BMW R 68
| BMW | |
|---|---|
 BMW R 68, model 1954: a specimen that was originally exported to the USA. Clearly visible: the manual shift lever and the unencapsulated cardan shaft |
|
| R 68 | |
| Manufacturer | BMW motorcycle |
| Sales description | BMW R 68 |
| class | Motorcycle , motorcycle team |
| design type | Sports Tourer |
| Motor data | |
| Displacement (cm³) | 600 |
| Power (kW / PS ) | ? / 35 |
| drive | Cardan drive |
| successor | BMW R 69 |
The BMW R 68 is a two-cylinder motorcycle with a displacement of 600 cm³ and a nominal output of 26 kW (35 hp) at 7000 revolutions per minute. The machine was developed from its 600 cm³ sister model R67 by increasing its performance. The standard equipment underlines the sporty note of the R68 with a narrow front mudguard, which made a stiffening bar between the fork legs necessary, as well as the special seat cushion, the so-called “racing roll”, above the rear mudguard, which is coupled to the saddle (and with it springing up and down) . The non-foldable additional footrests were not only intended for (sporadic) pillion use, but also allow the driver a more racing-like seating position.
history
In October 1951, the German motorcycle manufacturer BMW showed a new high-performance model in the style of the factory machine of the International Six-Day Tour at the IFMA in Frankfurt . A top motorcycle, a 160 km / h machine, which the experts quickly offered the title of " 100 mile racer " because it was the first German series motorcycle to crack the "100 mile limit" when it went into series production in the summer of 1952.
BMW motorcycles have always been 'facelifted' very carefully: in addition to many detail changes, there were three major 'facelifts':
- Mid-1952 (improved front fork, rubber sleeves)
- 1953 (Knecht air filter and sidecar connection)
- Model 1954, manufactured from September 1953 (solid hub wheels, light alloy rims and “torpedo” silencers).
In January 1955, the successor generation of the full swing arm models were presented at the Brussels Salon , the chassis of which once again reached the level of the competitors on the world market.
Only 1,452 examples of the R 68 were produced between 1952 and 1954, ultimately because of its exclusivity and the high purchase price of 3950 DM.
Top view of the BMW R 68, model 1954
R 68 boxer engine
BMW R 68, apparently with a modified exhaust system
BMW R 68 (1954) with Steib sidecar (1951)
technical description
The two-cylinder boxer four-stroke engine is installed lengthways. Its push rods ( OHV ) running over the cylinders in chrome-plated protective tubes operate the rocker arms in the cylinder head , and these actuate the overhead valves. The camshaft is driven by the crankshaft via a gear set running in an oil bath . The gear oil pump is located behind the front housing cover and is driven by the spur gears via a reduction gear.
Although the engine of the R68 looks very similar to the touring models R51 / 3 and R67 (/ 2, / 3), almost all parts have been adapted compared to the touring version. The compression was only increased moderately from 5.6: 1 to 7.5–7.7: 1 with the new sport pistons (now with a 20 mm piston pin). The cylinder head was completely redeveloped and had larger channels and valves as well as stronger ribbing, and the cylinder was also modified to accommodate the two additional fastening screws. The valve train has also been revised, a different camshaft (with an additional front bearing) controls the now needle-bearing rocker arm via more robust bumpers. Together with the large 26 mm Bing carburetors, the engine was ready for series production with a peak output of 35 hp instead of the previous 26 hp. In order to keep the premature failure of the main bearings due to bending vibrations of the crankshaft at high speeds, a barrel bearing (with slightly spherical bearing rollers) was installed as the rear main bearing . 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 also differs from the touring models in that it has a thinner friction disc and stronger springs, and the flywheel has been modified compared to the touring models due to changed ignition markings. The clutch is operated by a thrust bearing and a push rod that runs through the hollow main shaft of the gearbox. This model has the classic curved, chrome-plated kickstarter lever. On the right side of the gearbox - in accordance with the German standard DIN 73005 still valid in 1969 - there is also a manual gearshift lever; This manual shift option was last used by BMW in the "Gradweg suspension" models and makes it easier to find the idle position. Like the weaker sister models R 51/3 and R 67/2, the R 68 has a clockwise rotating cardan shaft with an external ring gear on the cardan drive; In the subsequent models, the direction of cardan rotation was changed due to the 3-shaft gear now used.
