A couple weeks ago, a Cub Scout leader called about the best location for adding weight to Pinewood Derby cars. Years ago when my sons were in Scouts, we came up with the theory that the best place to add weight was in the wheels. If my memory serves, we glued washers to the insides of the wheels. Our theory was that rolling weight would maintain momentum longer that static weight, not unlike a flywheel. Again, if memory serves we had good success with this arrangement. The cars were a little slower off the line but pulled away on the flat sections of the track.