Bill Dudley built this recumbent out of a 20″ bmx bike and a 27″ bike that he found on the side of the road. The only specialized part used was an idler from a commercial recumbent. Bill says the most difficult part was constructing a comfortable seat. It apparently works pretty well; He’s completed several metric centuries (100km/63 miles) on it. I think the first recumbent I ever saw as a child was the BEHEMOTH, which is in class of its own.