Yannick (Apriliabarth) and I have always disagreed about the method chosen by that Maverick designer, Danial Zimmermann to drive the single vertical rotary disc drive of his F3 engine.
This advanced 500cc twin water cooled racing two stroke engine was Zimmermann's very first self-built engine. After spending several years designing four stroke car engines I learned the value of 'reverse engineering' (copying!) the engines of other manufacturers. Sometime all we had was a few photographs. We had to guess an engine's dimensions, how it worked and what went on inside. Today this is called Forensic Photograph Analysis.
But even with such a background, I am still ambivolent about some areas of Zimmermann's F3 engine design. For example, why is the side-cover (disc valve cover) of this F3 engine not circular? But let me say that I have absolutely no doubt that Zimmermann used skew-gears and a cross shaft to drive the rotary disc valve on this F3 engine. NO DOUBTS at all for me.
But over the years that I have studied rotary disc valve two-stroke racing engines, I have noted various methods of driving the rotary disc valve:
1. Crankshaft mounted inside the crankcase - (the very simplest method) as per the original Sun Vitesse and the German Ziro and many others. This method used a thick (possibly 6mm thick) rotary valve mounted inside the crankcase where it ran close to the crankcase wall. The disc was heavy, unbalanced and did not seal well. Zimmermann had an idea...
2. Crankshaft-end-mounted running externally in its own housing - (the most common method) used by the vast majority of this type of engine such as Zimmermann's 1952+ ZPH, 1953+ IFA/MZ 125, 1961+ Suzukis including the ubiquitous Suzuki RG500 square four ... and the Italians.
3. Mounted at 90 degrees to the crankshaft and driven via a skew gear (this is much rarer) and includes the 1950 Zimmermann 500cc Twin F3 racing engine, the 1970-80s Rotax twins (e.g. 454).
4. Mounted horizontally driven by bevel gear from the crankshaft - 1953 Zimmermann 125cc-Twin racer
5. Mounted horizontally driven by toothed belt such as the König and Fath racing engines
6. Mounted inside the engine and crankshaft driven - 1950s+ Trabant. I know no other example of this type.
So because possibly 99% of all rotary valve engines are of Type 2 above, it is crazy to think that this is the only way.
Daniel Zimmermann (who incidentally had NO involvement with the design or development of the Trabant engine) was very creative and used methods 1, 2 and 3 in the few years he was designing his two stroke racing engines. Apriliabarth and I discovered the photographic and the physical proof of these methods during our researches in the old East Germany. Indeed, Apriliabarth and I have seen and handled Zimmermann's beautifully machined motorcycle engines and his racing outboard engines (also using rotary disc valves).
It is easy to discount a new idea without thinking through its design details. Engineers like Daniel Zimmermann were immensly creative designers and produced ideas from their lateral thinking 'outside of the box'. He didn't care that the idea he had was brand new and had never been used before.
Yannick's biggest mission in life is to find Zimmermann's 1950/51 Formula 3 500cc car or at least its engine! I beg any one of you who reads this, the very best, most technically experienced gathering of motorcycle enthusiasts and engineers, to let Apriliabarth know any ideas you may have about his passion, the 1950/51 Zimmermann 500cc F3 car.