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|Class||standard, sport touring|
|Engine||longitudinal DOHC I4, 987 cc (60.2 cu in)|
|Bore / stroke||67 mm × 70 mm (2.6 in × 2.8 in)|
|Top speed||137 mph (220 km/h)|
|Power||90 hp (67 kW) @ 8000 rpm|
|Torque||63.3 lb·ft (85.8 N·m) @ 6000 rpm|
|Transmission||5-speed foot shift, shaft drive|
|Frame type||tubular steel, open cradle with engine as stressed member|
|Suspension||telescopic forks, single-sided swingarm|
|Weight||536 lb (243 kg) (wet)|
As the 1970s 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 needed over the amount of fuel entering the combustion chamber. From an engineering standpoint this was easier to achieve with more cylinders at an overall smaller displacement.
- The market-led development of bikes was leading to the Japanese factories developing smoother and quicker machines based around a four-cylinder format.
- Bike comparison in the media at the time was based around top speed, and a four-cylinder when fully developed created more power.
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.
The need for a quick development time scale of a clean burning four-cylinder engine and the fact that Honda had developed a flat-four boxer for the GL1000 Gold Wing, 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 choices. 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 from every other manufacturer, the choice was made to lay the engine flat in the frame to keep the centre of gravity low and improve the bike's handling. This is known as a longitudinal four because the crankshaft is in line with the direction of travel of the motorcycle. This configuration, an inline four cylinder engine with a longitudinal crankshaft had not been used since pre-WWI-era motorcycles such as the FN Four and Pierce Four, and overcame the problem of necessitating a horsepower-robbing 90-degree turn to the driveshaft. The second development took place around engine management, with Bosch L-Jetronic fuel injection, and later, Motronic fuel injection replacing carburettors, increasing power, broadening and smoothing the powerband and reducing fuel consumption.
These engineering choices meant that from start they had a solid-performing and marketable product, and the K100 was on the market and being sold within four years.
The engine has its four cylinders arranged so that the crankshaft is on the right-hand side of the motorcycle, with the cylinder heads, camshafts, injectors and spark plugs on the left-hand side. This arrangement keeps the centre of gravity relatively low, which benefits handling; and the space behind the front wheel available for the radiator.
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. BMW preferred a shaft-drive for smoothness, and a single-sided hollow swing arm enclosing the drive shaft provided right side drive through the gearbox and to the rear wheel. The 4-into-1 all stainless steel exhaust exited on the left hand side.
Brakes were twin-pot Brembo onto undrilled discs. Two different forks manufactures were used: Showa with an outer upper tube diameter of 1.612 in (41 mm) and Fichtel and Sachs measuring 1.627 in (41 mm). In later models, the standard swingarm was replaced with a Paralever just as on the K1.
Various models of the K100 were produced.
- K100, with no fairing.
- K100RS, with sports fixed fairing and lower bars.
- K100RT, with full fairing for 'road touring'.
- K100LT, with a higher screen and additional equipment as standard for 'Luxury Touring'.
All models have dual front, and single rear disk brakes. The RS model has a longer gear ratio than other models.
The K-series offered additional refinements including: all stainless steel exhaust, rust-resistant aluminium fuel tank (although tanks do corrode at the low points on right and left sides), anti-lock brakes or ABS on later-year models, adjustable headlight, high capacity 460 watt alternator, Hella accessory plug-in, self-cancelling signal lights.
Although sales were initially modest, buyers eventually warmed to the multi-cylinder BMW, and the three-cylinder derivation in the form of the BMW K75. The K100 was a relative sales success, stemming the losses to the Japanese and changing the media and public perception of BMW.
The four-cylinder engine suffered from secondary vibration, but the three-cylinder K75 was far smoother. The engineers had anticipated this, and had designed in excellent vibration isolation, but it was the only technical glitch.
The competition were never far behind in performance on launch, updates were modest, while engine performance was stepped up with the September 1988 launch of the radically aerodynamic BMW K1.
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