Motorworld’s newspaper №72

Despite the World War II, heavy activity on motorcycle design development in the Soviet Union had been kept up. But dozens of engineering designs & plans didn’t come down to us. Historical data is poor so we cannot pay tribute to all Russian geniuses of engineering science. To fill the gaps, we have to break through enormous amount of work. Alexey Popov, a restorer, made two replicas of M-73 at his workshop. M-73 is an experimental Soviet motorcycle having sidecar wheel drive. At this, each bike has a sidecar wheel drive made from scratch using the only survived drawing, which was a part of the test report by the General tank-automation command.

It should be noted that there were no all-wheel drive motorcycles in the Red Army in 1944. But a lot of motorcycles having sidecar wheel drive were captured. Generally they were BMW and Zündapp. These German machines made such a good showing that the Red Army command set a task to design & test the similar machine for army’s own needs.

The early tests were conducted in 1944. The machines under test were based on the production model М-72. New motorcycles had the following differences:

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

One preproduction prototype was equipped with a mass produced M-72 engine. Another one had an overhead valve engine from the M-75 model. Along with two new M-73 motorcycles, a mass produced M-72 was put under the test. Both M-73 prototypes 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.

On heavy-textured soils (muddy & sandy cross-country tracks), prototype models showed increased off-road capabilities compared to a production model. Steering response of M-72 was dramatically impaired on a muddy worn-down road – a motorcycle was drawn in the muddy track making further moving impossible. But an M-73 successfully got over such a road with no trouble.

Nevertheless, the prototype had some design weaknesses – several assemblies & parts failed or broke by the end of the test. Recommendations were drawn in the end stating for making some assemblies suitable for heavy-duty applications should the machine been cleared for serial manufacture.

But this has not happened. According to the historical data, it was agreed that the machine had no any significant advantages compared to a production model (i.e. standard M-73), and, along with an increased price and manufacturing challenges rendered it an economically unsound project.

The M-73 project was revived in 1947 (read about this in our article describing M-73 with an overhead valve engine) when 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). The off-road capability of M-73 has been compared with a mass produced M-72 and BMW R-75 that showed the best results excluding the boggy test on which M-73 was a winner (providing the sidecar wheel drive was on).

On completion of testing in 1947, it was decided that the sidecar wheel drive for mass produced М-72 machines makes no sense due to the fact a series-produced M-72 engine has sufficient capacity.

Years of manufacture 1944, 1947
Quantity produced, units 2
ENGINE AND TRANSMISSION
Type Twin-cylinder, opposite, 4-stroke, sidevalve
Engine capacity, cc 746
Bore and stroke, mm 78 х 78
Engine rating 22 hp at 4400-4900 rpm
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