Ever since the creation of Yamaha's motorcycle division in 1955, Yamaha had raced and they had won. The 1955 & 1956 Mount Asama 125cc races were won on essentially tuned up 125cc YA1 road machines, but Yamaha could see that they would need to produce a fully race dedicated machine in future in order to maintain their competitive dominance.
A small team of Yamaha engineers looked towards Europe to study contemporary Continental motorcycle designs, in order to look for inspiration for their new 250cc, as they had previously done with the YA1, taking inspiration from the DKW RT125. The team were suitably impressed with the twin cylinder Adler MB250 and used this as the basis for the new road going YD1 production street model, but as well as this they unsurprisingly also studied Adler's racing variant, the very fast and successful RS250 to develop Yamaha's own racing models. The influence is easy to recognise.
Two new machines were developed ready for 1957, a 125cc single & 250cc twin cylinder machine. The two machines were based upon similar cycle parts and a similar engine / gearbox unit. Each of these machines were also made with a square 54 x 54mm bore & stroke and a short stroke variant at 56 x 50mm.
Above: the final air cooled version of the Adler RS250, clearly the influence for the Yamaha's YD Racers. Below shows Noguchi's YD-B shortly after the Asama Races with a smaller YA fuel tank fitted, the similarities between frame, engine and wheels can be noted.
Below: The Adler RS250 air cooled engine, the origin of the Yamaha racing two stroke twin that was to dominate 250cc racing for the next 40 years.
Adler pictures courtesy of www.adler-veteranen.de and Dieter Jorzick/Johann Kleine