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Wheels rotate at incorrect speed

Discussion in 'General Discussion' started by B25Mitch, Jul 17, 2014.

  1. B25Mitch

    B25Mitch
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    BeamNG Team

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    In the recent update we've seen better friction introduced, so that there is now a difference between static and kinetic friction. This is great, and it's resulted in a big improvement to low-speed handling, but there is still a huge underlying problem that prevents it from improving vehicle handling at high speeds:

    Powered wheels always spin too fast. This becomes worse and worse the faster you go, and at 200km/h you'll see the rear wheels spinning 5% faster than the front ones.

    On the other hand, Unpowered wheels spin too slowly. Examine the screenshot below:


    (imported from here)

    I had been holding a constant indicated speed of 220km/h for several seconds. All wheels should be turning at the same rate, but there's a 14km/h difference between the rear and front wheel speeds. Meanwhile the airspeed is somewhere in between, at 212km/h.

    The problem here is that if the rear wheels are spinning 8km/h faster than the airspeed, and the front wheels 6km/h slower, none of the wheels are experiencing static grip. It's no wonder people are complaining of floatiness at high speed - the car is technically drifting in a straight line.

    I imagine the devs are probably aware of this, but the community needs to know as well so we can offer more useful feedback and suggestions. Perhaps those of us who are experimenting to improve handling can try tackling this issue.
     
    #1 B25Mitch, Jul 17, 2014
  2. Aboroath

    Aboroath
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    I've noticed a discrepancy in what the speedometer reads and real world
    percieved speed. For example when I drive the D15 on the monza track the speedo
    says 65mph and I swear to god I'm doing 30mph. Other tracks exhibit the same thing.
    Perhaps not related to the front/rear wheel speed issue but annoyingly similar.
     
    #2 Aboroath, Jul 17, 2014
  3. CTJacob

    CTJacob
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    This might be a perception issue with maps that are on a bigger scale than normal.
     
    #3 CTJacob, Jul 17, 2014
  4. B25Mitch

    B25Mitch
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    There are a ton of factors that could influence this.

    1.) The issue mentioned in the OP.
    2.) Wheel-spin while drifting.
    3.) Lack of detail on the track to gauge speed by.
    4.) The track is incorrectly scaled.
    5.) Absence of tactile forces to detect acceleration and speed.
    6.) Absence of peripheral vision to sense speed.

    Check your airspeed indicator if you want to rule out the first two.
     
    #4 B25Mitch, Jul 17, 2014
  5. Hati

    Hati
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    Nice catch, and it does seem to be the case that the car is permanently drifting.

    if you look up close in slow motion you can see the individual nodes "slip" when the vehicle has finished rolling over them; probably due to the beam springing back suddenly and the pressure not being on the node to hold it in place; have a look. The problem might be fundamentally unsolvable.

     
    #5 Hati, Jul 17, 2014
    Last edited by a moderator: Oct 21, 2015
  6. deject3d

    deject3d
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    on a slightly related note, i've noticed that power delivery to wheels is pretty much instantaneous. if you put powered wheels in the air, give 100% throttle, and shift through the gears, the torque curve becomes meaningless and wheels will nearly instantly change their velocity according to the gear ratio. i've not exactly attempted this in a real car, but i'm pretty sure it's not supposed to work like that.

    love seeing your posts, mitch.
     
    #6 deject3d, Jul 17, 2014
  7. Hati

    Hati
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    Actually, the only thing I can think that might help is if you allow the node to "dig into" the ground very slightly. The way a road surface is engineered is that there's a variety in the hardness of the materials used which creates many peaks and troughs through wear, its why you have some amount of grip even in the wet. The rubber of the tire sits on those peaks. Under pressure you could probably think of as the road penetrating the surface of the tire very slightly even though in reality its just deforming around them. The road offers a series of points and your tire is a flat surface with some give to it. In beamNG its reversed, the tire is made of the sharp points and its the road offering the flat surface. If the nodes are grabbing into the ground rather than drifting over an impenetrable flat surface it could help. The effect of this has to be miniscule, fractions of a millimeter but you'll notice that the slippage of the nodes when you drive is also quite miniscule, it just adds up over the surface area of the wheel.

