Search Unity

  1. Megacity Metro Demo now available. Download now.
    Dismiss Notice
  2. Unity support for visionOS is now available. Learn more in our blog post.
    Dismiss Notice

[Car Physics] Clutch bite point, Differential speed

Discussion in 'Physics' started by NDSno1, Feb 11, 2018.

  1. NDSno1

    NDSno1

    Joined:
    Dec 20, 2014
    Posts:
    223
    Hi all,
    So I suddenly came to a few questions while modeling the clutch.
    1 - From reading around car forums and my own driving experience, when driving a stick shift with clutch, there is a certain clutch pedal position where you can feel the clutch disks contacting each other from the vibration of the clutch pedal, and the wheels would slowly spin. This is especially important when launching from uphill, as you have to use and feel the clutch to hold the car. So my question is: what is happening at the clutch at this stage? I'm guessing based on the model I'm doing:
    engineSpeed += (engineTorque - clutchResisTorque) / engineInertia
    and the torque driving the wheels is:
    clutchResisTorque * gearRatio * diffBias.
    At the bite point, engineTorque == clutchTorque, so the netTorque is 0 which result in no acceleration at the engineRPM. At the wheel side, because clutchTorque is too low to overcome the rolling resistance, so netTorque is also 0 resulting in no acceleration.
    This is just my theory, please correct me.
    2 - This one is different from the one above but related to my clutch model, and I don't want to spam the forum so sorry for the wall of text. This time is about the speed of the drivetrain relating to the differential. Overall, the diagram is
    engine -> clutch -> gearbox -> drivetrain -> differential -> wheel
    Is the speed of the drivetrain always be the average speed of all the powered wheels? Is the speed different with different types of differential (open, locked, limited-slip)?
    In case of torque going to each wheels, the most common I've seen in Unity is the open type, where the torque is just splitted 50-50 between the wheels all the time.
    Locked and limted slip are rare and the only I've seen is splitting torque based on the difference between the speeds of 2 wheels
    (leftSpeed - rightSpeed) * someConstant
    I'm planning to implement this model. However, I'm still having difficulty with actually come up with the bias ratio to split the torque from the drivetrain to the wheels. Can someone please give some hints or ideas?

    Thank you very much in advance, and sorry for the wall of text.