BMW R 51
(1952)1.JPG)
The BMW R 51 was a twin-cylinder motorcycle of the manufacturer BMW with a 500 cc - boxer engine and a power of 17 kW . It was built from 1938 to 1941.
Parallel to the sporty 500 series, a side valve-controlled 750 series with a slightly lower output was built under the designation R 71 , with high torque, preferably suitable as a sidecar trailer .
history
On February 18, 1938, BMW presented a new series of large-volume motorcycles with rear suspension at the 28th International Automobile and Motorcycle Exhibition (IAMA) in Berlin: the R 51 , R 61 , R 66 and R 71 models.
development
BMW had implemented four engine variants in one chassis - R 51 , R 61 , R 66 and R 71 - and thus created four motorcycles in different classes from 500 to 750 cm³.
"The similarity of the engines, such as the use of the same, all-wheel suspension chassis for all four machines, made it necessary to summarize them in just one manual, which at the same time gives an interesting overview of the entire BMW range in the large class."
previous version
The predecessor model of the R 51 was the R 5 , which was built until 1937 .
Design features
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 model, the BMW R 5, the engine is mounted in the frame with the engine quick-release axles. The muffler is attached directly to the engine axle and frame with clamps.
engine
The motor of R 51 has two chain-driven camshaft for hanging valves. In addition to the engine lubrication, the engines of the R 51 also have oil fillings that have to be checked and changed separately for each cylinder head. The only major difference to the engine of the R 5 is an upper engine support.
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 R 6, 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 bumpers actuate the hanging valves via rocker arms in the cylinder head. The rocker arms are like the two connecting rods Needle roller bearings; the crankshaft runs in ball bearings. The camshaft is driven by the crankshaft via a timing chain . The oil pump is located in the oil pan and is a simple gear pump that is driven by the camshaft via a worm gear reduction .
Ignition and alternator
The direct current alternator with charge regulator driven by the timing chain is visible on the motor housing . The ignition system is a battery ignition with an ignition distributor and manual adjustment of the pre-ignition, driven by a camshaft. The lead-acid battery is attached to a holder behind the gearbox without being encapsulated.
Carburetor, air filter
The two carburetors are Fischer-Amal round slide carburetors with a conical needle. 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 flywheel, which holds the dry single- disk clutch , sits on the rear crankshaft stub . The clutch is operated with a thrust bearing via a push rod that runs through the hollow main shaft of the gear unit. There is also a manual gearshift lever on the right side of the motorcycle.
transmission
The R 51 had a foot-shifted four-speed gearbox with a cardan shaft on the right side of the sprung rear wheel. In addition, there was a hand lever on the right side of the gearbox, which was particularly recommended for quickly finding idle. The tunnel gearbox was flanged directly to the motor housing. The four-speed input shaft was driven directly by the single-disc dry clutch in the crankshaft's flywheel.
Rear wheel drive
The rear wheel was driven by a cardan drive with the following elements in the power flow:
- Hardy washer on the output shaft of the gearbox to compensate the angle between the output shaft and the following cardan shaft
- Unencapsulated cardan shaft with universal joint fork on the output side - the visible part of the cardan drive
- Encapsulated universal joint on needle bearings for angle compensation between the cardan shaft and the following wheel drive
- Slidable universal joint fork for length compensation on the input shaft of the bevel gear axle drive
- Input shaft with pinion for driving the crown wheel in the drive housing - power deflection by 90 °
- Needle-bearing driving flange in the drive housing with splines for driving the rear wheel
landing gear
The double-loop tubular frame was welded from steel tubing. For the first time, BMW used straight-travel suspension for the rear wheel on the R 51, R 61, R 66 and R 71 models .
bikes
The R 51 is equipped with narrow half-hub brakes; the poor deceleration values of such "can-lid brakes" are to be viewed with caution nowadays and to be taken into account through a proactive driving style.
Technical specifications
| Parameter | Data of the R 51 |
|---|---|
| drilling | 68 mm |
| Hub | 68 mm |
| Displacement | 494 cc |
| Compression ratio | 6.7: 1 |
| power | 24 hp (18 kW ) at 4900 min -1 |
| Top speed | 135-140 km / h |
| Empty weight | 182 kg |
| Tank capacity | 14 liters |
Racing version R 51 RS
In 1939 17 units of the racing motorcycle, which can be bought for private drivers, were built. The weight has been reduced to 148 kg through the use of aluminum instead of steel and magnesium instead of aluminum and savings in approval-relevant components. The output of 36 hp at 7000 rpm enabled a top speed of approx. 185 km / h.
On the occasion of the “Las Vegas Motorcycle Sale” on January 6, 2011, the Bonhams auction house auctioned an originally preserved R 51 RS from the USA. The auction raised US $ 130,200 (~ € 96,000).
Successor version
The successor to the R 51 after the Second World War was the only slightly modified BMW R 51/2 .
See also
Web links
- BMW R 51. In: BMW History. BMW AG, accessed on January 26, 2017 (dossier of the BMW Group Archives).
Individual evidence
- ↑ BMW presents the model series R 51 / R 61 / R 66 / R 71. In: BMW history. BMW AG, February 18, 1938, accessed on January 26, 2017 (document in the BMW Group Archive).
- ↑ a b c d e manual for the BMW motorcycles R 51, R 66, R 61 and R 71. In: BMW history. BMW AG, April 1938, accessed on January 26, 2017 (manual with pictures, 48 pages).
- ↑ Spare parts list for the BMW R 51, R 66, R 61 and R 71 motorcycles. In: BMW Geschichte. BMW AG, April 1938, accessed on January 26, 2017 (spare parts list with pictures, 87 pages).
- ↑ BMW R 51 RS. In: BMW history. BMW AG, accessed on January 26, 2017 (dossier of the BMW Group Archives).
- ↑ 1939 BMW R51RS. Bonhams, January 2011, accessed on January 26, 2017 (English): "Rare, factory prepared competition unit, ex-Rody Rodenberg"
| class | Type | 1920s | 1930s | 1940s | ||||||||||||||||||||
| 3 | 4th | 5 | 6th | 7th | 8th | 9 | 0 | 1 | 2 | 3 | 4th | 5 | 6th | 7th | 8th | 9 | 0 | 1 | 2 | 3 | 4th | 5 | ||
| up to 250 cm³ | Touring motorcycle | R 2 | R 20 | |||||||||||||||||||||
| R 23 | ||||||||||||||||||||||||
| Sport motorcycle | R 39 | |||||||||||||||||||||||
| up to 500 cm³ | Touring motorcycle | R 32 | R 42 | R 52 | R 4 | |||||||||||||||||||
| R 3 | R 35 | |||||||||||||||||||||||
| Sport motorcycle | R 37 | R 47 | R 57 | R 5 | R 51 | |||||||||||||||||||
| up to 750 cm³ | Touring motorcycle | R 62 | R 11 | R 12 | R 71 | |||||||||||||||||||
| R 6 | R 61 | |||||||||||||||||||||||
| Sport motorcycle | R 63 | R 16 | R 17 | R 66 | ||||||||||||||||||||
| Prototypes, racing bikes | R 7 | |||||||||||||||||||||||
| WR 750 | R 51 RS | |||||||||||||||||||||||
| Military motorcycles | R 12 | |||||||||||||||||||||||
| R 75 team | ||||||||||||||||||||||||
