Another Brick in the Wall: 1986-1992 BMW K75S

Best bets on tomorrow’s classics: 1986-1992 BMW K75S.

Left Front View of BMW K75S

Another Brick in the Wall: 1986-1992 BMW K75S

Image courtesy BMW

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1986-1992 BMW K75S
Claimed power: 75hp @ 8,500rpm
Top speed: 115mph (period test)
Engine 740cc liquid-cooled, DOHC inline triple
Weight (dry): 504lb
Fuel consumption: 35-55mpg
Price then/now: $5,950 (1987)/$2,900-$4,800

In the late 1970s, BMW’s overhead valve boxer twins were looking rather dated alongside the feisty 4-cylinder offerings from Japan. They were also down on power, performance and sporting appeal compared to their Asian competition. What to do? BMW needed an all-new design that reflected their reputation for engineering innovation and sophistication. An inline four would give them performance and smoothness, but simply borrowing the current favorite Japanese engine layout was verboten! So in a brilliant piece of (literally) lateral thinking, BMW’s engineers took an inline four from a Peugeot 104 car, laid it on its side lengthways and hooked it up to the transmission and shaft final drive from an R80 boxer twin. It worked. The Flying Brick was born.

The 1983 production K100 engine was of BMW’s own design, though, with dual chain-driven overhead camshafts, two-valves per cylinder and liquid cooling. Fueling was by Bosch LE-Jetronic injection, while inductive electronic ignition provided the sparks. Like the boxer twins, the engine drove a dry single-plate, engine-speed clutch and 5-speed transmission through a monolever single-sided swingarm to a hypoid final drive gear in the rear hub. The drivetrain was suspended from the steel tube backbone frame as a stressed member. A single spring/shock unit, adjustable for preload, formed the rear suspension. At the front, the telescopic forks contained damping in one leg only, the other acting simply as a spring guide.

Introduced in 1986, the K75 was essentially a K100 with the front cylinder missing, and a pair of balance shafts in the engine to quell vibration from the rocking couple of the triple’s 120-degree crank. And like its liter-class brethren, what impressed testers at the time was how BMW had managed to preserve the character of their boxer twins — which Cycle magazine defined as “civility, simplicity and excellence” — in the K-series bikes.

When launched, the K75 came in three flavors: The naked K75, the touring-oriented K75C and the sporting K75S, which differed from the plain Jane and C versions by adopting the 17-inch rear wheel and disc brake from the K100 instead of an 18-inch wheel and drum brake. Inside the K75 engine went higher compression pistons than the K100 (11:1 instead of 10.2:1), which together with changes to intake and exhaust systems gave 75 horsepower compared with the K100’s 90 ponies.

So with the K75S also being 22 pounds lighter and therefore nimbler than the K100, it’s not surprising it was designated the K-range’s sport bike. And like the other K-bikes, the S got equipment that would be considered adequate even today, like a 460-watt generator and full instrumentation with an LCD clock and hazard flashers.

The K75S’s sporting ambitions were expressed visually by the wide (for the time) 130-section rear tire and slender, compact, frame-mounted fairing. “The S wears the most carefully sculpted fairing ever seen from Germany, a masterpiece of integrated finish and aerodynamic function,” Cycle gushed.

But the upgrades were more than skin deep, most noticeably in the suspension: “Its fork is superb,” Cycle said, noting that the other K-bikes would dive noticeably under braking, sometimes even bottoming out at the front, while the K75S’s fork was “smooth, solid, responsive enough to keep the front tire planted in bumpy turns, firm enough to prevent bottoming under heavy braking … the S bike’s combination of firm, balanced suspension gives the rider control unparalleled in BMW’s fleet.” Cycle also noted that a stiffer rear suspension prevented the “jacking” that affected other BMWs under hard acceleration.

