Tire slip in the rolling direction. Acceleration slip is negative, braking slip is positive.

	// Prints "braking slip!" when tire slips badly.
	function FixedUpdate() {
		var hit : WheelHit;
		var wheel : WheelCollider = GetComponent(WheelCollider);
		if( wheel.GetGroundHit( hit ) ) {
			if( hit.forwardSlip > 0.5 )
				print ("braking slip!");
		}
	}