[Excaliber]

Steve, is that tire you speak off "solid" for the most part? A low drag and the best rolling wheel/tire is almost a must. Rock hard tires come to mind. As David said, he was winded and tired after just a short run in one of the chairs. Light weight, for the same reason.

Man this is a really complicated exercise. Light but strong, cost effective, easy to mass produce, rolls easy. For such a simple machine it's an enormous task.