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BMW R73 with French Roots: What is it? PDF Print E-mail
Written by BMWVMCA NEWS   
Saturday, 11 June 2011

 

Text and photos by Stefan Seipel: Uppsala, Sweden, with contributions by Scott Williams: California.

 

     This is the story of a very “individual” restoration of a BMW which began in April 2004. What started with a rather pathetic combination of various BMW pieces, ended with a really fun to ride bike of classical elegance. For the perfectionists I should say upfront, that this bike was never intended to resemble an original BMW production model like a R66 or R71. Nevertheless, I followed a design and technical concept that is rooted in the history of French motorcycle production immediately after the second world war: At the end of WWII, a huge spare part depot at the Heeres-Kraftfahr Park nearby Neuilly-sur-Seine (in the outskirts of Paris) was left behind after the Germans had retreated. After the war a French company by the name “Centre de Montage et de Réparation” -in short CMR -was founded to build motorcycles from the stock of new spares.

 

     These were mostly parts to complete R12 and a few R66 and R71. But there were also replacement engines for the R75. Out of those parts it is said [1] that approximately 300 exemples of the R12 were produced, as well as few R71 and R66. All of them completely made from original BMW components.

 

    As more and more vital components of were used up, CMR started to produce an own chassis to make use of the remaining R75, whereby they followed the well-proven R71 design. In thisprocess, 80 hybrids came to existence and they were called the BMW-CMR-R73. All this happened right after end of WWII and years before BMW put heir first post-war R51/2 flat twin into production.

 

     The CMR hybrids used the drive-train from the R71/R75, but the frame and all sheet metal parts were made all new and slightly different in design compared with original BMW parts. CMR designed its own emblem, which combined the colors of the French flag with a symbol similar to the BMW logotype [7]. Just like the Russians continued to further develop their line of products stemming from the early Molotow M72 to the Urals and Dneprs, so did the French: CMR later transformed into CEMEC who produced a bike called L7 [7]. CMR was later bought by Ratier [4]. Most of these bikes produced by CMW and CEMEC were used by French authorities (mainly police). As BMW was allowed to resume production of more powerful motorcycles from 1950 on, the R51/2 became a popular bike used by French Police.

 

     Undoubtedly true is that the R71 equipped with an R75 engine was a very popular motorcycle also outside France. For instance in Germany, private racing pilots and professionals appreciated the combination of cheap and robust prewar components. In particular did the 750ccm OHV engine from the R75 offer quite some potential to squeeze out many more horse power than was available from the factory. In fact, the R73 design was apparently still successful in side-car and solo races into the 1950’s. Sidecar rigs based on the R71 chassis and using the R75 engine were a common sight on sidecar race tracks and the engines were downsized with smaller cylinder barrels to become competitive sidecar racers in the half-liter category. The engine is also robust enough to be run with large barrels. A 905 ccm engine was run by Klankermeier and later Wiggerl in the one liter category [1]. There is another excellent book on BMWs by Heinz Härtel (unfortunately out of print for many years) with pictures showing BMW riders using the “R73”.

  

     Among those were e.g. Hillebrand and Prätorius (sidecar combination) and Alois Huber (solo) [2]. It is from this book that I originally got the inspiration of re-building an R73 many years back.

 

     A contemporary “R73” sidecar racer is on display at the BMW Museum in Munich. Unfortunately, there is little officially published material on the BMW-CMR R73. The closest I could get until now is a technical review about the R73 in a French motorcycle magazine from 1950 [3], which I found on French eBay some time ago. It summarizes in one page the R73 specifics with a few pictures of two different R73s . It then follows a detailed technical documentation of the R75.

 

      This is the only contemporary source I am aware of that illustrates authentic R73s. According to those pictures there existed both a single-carburetor version as well as a double-carburetor version. Among the technical details is mentioned, that the compression ratio of the engine was increased from 1:5.5 to 1:6.6 owing to a re-designed cam-shaft width wider lobes. I was lucky to find such a cam-shaft along with two different types of individually manufactured valve covers from the former eastern German racing community. The difference between the original specification cam and the re-designed version can be seen clearly on the pictures. Similarly designed cams have also been produced by Schleicher, the specialists from Munich for cam shafts, and they are still available as new parts. If we consider the compression similar to that of a R66, it is fair to assume that the R73 with a 25% more displacement even in a “civilized” manner should easily produce something like 35hp, even without porting heads and polishing the valve train and web.

