BMW K100

BMW K100RS

The BMW K100 is a 1000 cc touring motorcycle that was manufactured by BMW from 1983 to 1992. It was the bike that took the public's perception of BMW as just a manufacturer of flat-twin boxer engines.

Background

As the 1970's came to an end, BMW faced three problems from developing its flat-twin boxer engine further:

• Developing European Union emissions regulations, meant that more control was need over the amount of fuel entering the combustion chamber. From an engineering view point, this was easier to achieve with more cylinders of lower capacity
• The market-led development of bikes was leading to the Japanese factories developing smoother and quicker machines based around a 4cyclinder format
• Bike comparison in the media at the time was based around top speed, and a 4cyclinder when fully developed created more power, and should go quicker

In combination, this meant that BMW's marketing to users of a superior bike, allowing them to price at a premium, was being quickly lost, resulting in a loss of sales and market share.

Origin

In the late 1970s, following several years of declining sales, BMW engineers began work on developing a replacement for the aging flat-twin engines. The design team was led by Josef Fritzenwenger and Stefan Pachernegg.

Engine

The need for a quick development time scale of a clean burning 4cylinder engine, and the fact that Honda had developed a flat-4 boxer for the Honda GL1000 Goldwing, guided the design team led by Josef Fritzenwenger and Stefan Pachernegg to an existing liquid cooled Peugeot car engine.

To speed development but improve the engine, the team made two key developments. Firstly, as a car engine it was relatively large (most specifically wide, but also tall) compared to a motorcycle engine. As BMW needed something different to every other manufacturer, the choice was made to lay the engine flat in the frame, to keep the weight low and hence easily manageable. This is known as a longitudinal four because the crankshaft is in line with the direction of travel of the motorcycle. This configuration, although technically not new for motorcycles, had not been seen for many years, and over came the problem of splitting the crankshaft of the engine, and hence speeded development.

The second development took place around engine management, with Bosch Motronic fuel injection replacing carbureters, increasing power, power band and reducing fuel consumption.

These engineering choices made, meant that from start of the project to a marketable product, the K100 was on the market and being sold within four years.

The same team would later develop a three cylinder version of the engine for the 750cc BMW K75, and an improved four valve per cylinder head for the aerodynamic BMW K1.

K100

K100 engine closeup

The original engine has its four cylinders arranged so that the crankshaft is on the right-hand side of the motorcycle, with the cylinders, pistons, camshafts, injectors and spark plugs on the left-hand side. This arrangement keeps the centre of gravity relatively low, which benefits handling; and the sapce behind the front wheel available for the radiator, resulting in a reduced width.

In addition, since the crankshaft is now on the right-hand side, access to the engine becomes much easier than in a conventional design, where the crankshaft is at the bottom (hence the term "bottom end"). BMW preferred a shaft-drive for smoothness, so a standard swing arm provided right side drive directly to the gearbox and onwards to the rear wheel.

Brakes were twin-pot Brembo onto undrilled discs.

External links

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