IndexOutOfRangeException: Array index is out of range.

this is TractionHelper.cs error at line " oldGrip = front[0].grip; "

public class TractionHelper : MonoBehaviour {

	// assign car's front wheels here.
	public Wheel[] front;

	// how strong oversteer is compensated for
	public float compensationFactor = 0.1f;	
		
	// state
	float oldGrip;
	float angle;
	float angularVelo;
	
	void Start () {
		oldGrip = front[0].grip;
	}
	
	void Update () {
		Vector3 driveDir = transform.forward;
		Vector3 veloDir = rigidbody.velocity;
		veloDir -= transform.up * Vector3.Dot(veloDir, transform.up);
		veloDir.Normalize();
		
		angle = -Mathf.Asin(Vector3.Dot(Vector3.Cross(driveDir, veloDir), transform.up));
		
		angularVelo = rigidbody.angularVelocity.y;
				
		foreach( Wheel w in front)
		{
			if (angle * w.steering < 0)
				w.grip = oldGrip * (1.0f - Mathf.Clamp01(compensationFactor*Mathf.Abs(angularVelo)));
			else
				w.grip = oldGrip;
		}
		
	}
}

What msdn says: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index. 1) check length of this array on startup. 2) Are u sure that u have assigned array in editor? if in other script, than thisone can be executed before your asigning script.