BMW R 51/2
The BMW R51 / 2 is the first boxer - motorcycle that BMW built after the Second World War. It was only produced in the years 1950/51 in a small number of 5000 copies. The successor model , the BMW R 51/3 , was produced in far larger numbers from 1951.
history
BMW presented the R 51/2 to the public on October 18, 1949 at the Bayerischer Hof Hotel in Munich . The predecessor of the R 51/2 was the BMW R 51 model, built from 1938 to 1940, with essentially the same chassis, telescopic fork and straight-travel rear suspension.
Design features
The frame is welded from tubular steel with laterally attached ball heads for sidecar use. The S 350 or the TR 500 from Steib are suitable sidecars . A special gear ratio of the cardan drive is required for sidecar operation .
The fenders, tank and rims are made of sheet steel . The engine block, like the gearbox and the cardan gear housing, is made of cast aluminum . As with the previous models, the BMW R 5 and the BMW R 51, the engine with the engine quick-release axles is mounted in rubber bushings. The muffler is attached directly to the engine axle and frame with clamps.
engine
The engine of the R 51/2 is the last version of the pre-war two-cylinder with two camshafts above the crankshaft, which are driven by a timing chain ; the engine of the successor model R 51/3 has only one camshaft driven by spur gears . There is only one oil filling left in the engine; The predecessor engines of the R 5 and R 6 had separate oil fillings for each cylinder head in addition to the crankcase lubrication.
The engine block , like the gearbox and the cardan gear housing, is made of cast aluminum . As with the predecessor models BMW R 5 and BMW R 51, the engine is a longitudinally installed two-cylinder boxer four-stroke engine with bumpers running above the cylinders in chrome-plated steel tubes ( OHV engine). The push rods operate the rocker arms in the cylinder head, which in turn operate the overhead valves. Like the two connecting rods, the rocker arms are mounted in bronze bushings; the crankshaft runs in ball bearings. The oil pump is located behind the front housing cover and is a simple gear pump that is driven by a camshaft via a worm gear .
transmission
On the rear stump of the crankshaft sits the heavy flywheel (with the ignition timing mark that can be seen in the peephole), which holds the dry single- disk clutch . The clutch is operated with a thrust bearing via a push rod that runs through the hollow main shaft of the gear unit. This model has the classic curved chrome kick starter lever. There is also a manual gearshift lever on the right side of the motorcycle.
Ignition and alternator
The R 51/2 is the last model with an externally mounted alternator, which is on top of the engine block. The DC alternator with charge regulator is driven by the timing chain and also serves as a chain tensioner. The splash-proof encapsulated ignition distributor and ignition coil are located on the left camshaft behind the cast aluminum cover . This design can also be seen for the last time in the R 51/2 model. The break contact of the battery ignition is controlled by the left camshaft. The ignition timing is set manually, as there is no automatic ignition timing adjustment . The successor model R 51/3 has a magneto ignition without an ignition distributor, but with automatic centrifugal adjustment. The lead-acid battery is not encapsulated and is attached to the lower frame tube under the saddle.
Carburetor, air filter
The two carburetors are Bing float chamber carburetors with a conical needle in the round slide. The wet air filter element sits on the gear housing, a steel mesh on which the dust clings when the air is sucked in, which is washed out for maintenance purposes and then wetted with oil.
drive
The drive train requires an elastic torque transmission element at the transmission output. For this purpose, a black textile-reinforced rubber four-hole disc ( Hardy disc ) is pushed onto the two -finger flange of the gearbox, which transfers the torque from the gearbox output shaft to the two-finger mount on the cardan shaft. Due to the high centrifugal forces of the rotating rubber disk, the front of this hardy disk is equipped with a chrome-plated steel ring.
The cardan shaft to the rear wheel is high-gloss chrome-plated and runs freely. The universal joint of the cardan shaft attached to the bevel gear is covered with a chrome-plated screw cap to protect it from dust. A rubber seal that rotates with the shaft runs against this screw cap and keeps loose dirt out of the universal joint.
The cardan shaft is provided with a cardan joint and attached to the bevel gear by means of teeth. The deflection gear is a bevel gear with 90 ° deflection. The gear wheels of the deflection gear run in a special heavy gear oil for high flank pressures (hypoid oil).
bikes
The R 51/2 is equipped with narrow half-brake hubs. The successor model , the R 51/3 , only got full aluminum hub brakes at the front and rear with significantly improved deceleration values from 1954.
Technical specifications
| Parameter | Data of the R 51/2 |
|---|---|
| drilling | 68 mm |
| Hub | 68 mm |
| Displacement | 494 cc |
| Compression ratio | 6.3: 1 |
| power | 24 hp (18 kW ) at 5800 min -1 |
| Top speed | 140 km / h |
| Empty weight | 185 kg |
| Tank capacity | 14 liters |
See also
Web links
- BMW R 51/2. In: BMW history. BMW AG, accessed on September 19, 2016 (dossier from the BMW Group Archives).
- Spare parts list BMW Motorrad R 51/2. In: BMW history. BMW AG, September 1950, accessed on September 19, 2016 (spare parts list with pictures, 28 pages).
Individual evidence
- ^ Presentation of the R 51/2 in the hotel "Bayerischer Hof" in Munich. In: BMW history. BMW AG, October 18, 1949, accessed on June 19, 2016 (document in the BMW Group Archive).
- ↑ Manual BMW Motorrad R 51/2. In: BMW history. BMW AG, April 1950, accessed on June 19, 2016 (manual with pictures, 67 pages).
