basic car physics

Hello to all you guys!
I made this script but I have a problem:

Code (csharp):

1.  
2. #pragma strict
3.  
4. var RuotaAD : WheelCollider;
5. var RuotaAS : WheelCollider;
6. var RuotaPD : WheelCollider;
7. var RuotaPS : WheelCollider;
8. var maxTorque : float = 40;
9.  
10.  
11. function Start () {
12.     rigidbody.centerOfMass.y = -0.0001;
13. }
14.  
15. function FixedUpdate ()
16. {
17.     RuotaPD.motorTorque = maxTorque * Input.GetAxis("Vertical");   
18.     RuotaPS.motorTorque = maxTorque * Input.GetAxis("Vertical");   
19.     RuotaAD.steerAngle = 10 * Input.GetAxis("Horizontal");
20.     RuotaAS.steerAngle = 10 * Input.GetAxis("Horizontal");
21.  
22. }
23.  

car loses grip when running and just takes a bit of speed
running but on two wheels, not four.
and then again increasing the speed, rolls!
also changing the mass of the rigidbody is the same.

help me please!
thank you very much!

 

never?!?!

 

Have you look into Unity car example?

http://u3d.as/content/unity-technologies/car-tutorial/1qU