What amazes me the most when I look at classic BMW motorcycles from the early 20th century is how similar their basic design is to modern models. A low-slung boxer engine with shaft drive had always been the fundamental design that differentiates BMWs from other bikes—except when it wasn't. Facing increased competition from the Japanese Big Four and tightening emission standards, BMW was forced to try some different designs in the early 1980s, starting with the K-bikes.
YouTuber The Illestrator (that's how he spells it) recently picked up a rough 1986 K100RS with plans to convert it into a unique cafe racer. He's made a number of impressive videos about the history of various makes and models, and here he covers the early BMW K-bikes as a lead-up to his build series. Skip ahead to 0:50 to get to the meat and potatoes about the Beemer.
Like the Japanese competition, BMW decided to go with an inline-4 engine for its new bike but laid it down on its side. This keeps the bike's center of gravity low like its traditional boxer engines. It's also great packaging for a shaft drive, requiring only one 90-degree bevel drive rather than two for a chain, ensuring that more power actually reaches the back wheel. The engine looks like a giant metal box sitting at the bottom of the bike, which earned these bikes the nickname of "Flying Brick."
BMW produced many variations of the K100, as well as the smaller K75, which used the same engine with one cylinder lopped off to make an inline triple. This basic design also led to the BMW K1, not to be confused with the modern Yamaha R1. This was the most aerodynamic bike of its time, with so much bulbous plastic bodywork that it almost looks like my Honda PC800 with a skinny butt.
The most notable feature of this engine was its use of fuel injection. Though common today, it was practically unheard of in the early 1980s. Even my PC800, which was considered super advanced for its time, still relied on good old fashioned carburetors.
Ironically, despite all of the technology in the K-bikes, the Japanese still beat out the Beemers on horsepower. This was not because of technology, but a German agreement not to build bikes with more than 100 horsepower. It wasn't until the introduction of the K1200 in 1997 that BMW finally broke this barrier. The modern K-bikes are obviously much more advanced than the original ones. Today's K1600s use a smooth inline-6, but it still sits fairly flat like the original K-bikes, not vertical like the old Honda CBX.
BMW figured out how to keep its boxer engines powerful and clean-burning over the years. They are available in many modern models. But the K-bikes were an important departure from this formula, one that led to a great deal of innovation in BMW's high-tech designs.