Head on collision vs skinning the wall.

Trying to get the force of an impact in a low altitude racing game. I wish to make my collision detection more robust. RIght now I get the same magnitude returned regaurdless of the colliding objects attack angle.

I am using:

function OnCollisionEnter(collision : Collision) {
    if (collision.relativeVelocity.magnitude > 16){
    // go boom
   } else {
    // make sparks
   }
}

It detecects collision and things blow up well.

However when a collision is very shallow in angle it is still detected as big collision, not as sliding down the wall. I recieve the same relativeVelocity.magnitude for hitting a wall head on as for glancing it with at a very light attack angle. How should someone calculate collision magnitude adjusted for the angle of the incoming force.

EDIT: this is working, thanks to efge's answer:

function OnCollisionEnter(collision : Collision) {
    var tempCount=0;
    var angleTot=0;
    var damageTot=0;
    for (var contact : ContactPoint in collision.contacts) {
        // finds the avrg of the difference of the angles of 
        // the Contact points from our transform's forward
        tempCount++;
        var angle = Vector3.Angle(contact.normal, transform.forward);
        angleTot+=angle;
        damageTot+=collision.relativeVelocity.magnitude;
    }
    Debug.Log ("avg angle:"+angleTot/tempCount+"  avg force:"+damageTot/tempCount+"  Adjusted force:"+damageTot/tempCount*angleTot/tempCount/180);
    //blow up or not based on adjusted force
}

it might be better to calculate baced on traveling direction (forces) than player rotation, but this feels good.