Illustrated history of BMW motorcycles. Roy Bacon. 1995.

BMW were unique in keeping to a single concept for their motorcycle for 60 years. Certainly there were improvements, many in the vanguard of progress, but the basis of the first, real motorcycle of 1923 was still there in 1983 and runs on to today. When they did change by adding line two, it was equally individual, even if it did become known as the ‘flying brick.’