The BMW R 68 has an automatic ignition adjustment via a centrifugal governor. If the fuel quality was poor, however, the high-compression engine tended to fuel ringing (" engine ringing "). Therefore, the R 68 was also equipped with manual ignition adjustment : the ignition adjustment lever is located on the left handlebar grip. With it, the ignition point (angle before reaching the top dead center ) can be adjusted manually. With today's fuels, manual correction of the ignition point is no longer necessary.
The two-cylinder boxer engine of the BMW R 68 is fastened with 2 bolts in the double loop frame. The splash-proof encapsulated direct current alternator with charge regulator sits behind a housing cover on the front crankshaft stub . The lead accumulator sits unencapsulated on a holder on the left behind the gearbox.
The adjustments of the chassis to the high engine power were clear. It was limited to the development of special balancing weights and suitable sports tires for the wheels. As soon as the engine power of the sports machine was used to the full, the inadequate chassis technology became a disadvantage as a weak point of the machine. Compared to the international competition, the fast and very expensive BMW still had no swing arm and, with its completely inadequate straight-line suspension on the rear wheel, easily lost contact with the ground on bad roads. This now unmistakable weakness of the chassis in combination with the powerful engine brought BMW a lot of criticism.
The two carburetors are Bing float chamber carburetors with a conical needle in the round slide (designation: 1/26/9 and 1/26/10). The wet air filter element sits on the gear housing. The air filter and carburettor are connected by chrome-plated suction pipes without external rubber elements. The air filter is a steel mesh wetted with oil, on which the dust clings when the air is sucked in and which has to be washed out for maintenance.
The drive train has a black textile-reinforced rubber four-hole disk ( Hardy disk ) at the transmission output, which is pushed onto the two-finger flange of the transmission. It transfers the torque from the gearbox output shaft to the two-finger mount on the cardan shaft . The hardy disc is strapped with a chrome ring.
The non-encapsulated, chrome-plated cardan shaft for the rear-wheel drive is attached to the bevel gear in the cast aluminum body with a cardan joint . The deflection gear is a bevel gear with 90 ° deflection. The gears run in a special heavy gear oil for high flank pressures (hypoid oil).
See also
literature
- BMW Boxer R51 / 2 to R68, published in 2019 by Bodensteiner Verlag, ISBN 978-3-9816013-5-0
Web links
- BMW R 68. In: BMW History. BMW AG, accessed on July 1, 2016 (dossier from the BMW Group Archives).
- Handbook Motorcycles R 51/3, R 67/2, R 68. In: BMW History. BMW AG, July 1952, accessed on July 3, 2016 (manual with pictures, 51 pages).
- Spare parts list BMW Motorrad R 68. In: BMW history. BMW AG, June 1953, accessed on July 3, 2016 (spare parts list with pictures, 50 pages).
Individual evidence
- ^ Presentation of the BMW R 68 and R 25/2 at the IFMA in Frankfurt. In: BMW history. BMW AG, October 1951, accessed on January 9, 2019 (document in the BMW Group Archive): "At IFMA in Frankfurt, BMW is presenting the first German" 100-mile racer "with the 160 km / h fast R 68."
- ↑ Price list no. 2/1951. In: BMW history. BMW AG, October 1951, accessed on January 9, 2019 (document in the BMW Group Archive).
- ↑ From the R 67 to the fastest German series motorcycle, the BMW R 68 . In: Automobiltechnische Zeitschrift . 10/1952, pp. 235-236.
