WheelHit.forwardSlip  

var forwardSlip : float

Description

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!");
    }
}