Stumpy, you let your plants get too tall. The most growth that you should ever see is about 4", if that much. Only leaves must be showing, not vines. The part which forms the potatoes is not the vine but a special portion of the plant between the feeder roots and the aboveground growth. The plant must be tricked into not developing a stem but rather to keep extending the production zone. Once the stem is allowed to form, it doesn't matter if you have a stack of 20 tires as all you are doing is burying a non-producing stem.
Martin