun! I'm sorry.

    I...

    [​IMG]

    :p
     
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  10. vilivili

    vilivili Registered

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    that's perfectly true.
    But I'm thinkin the last time I was on racetrack. I think there's plenty of grip even the tyres are sliding. I'm suspicious if those numbers (3.0 & 1.92) are bit too far away from each other.

    Also cars equipped with anti locking brake system (abs) stop using static friction. average car can do from 100kmh to 0 in 40 meters or so.
    So disabling abs means tyres will lock and car is stopped using motion friction. If motion friction coefficient is 64% of static friction coefficient the distance required to stop the car is 40m/0.64=62.5m
    so 40m abs on
    and 62.5 abs off
    I think something's wrong and it is the motion friction coefficient.
    atleast I'd like to hear where is this number from?
     
  11. Lazza

    Lazza Registered

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    Do you have access to any real data from such a braking comparison? Bearing in mind you're not comparing ABS to non-ABS (which will tend to favour the non-ABS for a skilled driver), but 'ABS or similar' to 'all wheels completely locked up'.
     
  12. jtbo

    jtbo Registered

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    I don't know too much about racing tires, but I do know that they share round shape and black color with street tires, pretty much everything else is different.

    To make good comparison you could increase brake torque so that you instantly lock tires, then set ABS so that it does not add grip (which it does by default setting) and set efficiency to 1.0 which means full brake pressure modulation is allowed from 0 to max, instead of only fraction of pressure.

    That way you would get some reliable results.

    After you do that test, you can then compare results to your grip difference calculation and check if tires actually get so much reduction, often things just are not quite how they seem to be.

    Try these settings and use high abs:
    ABS4Wheel=1
    ABSGrip=(1.00, 0.0)
    ABSLevel=(0.5, 1.0) // ABS Setting LOW, HIGH

    Before that set brake torque to at least twice, or more so that you get instantly locked wheels, ABS will do it's thing anyway and it is easier for you to test locked wheels when they will truly get locked, so there is no contamination of data from partly rotating wheel.

    I have done my brake testing with GTR in dev mode, perhaps one of the most criticized vehicles and has fairly big difference between numbers.

    You are probably using Motec to check braking distances?

    To remove realroad variance in grip, you could also use specific test track, our test facility is good in there that you get constant grip so that does not mess with your results http://isiforums.net/f/showthread.php/8510-Simple-flat-area-setup-for

    It is very important to make sure that there are no other variables affecting results.

    I have only data for winter tires easily on hand now:
    [​IMG]

    47m locked wheel and 35m ABS (what ever percentage that makes), street winter tire, summer tire is perhaps bit better than that, racing slicks probably complete different kind.
     
  13. KeiKei

    KeiKei Registered

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    I'm pretty surprised to see those values as I've always thought skillful non-ABS braking is the most efficient way in every situation. I think those table values could be explained by the fact that street cars have too much front-biased brake balances. Only odd values in those are that on asphalt ABS braking is shorter than skillful non-ABS braking.

    For those who don't speak finnish:

    Nastarengas: Studded tire
    Kitkarengas: Non-studded tire (friction based)
    Asfaltti: Asphalt
    Lumi: Snow
    Jää: Ice
    Lukkojarrutus: Lock braking
    Jarrutus: Braking
    Optimi: Optimal
    Ei: No, non
     
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  14. vilivili

    vilivili Registered

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    unfortunately I didn't found any measurement for tyres locked up braking distances. but some interesting discussion was found which generally ends up no big difference on dry conditions.

    www.ffcars.com/forums/35-autocrossing-prosolo/215536-abs-vs-non-abs-stopping-distance-question.html
     
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  15. jtbo

    jtbo Registered

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    I think locked wheel braking is generally quoted to be 25-30% longer, for high performance racing slicks however tends to be always different rules.

    For example drag racing tires perform lot better when sliding, friction coefficient is better when sliding, so it is impossible to do any assumptions.

    Also old bias ply tires got shortest braking distance with locked wheels.

    Radials generally get longer braking distances, but tire can be made so many ways really.
     