 

     The bike presented here is the result of combining the beautiful design of prewar engines and sheet metal with the power and robustness of a mid 1960’s motorcycle. The choices of the various BMW components used in the rebuilding were hence guided by aesthetic criteria and with the ambition to create a touring bike that can keep up effortlessly with modern traffic speeds over longer distances.

 

Stefan Seipel's R73

 

      With the help of a good friend in the Norwegian Vintage Motorcycle club, the basis for this project arrived at my garage from Norway in the beginning of 2004. According to the previous owners it had much earlier been imported from Denmark. That is about all I know of the history of this bike. I kept only the frame with engine and forks. The fenders as well as the headlight are reproductions of the corresponding R71 parts. I was lucky to find an original Meier Rennsport tank in the town where I live. Those gas tanks were popular in the early 1950s and I have seen them in several contemporary photographs being mounted mostly on R51/2 and R51/3. This tank was available without toolbox (as seen on my bike)and with a toolbox on top.  The gearbox is the early version of the R5 without central air filter housing; its internals are unaltered and according to factory specifications. There is no technical reason for this choice; but I loved its elegant shape in combination with the separate “ear”-type air filters on the carburetors.

 

     Rather than using the inverted design, I chose a regular style foot-shift mechanism from an R51 gearbox.  A low gear ratio in the final drive assures good utilization of the high engine torque at modest revs. This is accomplished by means of a solo-version R67/2 final drive with a 32/9 gear set. At this moment I have no racing ambitions. So, besides the factory new cam shaft by Schleicher there are no further modifications of the top-end or pistons. Also, the carburetors of type Bing 1/25/1+2 are only marginally larger than factory specification. From the first 300 kilometers of gentle riding, I experienced very respectable power. I mostly ride an R66 and an R5 which may here serve as adequate objects of comparison. The engine of the R73 revs apparently less than the R5 but similarly to the R66. However, torque is impressive and obviously better than with the R66. On small winding roads the R5 requires much shifting between 3rd and 4th gear, as it accelerates better at higher revs. With the R73, in contrast, you can accelerate in 4th gear from almost normal speeds.

 

     The summer in Scandinavia is very short, so I did not manage to take the bike out for longer day-trips of 200-300 kilometers. The longest testing track I tried out at higher speed was a straight section of 20 kilometers on an open road, where I constantly kept the bike at 105 km/h. At this pace, the engine sounded relatively quiet (compared with the usual noise levels of the pre-war engines) and I felt there was much more to come once the new renovated engine bike is run-in.

 

      Another example of an R73 in the US is owned by Scott Williams, California. Below please see one picture from Scott. If there are club members/readers who do have more information on the technical details or the development and history of the CMR bikes, I would be happy to get in touch with them. I can be reached at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

 

Scott Williams' R73

 Footnotes to R73 Story:

[1] Stefan Knittel, "BMW Motorräder 75 Jahre Tradition und Innovation", 1. Auflage, Motorbuch Verlag, 1997, ISBN: 3-613-01829-2

[2] Heinz Härtel, "BMW Motorräder – Typen und Technik", Ariel Verlag , Frankfurt am Main, 1973. ISBN : 3-7606-0101-10

[3] “Etudes des BMW R75 et R73 - Caractéristiques des R73", in Revues Technique Motocycliste, Numéro 25 - Janvier 1950, 17 – 38

[4] http://en.wikipedia.org/wiki/CMR_(motorcycle_company>http://en.wikipedia.org/wiki/CMR_(motorcycle_company), accessed 2010-11-01.

[5]  http://www.worldlingo.com/ma/enwiki/en/CMR_(motorcycle_company>http://www.worldlingo.com/ma/enwiki/en/CMR_(motorcycle_company), accessed 2010-11-01.

[6]  http://www.motards-de-la-gendarmerie.info/histoire_cmr_cemec_ratier.html>http://www.motards-de-la-gendarmerie.info/histoire_cmr_cemec_ratier.html, accessed 2010-11-01.

[7]  http://www.moto-histo.com/BmwR73/BmwR73.htm>http://www.moto-histo.com/BmwR73/BmwR73.htm, accessed 2010-11-01.

[8] http://www.r73.de/>http://www.r73.de/

 

Last Updated ( Saturday, 26 January 2013 )
 
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