Sidecar racing is a part of motorcycle racing in which BMW has a deep history. They dominated the sport for years and produced both machines and riders that are now legendary. However, BMW's entry into the sport goes back far beyond the peak of their success. In the 1930s, BMW were not officially racing sidecars, but they did have customers that raced their machines. These private "racing combination" entries typically were not dominant.
in 1935, there are several theories for what happened. The first is that the factory did not like their machines racing but not winning. The second is that racing was just good marketing, and sidecars should benefit just like solo machines did. remember that in 1936 Ernst Henne set a land speed record in a streamlined BMW. The third, and many would say most credible, is that the 500cc engines was a better platform for the purpose. Regardless, the factory decided to get involved and created a 600cc supercharged combination operated by Stelzer and Kraus. They did not achieve any notable success.
Following the war, when racing resumed, the sidecar classes were split into 600cc and below, or 1200cc and below. The larger class used the new R75 engine. Wins continued to be sporadic and few during the next few seasons of learning. Then in 1949, Klankermeier and Wolz began to win consistently with a supercharged machine. In 1950, they won the German Sidecar Championship. In 1951, another BMW team, Kraus and Huser, won a convincing title using the 750cc machine bored out to 900cc. More importantly, they narrowly bested the dominant Norton at the international race at Hockenheim. This probably convinced the factory to look at the world championship, and they did so the next season.
Ironically, the international rules were adopted for the German championship, and BMW reverted to the solo RS engine. The next two seasons brought steady improvements, including a reduced capacity R75 engine. Once again, the factory studied and learned for 2 seasons, and then in 1954 BMW began a 20 year domination of the sidecar world championship.