    If you drive a heavy car, go look at your tires and you'll likely find bits of road stone stuck in the surface. It does get stuck in there.

    Its just a thought, but it could be worth looking into.
     
    #7 Hati, Jul 17, 2014
  8. Miura

    Miura
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    Tires are supposed to slip if they have any acceleration or deceleration force. Here's one of the first search results for "tire slip ratio", with some theory and data: http://www-cdr.stanford.edu/dynamic/WheelSlip/SlmillerGerdesACC.pdf

    Maybe we could measure the slip and see how close it is to real data.

    About airspeed vs wheelspeed, keep in mind that wheelspeed is calculated with the original radius of the tire, but the tire expands as it spins faster. So wheelspeed shows lower values than airspeed if the tire is not slipping.

    edit - Slip ratio doesn't mean the tire nodes should be slipping all the time. I also think some kind of extra adhesion could be good when the nodes are nearly stopped, to simulate the tire digging into the surface. Not sure if the current friction model can fully simulate that.
     
    #8 Miura, Jul 17, 2014
    Last edited: Jul 17, 2014
  9. Hati

    Hati
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    thats an interesting read. Learn new shit every day :p
     
    #9 Hati, Jul 17, 2014
  10. estama

    estama
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    Thank you for the paper reference :) .

    Also many thanks to the other people that do the experimentation.

    "Not sure if the current friction model can fully simulate that":

    It does through the static friction coef value. It is slightly increased to account for the "digging" phenomenon.

    About the wheels spinning at different rates. It might be due to the slip that miura posted the paper reference above, or to the different weight distribution between front/back wheels.

    Wheel speed is calculated as:

    angular_velocity * initial_wheel_radius

    If a wheel's real radius isn't the same as the initial_wheel_radius (happens all the time due to weight distribution, pressure, centrifugal), then the wheel speed calculation won't be correct. At extreme speeds the difference of real wheel radius to initial_wheel_speed_radius is quite big.

    You might wonder why we don't do the wheel speed calculation with the real radius. This is because the physics core only knows about the *hub* of the wheel (to power it). So the physics core cannot find the real radius, which is the distance from the center of the wheel to the ground.

    So the way to solve the wheel speed question is to add an additional sensor in the GUI to track the distance of axles to the ground.
     
    #10 estama, Jul 17, 2014
  11. 14ramosr

    14ramosr
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    I thought it would be something like that. Thanks for the explanation :D
     
    #11 14ramosr, Jul 17, 2014
  12. gabester

    gabester
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    The driven wheels at full throttle will always be spinning a little bit faster than undriven/airspeed because the torque is constantly generating slip. We may be able to counter this with some friction tuning, but as has been stated, this behavior is true of real cars as well.

    As for the in-car gauges: I've tuned those manually to read slightly high, like a real car (it is illegal for a speedometer to read low). It also compensates for the slight inaccuracy created by the tires expanding at high speeds.
     
    #12 gabester, Jul 18, 2014
  13. specialsymbol

    specialsymbol
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    I know what you mean. On the other hand, everytime I go to the track I feel like I'm going incredibly slow - until I hit a turn. Then I can't believe that I need to go even slower to make that turn.
    If you only play games your perception of racing is simply way off.

    I recall a report in a german motorcycle magazine (Motorrad) where they ran into trouble with the rear tyre of a Ducati on the Autobahn. It was after not even 100km ground down to the carcass. Turns out that you have somewhat around 10-15% slip on a motorcycle's rear wheel at high speeds - even going at constant speed (no accelerating). Cars are may be not that excessive, but still.

    On the other hand I remember the game 1nsane - there was some sort of constant slip implemented. So when the vehicle stopped it started sliding sideways or back or forth - even with the handbrake applied.


    Does anyone know if a change of set up parameters (camber, caster, toe, scrub radius) would make a difference in BeamNG?
     
    #13 specialsymbol, Jul 30, 2014
    Last edited: Jul 30, 2014
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