Wheels can't touch the ground

Hello, I'm doing the unity's car tutorial and I found something stange on my car.

I was followed the steps and when I check the code.

 if(w.collider.isGrounded)
         {
             if(w.steerWheel)
                 canSteer = true;
             if(w.driveWheel)
                 canDrive = true;
         }

It's never been entry.

Is something I missing to do?

-------------------------------------------7-5-2013 More strange things.

I was check the code which one of the collider.

 if(w.collider==MeshCollider)print("yes");else print("NO");
         print(w.collider);

Then show me (Unity Engine.Wheel Collider)

But the conditional statement is show me the reverse answer.

What's wrong with it?

-------------------------------------------7-5-2013 I try do check is it my model is wrong.

Then I create the simply model to check. Add the collider independently.

But the result is same.

What's Wrong!? please HELP!!!