BMW R75/5, /6 and /7 Airheads

With the end of WWII in 1945, BMW lost its aircraft engine and automobile business, and like many European manufacturers, the company focused on the need for two-wheeled transportation.
But European motorcycle sales peaked in 1954, as small cars like the VW Beetle and the new Fiat 500 became more affordable. BMW entered this market with a license built Isetta bubble car, but with limited success: more affluent customers wanted a “real” car. Then in 1960, BMW made a radical strategic decision to focus its automobile division on sports sedans, introducing the “neue klasse” range in 1962. The 1602 and 2002 models followed.

Meanwhile, BMW motorcycle sales tanked from 30,000 a year in 1954 to 5,500 in 1957. Development had more or less stalled too, with relatively minor changes. There was a switch from a telescopic fork, used from 1938 to 1954, to an Earles fork front end. Also, a separate shock absorber, aka, damper, was added to the existing rear suspension. It was becoming clear that an equally radical change was needed for the motorcycle business. It seems astonishing now, but BMW seriously considered scrapping motorcycle production completely. Management understood that a complete redesign was needed to bring its product up to date while incorporating modern manufacturing techniques. This would mean a new motorcycle production plant as well. Big bucks. And the new bikes would need to be sufficiently different to avoid being labeled heavy, stately and slow like the contemporary /2 range — this without compromising BMW’s reputation for sound engineering and product quality.

By contrast, the US motorcycle market was expanding in the 1950s and ’60s, and BMW saw an opportunity. Motorcycles were associated with performance, an image BMW wanted to nurture in its cars, so the company made the decision to stay in the bike business. BMW lured Hans-Gunther von der Marwitz from Porsche and gave him the job of re-inventing the boxer twins. Thus was born the “slash” series of BMW boxers, the /5, /6 and /7. Von der Marwitz’s basic design was so good, it was still the basis of the last 1000cc “airhead” BMW’s of 1996.

Though von der Marwitz retained the opposed-twin engine layout, he completely revised its internals, using a one-piece forged crank with car-type plain big end bearings (with con rods from the 6-cylinder car engine). The camshaft moved below the crank driven by chain, not gears. A handsome aluminum casting that also contained the crankcase, housed the alternator and starter motor (though the kick starter was retained) and airbox. The automotive-type disk clutch drove a four-speed gearbox with final drive, naturally, by shaft. The starter required 12-volt electrics supplied by a 180-watt alternator.

The frame was completely new too, with a dual-loop cradle attached to a tubular spine and with a separate rear subframe. Dual lever-adjustable Boge shocks controlled the swingarm, which also housed the final drive shaft on the right side. Gone was the Earles fork replaced by Sachs telescopic units. Wheels were wire spoked with aluminum rims. Von der Marwitz intended the frame to provide good handling and a comfortable ride; but the inherent flexibility of the bolt-up construction, a weedy pressed steel top fork yoke and long travel suspension gave the /5 a “loose” feel, earning it the nickname “Gummikuh, or “rubber cow.” At speed, the lack of rigidity could prompt wobbles or even, in at least one anecdote, a tank slapper. A longer swingarm, introduced during 1971, helped alleviate the problem.

The new BMW range (500cc R50/5, 600cc R60/5 and 750cc R75/5) retained many of the BMW idiosyncrasies, like a headlight nacelle, enamel tank roundel and plunger ignition switch, which Ian Falloon described as “rather quaint.” (BMW Boxer Twins, 2004). But the /5s were a country mile ahead of the /2–and effectively saved the motorcycle division from dissolution.

The new R75/5 was the first post-WWII 750cc BMW, and that same capacity was continued in the /6 range of 1974 and the /7 of 1977. In particular, the 750cc models, which appeared in all three main iterations of the “slash” series – including the “toaster” tank /5 of 1972 – set new standards for performance and comfort. And while the “toaster” styling was controversial, the /5 received a number of improvements around this time: the front fork was upgraded and the front wheel widened. To emphasize the new performance focus, US distributor Butler and Smith developed R75/5s to compete in the West Coast production race series. During 1972-73, Reg Pridmore took 15 wins out of 23 starts.

Which is best?

No question that the later machines, like the R75/6 and /7 of 1974-78 were better overall than the first /5s with improved handling, braking and power delivery. But along the way, something was lost. While the /5s could clearly trace their heritage back to the /2, it was more difficult to see the lineage in the later /6 and /7 models. That said, the bolt-up rear subframe was retained throughout.

The later 750s were also sold in smaller numbers than the /5 and /6, and their relative scarcity can make them more collectible. All of the 750 airheads are bullet-proof reliable with regular maintenance and are capable of high mileage without major overhaul. The five-speed transmission improved rideability, while the front disc brake on the /6 and /7 is more effective than the earlier drum. The substitution of tubeless cast alloy wheels added to the /7’s utility and made dealing with punctures easier.

So the choice really comes down to practicality, aesthetics, and personal preference: embracing heritage and tradition, or opting for modern convenience?

Contenders: Alternatives to the R75/5

1974-76 R75/6

For 1974, the 750 received a front disc brake (with dual discs an option), a five-speed gearbox, and electrics upgraded with a 280-watt alternator. But the big news was the introduction of the Hans Muth-designed 900cc R90S and a 900cc R90/6, so the R75 was no longer the biggest capacity in the range. And while the crankcases were strengthened to accommodate bigger cylinder barrels, it’s worth noting that all the “airhead” BMWs from 500cc R50/5 to the eventual 1000cc R100 retained the same stroke of 70.6mm.

On the cosmetic side, the /6 inherited a new fuel tank and seat, with an instrument binnacle replacing the headlight nacelle. In fact there were many engine internal upgrades to increase strength and durability (such as needle roller rockers) to accommodate larger capacities and power outputs. At the same time, the swingarm was strengthened, brake caliper and master cylinder ratios changed to reduce hand lever pressure, and a new Boge front fork fitted to suit a new 40mm caliper.

1976-78 R75/7

With the introduction of the R100RS and R100S, these top-of-the-line models received new cast alloy wheels and a rear disc brake, while the R75/7 retained wire-spoke wheels and the rear drum brake until 1977. It also benefited from the 280-watt alternator as opposed to the 240-watt item from the R90S. As before, the kickstart was still an option.

Not surprisingly, development was focused on the Hans Muth designed R100RS, and it seemed likely that the /7s would quietly fade away, but the sting in the tail arrived with the R80/7 replacing the R75 in the model lineup and producing an extra five hp.

It’s astonishing that the new format and architecture had taken BMW Motorrad from likely closure in the late 60s to a performance powerhouse and design leader in just seven years.