The other Collider the wheel is hitting.

	// When attached to the WheelCollider, modifies tire friction based on
	// static friction of the ground material.
	function FixedUpdate() {
		var hit : WheelHit;
		var wheel : WheelCollider = GetComponent(WheelCollider);
		if( wheel.GetGroundHit( hit ) ) {
			wheel.forwardFriction.stiffness = hit.collider.material.staticFriction;
			wheel.sidewaysFriction.stiffness = hit.collider.material.staticFriction;
		}
	}