problem wheel position

i use this code for wheels,

 using UnityEngine;

using System.Collections;

public class wheel : MonoBehaviour {

 public WheelCollider _wheel;
 public GameObject smoke;
 
 // Use this for initialization
 void Start () {
 
 }
 
 // Update is called once per frame
 void Update () {
     _wheel.suspensionDistance=1.0f;
 RaycastHit hit;
     Vector3 colliderCenterPoint=_wheel.transform.TransformPoint(_wheel.center);
     
     if(Physics.Raycast(_wheel.transform.position,-_wheel.transform.up, out hit,_wheel.suspensionDistance+_wheel.radius))
     {
         transform.position = hit.point + (_wheel.transform.up * _wheel.radius);
         Debug.DrawLine(_wheel.transform.position,transform.position, Color.green);
     }
     else
     {
         transform.position = colliderCenterPoint - (_wheel.transform.up * _wheel.suspensionDistance);
         Debug.DrawLine(_wheel.transform.position,transform.position, Color.red);
     }        
     WheelHit wh;
     _wheel.GetGroundHit(out wh);
     
     if(Mathf.Abs(wh.sidewaysSlip ) > 2.0 )
     {
         Instantiate(smoke,wh.point,Quaternion.identity);
     }
 }

}

but it doesn't work when wheel touches ground,look at this image

at part of 2 means when car touches ground, position of all wheels are center of wheels, why?

_wheel.suspensionDistance=1.0f;