Motorcycle Classics

The BMW That Never Was: The R73

Even with all BMW models since 1923 being referred to numerically like the R32 and the R90S, there is one moniker you won’t find in BMW history books, the R73.

BMW motorcycle nomenclature has always been about “numbers.”

Since 1923, all models have been referred to numerically, such as R32, R69S, R90S, and R1200GS, with the “R” standing for rad or cycle in German. However, there is one moniker you won’t find in BMW history books, the R73.

black and white photo of a motorcycle with sidecar drifting around the corner of a dirt track in a race

In 1944 as the Wehrmacht troops began retreating from Paris, they left behind a motorcycle repair depot with a cache of replacement engines and parts for BMW motorcycles. Amongst them were approximately 80 unused 750cc R75M engines. Recognizing an opportunity, the enterprising French soon set up a corporation and began manufacturing motorcycles under the name CMR, short for Centre de Montage de Reparation. Having little use for lugging a heavy sidecar, the French adapted the 750cc engine for use in the lighter, civilian R71 frame. Once the supply of surplus BMW frames was exhausted, CMR produced a close copy with the slight difference of round instead of oval frame tubing. These machines would later become known as the R73, a mix between R71 and R75 (71+75/2=73). The moniker is akin to Norvin for a Norton-Vincent conversion or Triton for a Triumph-Norton conversion. The popular BMW plunger frame design was subsequently copied by numerous manufacturers around the world. In Russia, Ural produced a near-direct copy with their M72 motorbike. The Chinese Chang Jiang is a knock off as well. Indeed, many of the parts interchange. It is common for bikes that saw postwar service behind the iron curtain to be a mix of parts.

front view of a black and gray motorcycle

rear left view of a black and gray motorcycle showing the alloy sleeve on the rear brake drum

On the road and track

Like many post-war scenarios, surplus R75 engines were plentiful, similar to the flooding of the U.S. market with Harley Davidson and Indian Scout engines after the war. The R75 engines were large and reliable with abundant horsepower. It didn’t take long for them to find their way into racers, typically side-car outfits. The plunger frame’s transmission and differential were typically retained for use as the units from the R51-R71 were a direct fit. Depending on how the engine was mounted in the frame, a shorter drive shaft was sometimes required. There was a rarer postwar conversion in which the transmission and differential from a Zundapp KS-601 was grafted onto a BMW engine. The Zundapp transmission was considered more reliable. The large displacement BMW engines (750cc) were occasionally sleeved down to 500cc to qualify for the contemporary FIM classes.

black and white photo of two people riding a motorcycle with sidecar on a dirt track rear view of a black and gray motorcycle

In addition to road racing, speedway and scrambling were quite popular in the early 1950s. The usually torquey, low-revving R75 engines were hopped up with new cam grinds, and the cylinder heads were milled to increase the compression ratio. As part of the disarmament of Germany, BMW was prohibited from building any new engines in the first few years after the war. As restrictions eased, displacement was limited to 250cc. In order to compete, BMW also began using surplus R75M engines in some of their own bikes for continental competition. Today an over-bored sidecar outfit with a 905cc engine is on display in their factory museum! It was successfully campaigned in the one-liter class.

red motorcycle with blue sidecar with yellow circular number plates with 11 on them

left side view of a red motorcycle with a circular yellow 11 plate on it

These bikes became a reflection of the desire to return to normalcy after a tumultuous six years. Improvise and keep on with life; a motoring life is a good balm.
The “sweaty” BMW pictured here turned up in Greece a few years ago. Built using a post-war R51 plunger frame and war-time R75 engine, it is in the spirit of the R73. The front fender is a Meier aftermarket and the mufflers are Hoske megaphones. The racing outfit or “seitenwagen” was campaigned in the 1950s around Germany and garnered 36 ribbons during its career. It was upgraded with a Zundapp KS-601 transmission and rear end. MC

right side view of a gray and black motorcycle

  • Published on Dec 13, 2021
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