BMW R51/3

From Wikipedia, the free encyclopedia

R51/3 right side

The BMW R51/3 was BMW's first modern postwar 500cc motorcycle. It used BMW's time-tested boxer engine layout and exposed driveshaft. The R51/3 succeeded and modernized the 1950 R51/2, which was essentially a prewar design that was produced after the war.

1 History
2 Technical data
3 Gallery
4 See also
5 External links

[edit] History

Following World War II, Germany was precluded from producing motorcycles of any sort by the Allies. When the ban was lifted, in Allied controlled Western Germany, BMW Aktiengesellschaft (AG) had to start from scratch. There were no plans, blueprints, or schematic drawings. Company engineers had to use surviving prewar motorcycles to create new plans. In 1948, it introduced the 250cc R24, which was essentially a pre-war R23, complete with rigid rear end.

When larger machines were permitted, BMW introduced its R51/2 [1] in 1950, a model that was in production for only one year and that was essentially a prewar BMW produced after the war. The R51/3 was then introduced in 1951 for a production run of four years.

The 600cc R67, sister model to the R51/3 and almost identical visually, was also introduced in 1951, but it went through two revisions. The R67/2, with two more horsepower, came out one year later and was replaced in 1955 by the R67/3, which was in production through the 1956 model year.

In 1952, BMW introduced the 600cc R68 [2] [3]. With 35 HP and a compression ratio of 8.0:1, this was the first BMW production motorcycle that could reach and pass 100 MPH.

[edit] Technical data

The engine of the R51/3's predecessor, the 1950 R51/2, had an exposed generator, no magneto, and split valve covers

Compare the clean, unit-cast R51/3 engine above to that of the R51/2 above

	R51/3	R67 - R67/3
Design	Boxer Flat Twin
Engine	Four-stroke OHV
Model Years	1951 - 1955	1951 - 1956
Bore	68 mm / 2.68"	72 mm / 2.83"
Stroke	68 mm / 2.68"	73 mm / 2.87"
Displacement	494 cm³ / 30 in³	594 cm³ / 36 in³
Power	24 HP	26 HP / 28 HP
@ RPMs	5,800	5,500
Compression Ratio	1:6.3	1:5.6 / 1:6.5
Top Speed	87 MPH	93 MPH
Curb Weight	419 lbs.
Gross Vehicle Weight	783 lbs.
Fuel Tank Capacity	4½ gal.

[edit] Gallery

1952 R51/3 left side: note the half-width brakes	R51/3 plunger and exposed drive shaft	R51/3 "bell-bottom" front fender	1951 R67: note the rigid sliders on the front suspension, later to be replaced by rubber gaiters
Disassembled R51/3: note the full-width brakes from the last year of production	R51/3 identification plate