1970-1973: The BMW /5

The Slash Five bikes ushered in a bold new era for BMW. After more than six years on the drawing board this new platform would establish the direction of the marque’s motorcycle division for the next quarter century.

The /5 was a comprehensive reboot for BMW, with a completely redesigned chassis, engine, suspension and aesthetics.  The bikes came in three variations with 500, 600 and 750cc engines—the 50/5, 60/5 and 75/5, respectively. Besides some relatively minor spec differences the only thing that really separated the models was the bore of the cylinders.

As early as 1963 BMW’s development director—the perfectly German Günther von der Marwitz, who had then recently come over from a design post at Porsche—was test-riding prototypes of what would evolve into the Type 246 frame.

Von der Marwitz was an avid rider who rallied prototypes on the racetrack and had a fondness for the Norton Featherbed frame, which the Type 246 frame would bear more than a passing resemblance to. The Type 246 featured a double-walled backbone and double cradle frame made of conical tubing. With the exception of subsequent redesigned subframe/swingarm variations, the basic layout of the the /5’s frame would remain largely unchanged the next 25 years.

Freed of the design constraint to be sidecar-compatible, the /5 got a larger 19-inch front wheel, where previous models had 18” front wheels (were interchangeable with the rear or a sidecar). It also got a leading-axle telescopic fork, abandoning the twin-shock Earle’s front forks found on so many previous models.

The /5 engine received some comprehensive update as well, like new aluminum cylinders with bonded steel sleeves instead of the cast iron jugs of its predecessors, plus a new one-piece forged crankshaft that ran on overbuilt bearings and con-rods borrowed directly from the car division. Previous BMW designs used an open bath oil slinger setup, but the new /5 used a high-pressure oil pump system. While the Japanese competition was moving towards overhead cams BMW stuck with pushrod-operated valves, the tubes of which were repositioned underneath the cylinders for a cleaner, updated look.

Slash-5-cutaway

The new bikes got all new guts with 12-volt electrics powering a push-button starter (kick-start remained as a backup through 1974, and was an optional accessory after that.) The bikes also went to coil ignition with automatic advance; where previous BMW’s use 6-volt systems and magnetos.

Finally, the 75/5 started a long tradition of spec’ing CV (constant vacuum) carburetors made by Bing, an aviation brand for which smoothness and reliability are necessary, life-and-death virtues. (the 50/5 and 60/5 got older style TKTKTK Bings)

These telltale dome-topped Bings would become a familiar site on all BMW twins through the end of the airhead series in 1995. The smooth operating diaphragm equipped carbs, coupled with BMW’s trademark driveshaft, yielded the predictable power delivery that would become a benchmark of the brand.

The very first /5’s came with a big 22-liter (5.8 gal) touring tank, which thanks to the gas-sipping Bings gave the bikes incredible range. It was later offered with a smaller 17-liter (4.5 gal) with modernist chrome panels. Dubbed the “toaster tank,” these bikes have become synonymous with this early series, which today is highly collectible. But the toaster tanks were initially rejected by the U.S. motorcycle market. Consumers were so opposed to them that dealers had to sell retrofit kits to replace the chrome panels with black rubber kneepads.

Unlike its predecessors, the /5 came in a splash of colors—orange, white, blue, red, green, silver, and black, of course.

Updates were aplenty, but the /5’s DNA did retain some of the grace of earlier models, like a beautiful gauge and instrument light nacelle built into the headlight shell, the trade-mark hand-painted pinstripes along the tank and fenders, and the comprehensive factory toolkit under the seat that was good for both minor and major service work.

After decades of making comparatively archaic looking side-valve motorcycles, the /5 caught the motorcycle world’s attention.

As the UK-based Motor Cycle News gushed in 1970 review, ”The 750 BMW is a masterpiece of engineering which does not rely on sheer speed for its appeal. Quieter than many saloon cars, the R 75/5 will tick along in top gear at 20mph. It hums effortlessly at the British speed limit of 70. It will top the ton without coaxing. And it handles like a gentle giant”

OWNERS:
Black 60/5: Dan Greenbaum; Red 75/5: Angus Dykman; Blue: 75/5 Stuart Heys

Click Here to go back to the Main Menu of the interactive BMW Airhead Show, or follow the links below to the other bikes in the collection.

1970-1973: The BMW /5
1974-1976: The BMW /6
1974-1976: The BMW R90S
1977-1992: The BMW R100RS
1978-1995: The BMW R80RT and R100RT
1979-1987: The BMW R65
1983-1985: The BMW R65LS
1980–1986: The R80 G/S
1982-1984: The BMW R80ST
1987-1995: The R100GS
1992-1995: The BMW R100R