Yamaha’s rotary-engine motorcycle RZ201 was unveiled on the 19th Tokyo Motor Show in 1972 and surprised even many employees from the company, as most were totally unaware of this project. It was fated to never go into production
It was developed together with those of that other “bombshell”, the GL750 4-cylinder 2-stroke and the production machine TX750. The RZ201 had a wheelbase of 1.485 cm compared to 1.455 for the TX750. Weight was 220 kg against 230 kg respectively.
The “rotary” had the internal project code “YZ587” and the design sketches were approved by August 22, 1972. The Yamaha rotary engine utilized “CCR”/”OCR” (charge cooled rotor, but also oil-cooled rotor) The rotors had a radius of 83 mm and a diameter of 56 mm, resulting in a 660 cc displacement. Performance was 66 horsepower @ 6.000 rpm, comparable to the TX750’s 63 hp @ 6.500 rpm, (bore x stroke 80 x 74 mm).
As ignition Yamaha already used a full CDI system and most of the novel electrics used on the TX750 were also applied. An electric starter motor was also added.
A few RZ201’s were hand-made and according to some sources, 1 or 2 are still "hidden" in the factory to be discovered!
Yamaha Circuit Magazine. August 1981
Journalists too were drawn to the Yamaha stand by this unique model and motorcycle newspapers the world over proclaimed that its introduction marked the dawn of a new era in motorcycle design.
The RZ201, however, was never intended as a production model. It was simply an exhibition machine aimed at demonstrating the diversity of Yamaha technology. The fact that the machine was built to such a high standard of both technology and design fooled the motorcycle world into believing that it was a production prototype. The rotary piston engine was developed by Yamaha along with Yanmar Diesels who had already obtained a license to manufacture marine and light vehicle engines from the German patent holders NSU. Working within the confines of their patent license, Yamaha came up with several technical innovations, all designed to make this type of engine suitable for motorcycles. The twin rotary pistons, each displacing 330cc rotated in the direction of the vehicle advance to alleviate any vehicle roll caused by engine torque. The inlet and exhaust ports and the ignition plug were re-arranged in a more rational lay-out than the original design, saving a great deal of time and expense when carrying out routine maintenance. Another ingenious device was the combination port system that featured a periphery port working in conjunction with a side port. The combined effect of these two ports was to increase combustion efficiency over the low speed range while improving intake efficiency at higher speeds, giving improved performance throughout the rev range.
The final version of the rotary engine developed 68ps (Hp) at 6500rpm with a maximum torque of 7.8kgf-m at 4000rpm, and this power output was coupled to a five speed gear box and transmitted to the rear wheel by a silent chain.
A liquid-cooling system was adopted for the RZ201, while another technical innovation from Yamaha took care of engine lubrication. This was the "Charge Cooled Rotor" system which fed oil directly into the mixture from the carburetor to lubricate and cool the rotors. This system, which did away with the oil cooler required on conventional rotary engines, helped to make the power unit narrower, and so more suitable for use in a motorcycle.
It would have made a striking addition to the Yamaha range, and possibly sparked off a revolutionary new approach to motorcycle design. As it turned out, the Yamaha research and design department moved on to other things and, even without the help of the rotary engine, have kept the name of Yamaha to the forefront of motorcycle design.
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