There have traditionally been several races at each event for various classes of motorcycles, based on engine size, and one class for sidecars. Classes for 50 cc, 80 cc, 125 cc, 250 cc, 350 cc, 500 cc, and 750 cc solo machines have existed at some time, and 350 cc and 500 cc sidecars. Up through the 1950s and most of the 1960s, four-stroke engines dominated all classes. In the 1960s, due to advances in engine design and technology, two-stroke engines began to take root in the smaller classes.