i dont know why this is happening i followed an example for a raycasting supsension system

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wheel.cs:

public Transform graphic;

    public float mass = 1.0f;
    public float radius = 1.0f;
    public float maxSuspension = 0.2f;
    public float spring = 100.0f;
    public float damper = 0.0f;

    public float wheelAngle = 0f;

    private float circumference;

    private float contactPatchArea;
    private Rigidbody parent;
    private bool grounded = false;

    public bool IsGrounded
    {
        get
        {
            return grounded;
        }
    }

    void Awake()
    {
        circumference = (radius * 2) * Mathf.PI;
        parent = transform.root.GetComponent<Rigidbody>();
    }

    void FixedUpdate()
    {
        GetGround();
    }

    void GetGround()
    {
        grounded = false;
        Vector3 downwards = transform.TransformDirection(-Vector3.up);
        RaycastHit hit;

        // down = local downwards direction
        Vector3 down = transform.TransformDirection(Vector3.down);

        if (Physics.Raycast(transform.position, downwards, out hit, radius   maxSuspension))
        {

            grounded = true;
            Vector3 velocityAtTouch = parent.GetPointVelocity(hit.point);


            float compression = hit.distance / (maxSuspension   radius);
            compression = -compression   1;

            Vector3 force = -downwards * compression * spring;

            Vector3 t = transform.InverseTransformDirection(velocityAtTouch);

            t.z = t.x = 0;

            Vector3 damping = transform.TransformDirection(t) * -damper;
            Vector3 finalForce = force   damping;

            t = parent.transform.InverseTransformDirection(velocityAtTouch);
            t.y = 0;
            t.z = 0;

            t = transform.TransformDirection(t);

            parent.AddForceAtPosition(finalForce   (t), hit.point);

            if (graphic) graphic.position = transform.position   (down * (hit.distance - radius));

        }
        else
        {
            if (graphic) graphic.position = transform.position   (down * maxSuspension);
        }

        float speed = parent.velocity.magnitude;

        if (graphic)
        {
            graphic.transform.localEulerAngles = new Vector3 (graphic.transform.localEulerAngles.x, wheelAngle, graphic.transform.localEulerAngles.z); 
            graphic.transform.Rotate (360 * (speed / circumference) * Time.fixedDeltaTime, 0, 0); 
        }
    }

    void OnDrawGizmosSelected()
    {
        if (grounded)
        {
            Gizmos.color = new Color(0, 1, 0, 1);
        }
        else
        {
            Gizmos.color = new Color(1, 0, 0);
        }
        Vector3 direction = transform.TransformDirection(-Vector3.up) * (this.radius);
        Gizmos.DrawRay(transform.position, direction);

        Gizmos.color = new Color(0, 0, 1, 1);
        direction = transform.TransformDirection(-Vector3.up) * (this.maxSuspension);
        Gizmos.DrawRay(new Vector3(transform.position.x, transform.position.y - radius, transform.position.z), direction);
    }

Kart.cs:

Rigidbody rb;

    public float Throttle;
    public float SteerAxis;

    public float Acceleration = 2.0f;
    public float MaxSpeed = 10.0f;
    public float MaxReverseSpeed = 3.0f;
    public float Speed = 0.0f;
    RaycastHit hit;
    public AudioSource EngineSource;

    void Awake()
    {
        rb = GetComponent<Rigidbody>();
    }
    void Start () 
    {

    }
    void Update () 
    {
        Throttle = Input.GetAxis("Vertical");
        SteerAxis = Input.GetAxis("Horizontal");

        if(Throttle > 0)
        {
            Speed  = Acceleration * Time.deltaTime;
        }
        else
        {
            Speed *= 0.98f;
        }
        EngineSource.pitch = 1.0f   (Speed / MaxSpeed);
        Speed = Mathf.Clamp(Speed, -MaxReverseSpeed, MaxSpeed);
        transform.Rotate(new Vector3(0, 1, 0), 30.0f * SteerAxis * Time.deltaTime);
        Vector3 velo = transform.forward * Speed;
        rb.velocity = new Vector3(velo.x, rb.velocity.y, velo.z);\
   }

edit: it also starts to spin randomly after a while