A couple of issues did come to light during testing. After parking overnight on the sidestand, the K75S would blow blue smoke from the exhaust — the result of oil draining from the crankcase into the cylinder heads, which are on the left and the lowest point of the engine when on the sidestand. BMW’s answer was to recommend parking K-bikes on the centerstand. Rider magazine also suffered a broken rear shock mount, which collapsed the suspension, locking the back wheel.

Overall, though, testers loved the K75S. Summing up, Rider concluded that BMW had “produced the best argument yet for building their unique kind of motorcycle.” And what Cycle liked best about the K75S was how it “sharpens that special BMW experience, and, with better control, elevates it to a higher road speed.”

Contenders: Sports Rivals to BMW’s K75S

1986-1990 Kawasaki ZG1000 Concours
Claimed power: 108hp @ 9,500rpm
Top speed: 125mph
Engine: 997cc liquid-cooled 16-valve DOHC inline four
Weight: 631lb (dry)
Fuel capacity: 35-50mpg
Price then/now: $5,699 (1986)/ $3,500-$6,500

Few motorcycles become legends in their own time. Kawasaki’s Concours is one. Derived from the Z900 and Z1000R Ninjas, the Concours effectively created a new class of Japanese motorcycle — a sport-touring bike with the emphasis on touring. Yes, the CBX and Kawi’s own KZ1300 had evolved into touring bikes, but as Goldilocks would have said, the Concours was “just right.”

A bulletproof 997cc double overhead cam, 16-valve liquid-cooled four was mounted transversely as a stressed member in a triangulated backbone frame. Drive to the 6-speed transmission was by straight-cut gears with shaft final drive. Front suspension was 41mm telescopic forks, with Kawi’s Uni-Trak adjustable single shock at the rear. Wheels were cast aluminum with triple disc brakes. The whole thing was wrapped in a full touring fairing with removable hard bags standard. It worked well enough for Road Rider to contend that, “A well ridden Concours is going to surprise a lot of out-and-out sporting bikes in the twisties,” while offering a “happy joining of sporting characteristics and touring capabilities.” While awarding the Concours Bike of the Year for 1986, Cycle Guide’s only real criticism was the low windshield, which caused turbulent air at helmet level. They concluded, “… the Kawasaki will also set new standards of comfort in a category that’s no longer limited to RS BMWs and canyon racers with luggage.” The legions still on the road speak to that, and the bike continued in production through 2006 with minor upgrades.

1986-1993 Moto Guzzi Le Mans 1000
Claimed power: 84hp @ 7,800rpm
Top speed: 125mph
Engine: 949cc air-cooled OHV 90-degree V-twin
Weight: 507lb (dry)
Fuel capacity: 35-50mpg
Price then/now: $4,685 (1986)/$4,500-$7,000

The 1,000cc Le Mans first came to the U.S. in 1980 as the CX100, the increase in capacity from 850cc intended to compensate for tighter EPA emission rules. The full-meal deal for 1986 had 40mm pumpers and bigger valves, giving a quarter-mile time of 12.5 seconds and 110mph. It carried swooping bodywork and a 16-inch front wheel, both, it’s said, at the personal behest of then Guzzi owner Alejandro de Tomaso. Underneath was a familiar plot: an air-cooled, 88mm x 78mm, 90-degree OHV V-twin mounted longitudinally in Lino Tonti’s graceful twin-downtube cradle frame, a dry clutch and 5-speed transmission with shaft final drive. Stout 40mm forks and dual Koni shocks connected the plot to cast wheels, 18-inch rear and 16-inch front (18-inch from 1988), with triple-discs. Unlike the Concours and K75S, if you wanted luggage, you bought your own. And comfort? “You must be joking,” Cycle World said. “This is a Le Mans.”

Was it any good? One Dr. John Wittner and his team thought so. Suitably modified, their Le Mans 1000 won the 1985 AMA/CCS US Endurance Race Series against the might of Japan. The durability of the big V-twin is the stuff of legend, although it’s now almost impossible to get tires for the 16-inch front wheel. “Tops in its class,” Cycle World said. “But then, what else is in a class with the Moto Guzzi Le Mans 1000?” MC