In 1938, a completely new range was launched with the types BMW R 51, R 61, R 66 and R 71. The most striking innovation was the plunger rear-wheel suspension which had proved so successful in the Six Day Race held the year before. The flagship model was the BMW R 66, which was powered by a 30 hp 600 cc Boxer engine and had the accolade of being the fastest BMW motorcycle in the prewar period with a top speed of 145 km/h. The good reputation of the BMW R 51 and its predecessor BMW R 5 at the Six Day Races assisted the unit sales of the BMW R 66 but they did not approach the sales of the BMW R 51.