 With the exception of Bentley and Draper system (New Imperial and Brough Superior machines) and the HRD (later Vincent) system both developed and patented in the 1920s it is only since the 1980s that monoshock motorcycles have been the norm, the term "twinshock" is now used to categorize vintage motorcycles.This distinction is important in that it provides classes used for vintage motorcycle competition.For example, vintage motocross races are held for older motocross motorcycles.