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Monday, March 28, 2011

BMW R51/75

This post will serve as a followup to some recent articles in the VMCA club newsletter.  If you are a vintage BMW enthusiast, you owe it to yourself to join the two clubs here in the states devoted to vintage BMW motorcycles, the BMW VMCA and the VBMWMO.

At the onset of WWII, the German army was supplied with sidecar machines in the form of the R12, and to a lesser extent the R71.  Midway through the war, BMW introduced the purpose built R75.  A robust OHV pushrod engine with 750 cc displacement, coupled with a complex, low geared gearbox, sporting on and off road gearing, as well as reverse, the machine proved its worth on the battlefield. It was a true GS (Gelande Strasse).  There was ample power to pull 3 passengers, often with machine gun mounted.  Occasionally small utility or ammo trailers were fitted to the rear.

Photo from

At the conclusion of the war motorcycle production in Germany ceased.  It wasn't until 1948 that BMW was again allowed to produce motorcycles, however displacement was limited to 250 cc.  The R24, based on the prewar R23 design was produced for two years before Allied forces eased restrictions, and the twin cam 500 cc R51/2 was added to the BMW line-up. 

R24's fresh from the factory line in 1948.
 Despite the desperate conditions in Germany immediately following the war, motorcycle racing soon began to flourish.  While anti-German sentiment meant exclusion from international venues, within Germany racing continued. 

The racing motorcycles that survived the war (many by being hidden underground, or in hay lofts) were unearthed and circled Teutonic tracks once again. 

Those wishing to compete in larger displacement categories, or sidecar divisions, had no options as far as new production. Many ingenious riders took to modifying the well constructed R75WH sidecar motors and placed them in either pre-war or post war plunger frames. 

These bikes have been called many things, with monikers including R73 (a cross between an R71 and R75) as well as R51/75.  Such bikes were successfully campaigned in solo and sidecar classes.

R75 Seitenwagen, BMW Group Classic Collection, München
In the example above you'll note embossed OZ74 below the serial number on the crank case housing, indicating its production during war time.  The Amal Fischer carbs used on this bike required the use of a special made adaptor as shown. The gearbox no doubt houses a special ratio for sidecar racing.  This particular housing having started life as a civilian model.  It is easy to see where the air filter mounting has been cut off, being obsolete once carbs with velocity stacks have been added.  For this reason, the 1936 R5 style gearbox housing is favored for racing applications.

Also, note the faltenbalge used on the front forks and rear suspension as a weight saving measure. 

R75 Seitenwagen.  This machine was for sale a few years ago.
Typical of the period would be the use of open megaphone exhaust.  Special gas tanks were either fabricated, or sourced from local craftsmen in Munich, such as Lugauer, who supplied sheet metal to the BMW Works. Sidecar machines often employed larger capacity tanks, as expected.

R75 Seitenwagen, Zweirad Museum, Neckarsulm

Unlike many pre- and post war BMW motorcycles, the R75 motor does not easily drop into a plunger frame without some frame modification.  The front engine bolt on the R75 actually goes through a boss in the front engine timing cover, instead of the bottom of the crankcase as is typical for pre- and post war plunger motors. As such, one needs to weld to the frame a point of attachment for the front motor mount bolt.

Below are some pictures of a privateer racer with German pedigree.  For more information you can check out RCS Racing

Eilenriede 1954

R51/75 Rennmaschine, solo, 1950 (authors collection)
In a future post I will detail the subtle nuances of my R51/75, which are more than they would appear at first.  I will also provide some interesting history and some video of the bike in action.


  1. Thanks for the high-res pic of a beautiful machine.
    looking forward to seeing more

  2. Hi Scott,

    I am interested to get in email contact with you
    and to learn more about your R73.
    Please visit my website and find my email there.

    Best regards