Motorworld’s newspaper №71

In 1944, the General tank-automation command performed tests for motorcycles having a sidecar wheel drive. These machines were based on mass produced М-72 motorcycles. Two prototype models were manufactured (both had M-73 label): one was equipped with a standard M-72 engine and another one had a test M-75 overhead valve engine, which was based on the mass produced M-72 engine with cylinders & cylinder heads replaced. Interestingly, the test engine was partially built of German parts (cylinder heads, specifically). Another fact: that overhead engine showed an increased output & rpm but lower overall test results compared to a standard flathead M-72 engine.

None of these two motorcycles remained until today. 70 years passed since then, and now we have replicas built by Alexey Popov. Remember it was built from scratch using the only survived drawing as a part of the test report!

By 1944, the Soviet moto-engineers explored the captured German all-wheel drive BMW R-75 and Zündapp KS 750 and received an assignment from the Red Army command to create an own motorcycle with a sidecar wheel drive because there were no Soviet machines similar to the German ones. The new motorcycle was based on M-72, the Red Army’s war horse.

М-72 from the «Motorworld by V.Sheynov» collection

Zündapp KS 750 from the «Motorworld by V.Sheynov» collection

BMW R75 from the «Motorworld by V.Sheynov» collection

M-73 machines had the following differences (compared to a mass produced M-72):

  1. sidecar wheel drive;
  2. sidecar wheel brake;
  3. reverse gear ratio was 4.62 (M-72 had a 3.79 ratio).

Along with two new M-73 motorcycles, a mass produced M-72 was put under the test. Both machines had to test the capabilities and advantages of the sidecar wheel drive. According to the report, the test objectives were as follows:

  1. To determine the operational and physical characteristics (M-73).
  2. To determine the reliability of assemblies & parts (M-73).
  3. To determine the performance data (M-73).
  4. To determine the M-72 motor output when using a sidecar wheel drive.
  5. To perform a comparative assessment of the off-road capabilities between M-73 and M-72.

The sidecar wheel drive fell short of expectations on completion of testing. Although M-73 showed better off-road capability compared to the mass produced model, considerable gaps that should be eliminated were found (see the article describing a flathead M-73). Therefore, M-73 didn’t go into production.

In 1947, the Central design bureau of the General Directorate for the production of motorcycles and bicycles launched another run of the regular tests. As in 1944, two machines were put under the test: one machine was equipped with the M-72 engine, another one – with the M-75 engine (I7V). In order to determine an advantage of the sidecar wheel drive, the off-road capability of M-73 has been compared to M-72 (there were two test runs: with sidecar wheel drive on and off) and BMW R-75. BMW R-75 showed the best results excluding the boggy test on which M-73 was a winner (sidecar wheel drive was on). The advantage of M-73 was as follows: if two people would jump off the bike, it could proceed further without a stop while BMW R-75 immediately made a belly landing.

During the tests in 1947, the motorcycles has covered about 5,000 kms of asphalt, wetlands and sandy, muddy & country roads. Enormous stress the test machines were put under led to breakdown of components and assemblies that required an overhaul. Professionals of the Central design bureau of the General Directorate for the production of motorcycles and bicycles reached a conclusion that “because of design flaws revealed during the tests, M-73 cannot be cleared for production”. It’s interesting that the military of the General tank-automation command who performed an acceptance of the machine has reworded the conclusion as follows: “M-73 can be recommended for serial manufacture if the drive gear is improved followed by successful completion of tests”.

But that didn’t happen. The motorcycles built for 1944 & 1947 tests fell off the edge of the Earth. However you can always go to the Motoworld to see this engineering heritage.

Years of manufacture 1944, 1947
Quantity produced, units 2
ENGINE AND TRANSMISSION
Type Twin-cylinder, opposite, 4-stroke, OHV
Engine capacity, cc 746
Bore and stroke, mm 78 х 78
Engine rating Around 25 hp
Sparking Battery
Carburetor K-37
Battery 6 V
Clutch Single plate
Transmission 4-speed
FRAME AND WHEELBASE
Frame type Duplex tubular
Front suspension Spring-mounted, with hydraulic shock absorber
Rear suspension Candle-type spring-mounted, with no shock absorber
Brakes Drum type
Wheel size 3,75 х 19
DIMENSIONS
Length, mm
2 380
Width, mm
 1 690
Height, mm
 1 000
Wheelbase, mm
1 400
Ground clearance, mm
   132
Seat height, mm
790
Mass, kg
   355
Gas tank size, l
     21
Maximum speed, km/h
    82,3
Range, km