Ignore a Rigidbody's own children while colliding with another entity?

Hey unityAnswers, I've been trying to make it so that my car (which has wheel childrens) to set wheel friction stiffness to zero on collision with whatever terrain - But each time I set the code to respond "OnCollisionEnter(collision:Collision)", not only does the car collide with the terrain, but the wheels too! So the event triggers even when the wheels collide with the terrain. D=

Is there a way to exclude a Rigidbody's children except for it's own collider to check collisions against terrain?

My current code:

function OnCollisionEnter(collision:Collision)
{
FrontLeftWheel.sidewaysFriction.stiffness=0;
FrontRightWheel.sidewaysFriction.stiffness=0;
BackLeftWheel.sidewaysFriction.stiffness=0;
BackRightWheel.sidewaysFriction.stiffness=0;
}

I understand there's a function called "if(collision.gameObject.tag == "layertag")" but I have no idea how to implement it, or to achieve the results that I would like to come up with, outlined above.

You can use Physics.IgnoreCollisions, but it's not persistent (read the docs for more detailed explanation)

Or, since Unity3 you can use Layer Based Collision Detection, and it's scripted counter part - Physics.IgnoreLayerCollision.