Parent-child rigidbody issues with collider

Okay, I have a bike in air the scene, say Parent, and its various c$$anonymous$$ld objects like handle and wheels, Parent is a rigidbody, on Wheels(c$$anonymous$$ld) a wheelcollider is added. Rigidbody is added to wheel to make AddTorque function properly.Here come the issues

1) Without Rigidbody on the c$$anonymous$$ld, bike behaves as expected. 2)if rigidbody is added on c$$anonymous$$ld, the c$$anonymous$$ld will remain in air ( Parent is always rigidbody) 3) if gravity is checked, the c$$anonymous$$ld will keep falling through the terrain, although wheelcollider is added.

I need my bike to fall on terrain, with all its elements, and then move as i press the up arrow,

NOTE: I have to add rigidbody to c$$anonymous$$ld to make my script work, as AddTorque script works on rigidbody

not needed but here is my script

     void Update () {
             if(Input.GetKey("up")){
                     float v = Input.GetAxis("Vertical") * amount * Time.deltaTime;
                     GetComponent<Rigidbody>().AddTorque(transform.right * v);
             }
             if(Input.GetKey("down")){
                 float v = Input.GetAxis("Vertical") * amount * Time.deltaTime;
                 GetComponent<Rigidbody>().AddTorque(-transform.right * v);
             }