Parent-child rigidbody issues with collider

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

1) Without Rigidbody on the child, bike behaves as expected. 2)if rigidbody is added on child, the child will remain in air ( Parent is always rigidbody) 3) if gravity is checked, the child 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 child 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);
             }