my car isn't moving

hi, i'm just trying to learn how to use WheelCollider.motorTorque , and i saw this script somewhere around

     public WheelCollider fr;
 public WheelCollider fl;
 public WheelCollider br;
 public WheelCollider bl;

 public float speed = 10.0f;
 public float steerSpeed = 10.0f;

 public GameObject COM;

 int gear = 4;

 void Start ()
 {
     rigidbody.centerOfMass = COM.transform.localPosition;
     rigidbody.angularDrag = 10;

     br.motorTorque = 30;
     bl.motorTorque = 30;
 }
 
 void Update ()
 {
     if (Input.GetKeyDown("1"))
     {
         gear = 1;
     }

     if (Input.GetKeyDown ("2")) 
     {
         gear = 2;
     }

     if (Input.GetKeyDown ("3"))
     {
         gear = 3;
     }

     if (Input.GetKeyDown("4"))
     {
         gear = 4;
     }
 }

 void FixedUpdate()
 {
     fr.steerAngle = Input.GetAxis("Horizontal") * steerSpeed;
     fl.steerAngle = Input.GetAxis("Horizontal") * steerSpeed;

     br.motorTorque = Input.GetAxis("Vertical") * speed * gear;
     bl.motorTorque = Input.GetAxis("Vertical") * speed * gear;


     
 }

so i made a box and some clyinders to make them a car and an empety GameObject to be the COM and centered it to all the wheels to test this script(i didn't parent any thing)

and i tested it but the wheels doesn't move, the front wheel's WheelCollider is rotating well but it never moves, sorry for being so stupid >,< :)