3D rigidbody position doesnt seem to be updated correctly

Unity Engine
,
1

Here’s the link so I don’t have to try and describe it with my monkey brain. Code is below but I’ve tried everything. I’ve tried every combination of rigidbody settings and nothing helped. I’ve moved UpdateWheel() into FixedUpdate, Update and even both at the same time (accidentally). Making the rigidbody kinematic or turning use gravity off just makes the car float like a paper airplane.

Code used for inputs:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CarControllerV2 : MonoBehaviour
{
    public float motorTorque;

    public Transform FrontRight;
    public Transform FrontLeft;
    public Transform BackRight;
    public Transform BackLeft;

    private VehiclePhysicsV2 FL;
    private VehiclePhysicsV2 FR;
    private VehiclePhysicsV2 RL;
    private VehiclePhysicsV2 RR;

    private void Start()
    {
        FR = FrontRight.gameObject.AddComponent<VehiclePhysicsV2>();
        FL = FrontLeft.gameObject.AddComponent<VehiclePhysicsV2>();
        RR = BackRight.gameObject.AddComponent<VehiclePhysicsV2>();
        RL = BackLeft.gameObject.AddComponent<VehiclePhysicsV2>();
    }

    private void FixedUpdate()
    {
        //Vroom
        FR.motorTorque = motorTorque;
        FL.motorTorque = motorTorque;

        //Turn the steering wheel
        FR.steerAngle = Input.GetAxis("Horizontal") * 45;
        FL.steerAngle = Input.GetAxis("Horizontal") * 45;
    }
}

Code attached to the wheels:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class VehiclePhysicsV2 : MonoBehaviour
{
    private Transform dummyWheel;
    private Rigidbody rb;

    private Vector3 netForce;
    private Vector3 center;

    public bool isGrounded;

    public float motorTorque;
    public float brakeTorque;
    public float steerAngle;
    public float wheelAngularVelocity;
    public float wheelRotationAngle;
    public float wheelRadius;
    public float wheelMass;

    public Color GizmoColor = Color.green;
   
    public Vector3 Center
    {
        set
        {
            center = value;
            dummyWheel.localPosition = transform.localPosition + center;
        }
        get
        {
            return center;
        }
    }

    private void Awake()
    {
        dummyWheel = new GameObject("DummyWheel").transform;
        dummyWheel.transform.parent = this.transform.parent;
        Center = Vector3.zero;

        wheelRadius = 0.5f;
        wheelMass = Mathf.Max(1f, .0001f);
    }

    private void Start()
    {
        GameObject parent = this.gameObject;
        while (parent != null) //While the gamobject exists
        {
            if (parent.GetComponent<Rigidbody>() != null) //if there is a rb in this.gameObject.rb
            {
                rb = parent.GetComponent<Rigidbody>(); //Then set rb to this.gameobject.rb
                Debug.Log("Rigidbody set");
                break;
            }
        }
        if (rb == null) //If no rb found then error;
        {
            Debug.Log("VPV1 cant find a rigidbody. Add one to the wheel object.");
            //Game freezes and will not respond if under this condition. Force stop is required.
        }
    }

    private void FixedUpdate()
    {
        UpdateWheel();
       
       

    }

    void UpdateWheel()
    {
        if (isGrounded)
        {
            rb.AddForceAtPosition(netForce, transform.position);
        }

        dummyWheel.localEulerAngles = new Vector3(0, steerAngle, 0); //Set steering angle of dummy

        wheelRotationAngle += wheelAngularVelocity * Time.deltaTime; //Calc wheel rotation

        this.transform.localEulerAngles = new Vector3(wheelRotationAngle, steerAngle, 0); //Set Rotations of actual wheel

        transform.localPosition = dummyWheel.localPosition; //Set position of wheel

        wheelAngularVelocity += motorTorque / wheelRadius / wheelMass * Time.deltaTime; //Motor Torque

        wheelAngularVelocity -= Mathf.Sign(wheelAngularVelocity) * Mathf.Min(Mathf.Abs(wheelAngularVelocity), brakeTorque * wheelRadius / wheelMass * Time.deltaTime); //Brake Torque


    }

    public void OnDrawGizmosSelected()
    {
        Gizmos.color = GizmoColor;

        //Draw Suspension
        Gizmos.DrawLine(
            transform.position - dummyWheel.up * wheelRadius,
            transform.position + dummyWheel.up
        );

        //Draw Wheel
        Vector3 point1;
        Vector3 point0 = transform.TransformPoint(wheelRadius * new Vector3(0, Mathf.Sin(0), Mathf.Cos(0)));

        for (int i = 1; i <= 20; i++)
        {
            point1 = transform.TransformPoint(wheelRadius * new Vector3(0, Mathf.Sin(i / 20.0f * Mathf.PI * 2.0f), Mathf.Cos(i / 20.0f * Mathf.PI * 2.0f)));
            Gizmos.DrawLine(point0, point1);
            point0 = point1;
        }
        Gizmos.color = Color.white;
    }
}
2

You can’t force the position of a rigidbody by doing transform.localPosition = whatever and expect physics to work correctly, this should be common sense. To the physics engine, this is the equivalent of teleportation: linear/angular velocities and collision detection are ignored when setting the position/rotation of a rigidbody via directly manipulating its transform.

To make matters worse, your wheels are parented to the car rigidbody and are affected by gravity, so they’re falling in the car’s local space. Every frame they’re falling due to gravity, then being teleported back into place by your code. This results in the jittering you see in the video.

Overall, your car setup doesn’t make any sense whatsoever. Any specific reason you’re not using WheelColliders? Or if using MeshColliders, motorized joints to keep the wheels in place and rotate them?