Bevel boxers

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Bevel boxers

Royal road

Vertical shaft and desmodromic – for Franz Pohn this is the most consistent and elegant way of operating four valves per cylinder. Since BMW cannot fulfill such wishes, he built his dream boxer himself.

The Upper Austrian Franz Pohn from the idyllic Vocklabruck is actually a convinced BMW driver, as a BMW R 1100 RS in his fleet shows. But like many die-hard fans of the brand, the technology of the old and new Bayern boxers is not demanding enough for him. The RS 54 upright shaft racing machine from the fifties is at least conceptually the embodiment of the boxer for Franz Pohn. The idea of ​​transferring the concept of the prototype to a modern design has long fired his imagination, and so at the beginning of the eighties he simply converted an R 90 S to valve control with vertical shaft and desmodromic (MOTORRAD 3/1986).
But standing still is a step backwards, thought the former technical manager of a fittings factory, who is now enjoying his retirement, and was already brooding on a new project in 1987. The key data were clear: of course, it had to be a boxer engine with vertical shafts for the valve control. Four valves instead of two, as in the first conversion, were a matter of course, and since the technician is not only a BMW enthusiast, but also a Ducati fan and owner, these should again be operated desmodromically. Instead of springs, a second rocker arm operated by an additional cam closes the valve.
In contrast to his first work, which was based on the foundation of the bumper boxer, the inventor did not want to compromise on the successor. The engine therefore had to be a completely new design with all the consequences, that is to say, everything from the idea to the design to the manufacture of all parts should be developed in-house. And Franz Pohn had set himself another goal. Well informed about the development of the new boxer in Munich, he wanted to be finished with his project before the white and blue.
In 1987 his considerations – or as Franz Pohn put it: "I already spun it before" – took on concrete forms on the drawing board. True to the historical model, the crankshaft drives an intermediate shaft above it via gears. The bevel gears for the vertical shafts to the cylinders sit on it. They set one overhead camshaft per cylinder in rotation, which actuates the valves via forked opener and closer rocker arms. For a few parts, the builder used the tried and tested. So the crankshaft and the cylinders of the old boxer generation offered themselves. The latter set the displacement of 979 cm³ with a stroke of 70.6 millimeters and a bore of 94 millimeters. The engine mounts and the gearbox flange were also based on the Bayern boxer, as Fritz Pohn wanted to take over the Paralever chassis of an R 100 R and its gearbox in a slightly modified form.
Otherwise, the Austrian actually relied entirely on his own work. He even made the models for the casting molds himself. The mold for the cylinder head alone consists of no fewer than seven parts. After a few test casts, new knowledge flowed in, so that the case underwent several changes. Of course, the engine builder worked on all cast parts personally in his perfectly equipped workshop, and even parts such as rocker arms or camshafts were self-made.
From 1990 to 1994 several thousand hours passed before the engine was ready to start at the end of 1994. Franz Pohn had lost the race with BMW in Munich – although an accident set him back by a year – but the result justifies the somewhat longer development time of the one-man team. After the engine had been transplanted into the BMW chassis, the eagerly awaited first test run went without any problems. The designer reports with satisfaction: "A push on the button, and the engine is running." The approval of the Austrian traffic authority received the new design straight away. After some adjustment work, it was absolutely suitable for everyday use, as has now been confirmed with a mileage of several thousand kilometers.
AT.When Franz Pohn starts his own construction for MOTORRAD, the BMW system emits a sound familiar from the boxers, at best a slight howl indicates the existence of four pairs of bevel gears. The perfectionist wants to reduce this by reducing the backlash. He compares the performance with that of his R1100 RS and guesses between 90 and 95 horses, but it’s not all evening yet. In the absence of his better half, who has only seen her husband in the workshop in recent years, the qualified engineer confesses his next plans. On the one hand, he wants to replace the Dellorto carburettor with an injection system and install a double ignition, on the other hand, he dreams of more compact cylinder heads that will make the boxer, which is currently twenty millimeters wider than the BMW four-valve, narrower.
Anyone who has met Franz Pohn once is convinced that he will also put these projects into practice. Finally, he himself mischievously admits: "The second BMW was born out of the arrogance of the first." And a certain arrogance can still be seen in him even today with his 66 years. The motorcycle world can hope that a lot can be expected from PMW – the Pohn Motoren-Werke.

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