Another Brick in the Wall: 1986-1992 BMW K75S

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

| May/June 2014

  • Another Brick in the Wall: 1986-1992 BMW K75S
    Image courtesy BMW
  • 1986-1990 Kawasaki ZG1000 Concours
    Image courtesy Kawasaki
  • Contenders: Moto Guzzi 1000 Le Mans
    Image courtesy Moto Guzzi

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.

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