First off, Im not talking about motorcycles or bicycles here. Thats an article for another time. What I am talking about are the cycles in nature and human history. There is a 20-year cycle of hurricane frequency (global warming notwithstanding). There is an approximate 20-year cycle of wet and dry years in the state of Oregon. Until Ronald Reagan, there had been a 20-year cycle of presidential deaths in office, starting with William Henry Harrison in 1840 and continuing through John F. Kennedy in 1960.