The Little Boxer: 1969-1973 BMW R50/5

The BMW R50/5 and others in the “Slash 5” series represented a revolution in BMW motorcycle design.


| July/August 2015


BMW R50/5
Years produced: 1969-1973
Claimed power: 32hp @ 6,400rpm
Top speed: 97mph (est.)
Engine: 498cc air-cooled OHV flat twin
Transmission: 4-speed, shaft final drive
Weight: 451lb (wet)
MPG: 45-55mpg
Price then/now: $1,025 (1970/est.)/$1,500-$4,000

It seems almost unthinkable now, but in the mid-1960s, BMW came close to abandoning motorcycle production altogether. Sales were in decline, with potential buyers questioning whether they should pay as much for a motorcycle as a small car. BMW was also focused on developing its automobile business, which meant its motorcycle range got somewhat neglected.

The Bavarian maker enjoyed a reputation for expensive, solidly engineered and beautifully built bikes that were reliable, sedate tourers. The then-contemporary R50/2, R60/2 and R69S used a heavy, plunger-derived frame and Earles front fork (telescopic in the U.S. only), while featuring anachronisms like magneto ignition and a 6-volt electrical system powered by a DC dynamo.

In 1964, BMW made the crucial decision to continue making motorcycles, recognizing in the process that a complete redesign of its range would be required. Once committed, BMW lured engineer Hans-Günther von der Marwitz away from Porsche to complete the makeover. The bikes would be built at a new factory in Spandau, near Berlin.

The “Slash 5” series (R50/5, R60/5 and R75/5 of 500cc, 600cc and 750cc) was announced for the 1970 season, and represented a revolution in BMW motorcycle design. Though the basic flat-twin “boxer” engine layout remained, just about everything else was new. Gone was the built-up crankshaft and gear-driven camshaft of the previous range. The new engine used a forged one-piece crankshaft, and borrowed plain-bearing rods from BMW’s 2.8-liter 6-cylinder car engine. The duplex chain-driven camshaft now ran below the crank instead of above. Iron-linered light-alloy cylinders replaced the previous cast iron types and were capped with redesigned cylinder heads fed by dual Bing slide carbs (CV on the R75/5). The specification included 12-volt electrics with push-button start, though the kickstarter was retained. Power went through the traditional single-plate, engine-speed clutch to a 4-speed gearbox. The tubular steel frame used duplex tubes with the drive shaft built into the right-side rear swingarm.

The rear spring/shock units were adjustable for pre-load with a simple hand lever. Telescopic forks (a BMW innovation from the 1930s) replaced the Earles fork at the front end. Wheels were 19-inch diameter front and 18-inch rear, with 3.25 x 19-inch front and 4.00 x 18-inch rear tires. Brakes were twin-leading-shoe drum front and single-leading-shoe rear, both of 7.87-inch (200mm) diameter.

Whistlingman
7/26/2015 4:53:43 PM

I had a 71 R75/5 I bought in 84 with 33K miles. I traded the stock seat for a Saddleman because you could not ride over 200 miles sitting on that board. It was easy to maintain yourself and ran smooth as silk at 70 MPH. The front brake was for looks only. I would have converted to a later model disc brake if I had kept it, not being a purist myself. I sold it when it had 65K miles and started using oil. Now I ride a R100RS. Excellent brakes!


Yukoner
7/24/2015 1:10:08 PM

Interesting you should say that. The disc brake that followed the /5 had a horrible reputation. First disc from BMW and without holes in the disc, many retro fitted the drum. I'm just rediscovering the advantages of the drum brake and my Toaster has the best front drum brake I've ever encountered. My Honda 450 had one of Honda's first disc brakes and it functioned great, but my '68 Triumph had a single leading shoe drum which also seemed adequate to my needs. But this was was followed closely by Triumph's first disc. It seems to have been a pivotal era in engineering developments. The even bigger issue with the /5 was the wheel base, length. BMW extended the length by two inches in subsequent years, apparently to keep the bikes from experiencing a high speed wobble, otherwise called a "Tank slapper".


TONYC
7/23/2015 8:20:27 PM

While funky and cool, the slash 5s had an issue which caused me to get rid of my toaster tank 73. Fred Flintstone could stop faster than that drum-braked bike!






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