  16. jtbo

    jtbo Registered

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    Here is data from truck and bus caliber, you can see how locked wheels gets shorter braking distances with them, again tires are fairly different from race car or street car.
    http://deepblue.lib.umich.edu/bitstream/2027.42/879/2/15697.0001.001.pdf

    edit: from that document I see also peak COF for truck tires being 0.75 at oval track, sliding COF being 0.60, but still stopping distance being better with locked wheel.
    That is why one can't really judge things from those two values alone, there are many other parameters that change how much there actually is grip, good thing in rF2 is that these factors are modeled and are controlled by three sliding curves.

    Just would need to be able to crack how exactly to get real world values for those curves :p

    For truck tire for example with those numbers, I get sliding friction of 0.6, then I get peak multiplier, using those curves I need to set peak to proper velocity and multiply sliding base 0.60 so that I get 0.75 for that peak.

    For example SkipBarber has these values, also comments show additional information about this:
    SlidingAdhesionCurve=(-9.2, 0.4, -5.2, 1.68, -1.2, 0.2) // min sliding speed (log(10) aTv), grip multiplier (for min), peak sliding speed, grip multiplier (for peak), max sliding speak, grip multiplier (for max)
    SlidingMicroDeformationCurve=(-5.2, 0.3, -1.2, 1.8, +2.5, 0.3) // these values are blended following a cosine rule
    SlidingMacroDeformationCurve=(-1.2, 0.2, +2.5, 2, +6.0, 0.4) // to get the totals the adhesion, micro and macro curves are then multiplied by the surface types as defined in the TDF files, the defaults of which are 0.25 for adhesion, 0.5 microroughness, 0.25 macroroughness

    Can't say that I would still understand fully those, but maybe some day there is enough understanding from this that it can be put to Wiki so there would be bit less confusion.
     
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  17. Lazza

    Lazza Registered

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    The other thing with that document, jtbo, is that the comparison is between 'no locked up wheels' and 'some locked up wheels'. Given the number of wheels involved (and trailers!) it seems possible that having some locked up wheels means others (the ones not locked) are close to the optimum point. If no wheels are locked, those same ones must be further from that optimum point (since all brakes must be producing less force, hence no wheels are locked) and that could increase the braking distance somewhat. There are a lot of variables though.

    vilivili, that discussion is centred on ABS vs non-ABS, there is some anecdotal stuff regarding stopping distances with fully locked tyres but, as in rF2 presumably, the coefficients of friction are but a small part of the picture.

    It doesn't seem immediately easy to find tables with comparitive values (most rubber-asphalt listings only have sliding friction), I did find a couple of statements where fairly large differences exist:

    Modern bike tyre... sliding dropped friction from 1.2 to 0.8
    http://www.stevemunden.com/frictiontopics.html

    Plainly stated coefficients, rubber tyre on concrete drops from 1.0 static to 0.7 sliding
    http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-3/Friction.html

    I'm sure a lot of things can affect the measurements, but it would be rare for two materials to exhibit more or similar sliding friction to static friction. The effects of the motion of the tyre in a real situation might be very different, but again that's what the rF2 tyre model would be looking to simulate - still based on the usual coefficients. I don't think there's any reason to doubt that the listed coefficients are normal. Judging them based on how they feel - given the possibly hundreds of variables involved - is probably not the best approach.
     
  18. vilivili

    vilivili Registered

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    not the best approach but atleast an approach.
    I think rf2 tyres feel a bit better with fixed coefficients. and rf2 game engine (unlike rf1) now offers all the parameters needed for realistic tyres so maybe some skilled modders show us what are the "correct" values.
     
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  19. jtbo

    jtbo Registered

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    I would say it is approach that is no better than no approach to be honest.

    You can be feeling things that are completely unrelated to tires. Also focusing only to single aspect and changing that can easily mask real issue or imperfection whatever that is.

    I did describe method how to test tires so that you will get some real results and can see how much those parameters affect that you are focused on, that should be first thing to test, is it as much effect as you feel it is.

    It is so easy to adjust thing and get great feel in wanted situation, but then look at the data, check other situations and you have something that surely feels nice but performance is something from imagination, not much real.


    StaticBaseCoefficient=2.12
    SlidingBaseCoefficient=1.368
    Simply does not feel like ice despite big difference in numbers, ISI tires, only things that are wrong are feelings, they always lie and trick one.
     
  20. CdnRacer

    CdnRacer Banned

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    lol.

    Start @3.55 and listen to him describe the F3 car as knife edge grip. Just because you think you have one car or one tire figured out doesn't mean you should be working for Michelin. ;)

     
    Last edited by a moderator: Dec 24, 2012

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