||It has been suggested that BMW K75 be merged into this article. (Discuss) Proposed since September 2014.|
|This article needs additional citations for verification. (September 2010)|
|Also called||"Flying Brick"|
|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 PS (66 kW) @ 8000 rpm|
|Torque||63.3 lb·ft (85.8 N·m) @ 6000 rpm|
|Ignition type||Bosch LE-Jetronic|
|Transmission||5-speed foot shift, counter-rotating clutch, 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:
- Emissions regulations being developed in the United States and the European Union 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.
BMW needed to develop a clean burning four-cylinder engine quickly. While a flat-four engine would have been suited to their boxer tradition and experience, it would also give the appearance that they were copying Honda's GL1000 Gold Wing.
In 1977, Josef Fritzenwenger presented a prototype using a PSA-Renault X-Type engine from a Peugeot 104. The engine, which was installed in the 104 at a 72° angle, was laid flat in the frame with the crankshaft on the right, running parallel to the centre line of the frame. This layout, for which BMW submitted a patent application, was well suited to BMW's traditional shaft drive, needing only one 90° bevel drive to transmit power to the rear wheel. Using shaft drive with the near-vertical transverse engine preferred by the Japanese manufacturers at the time, would have needed two 90° bevel drives, doubling the power lost to the inefficiency of these units. The new layout also kept the bike's centre of gravity low, which improved the bike's handling, and made space behind the front wheel available for the radiator.
Design and development
Fritzwenger's concept was developed by a team led by Stefan Pachernegg based on criteria set out by R. P. Michel and K. V. Gevert. Martin Probst, who had earlier worked with the development of BMW's Formula Two engine, was responsible for engine testing and development.
As an automobile manufacturer, BMW had about twenty years of experience with liquid cooled overhead camshaft inline engines. This was carried over to the K100 engine, which used a Bosch LE-Jetronic fuel injection similar to that being introduced on their second generation 3 Series cars. Replacing the Bing carburettors traditionally used on BMW motorcycles, the fuel injection system increased power, broadened and smoothed the powerband, and reduced fuel consumption, partly by shutting off fuel under deceleration to 2000 rpm. The engine also featured a breakerless electronic ignition system.
The engine was positioned with the crankshaft on the right-hand side of the motorcycle and the cylinder heads, camshafts, injectors and spark plugs on the left-hand side. This improved access to the engine over that of a conventional design, where the crankshaft would be at the bottom and the cylinder head and associated parts would be between the engine block and the upper frame.
The three-cylinder BMW K75 was developed alongside the K100. The K75 engine had a crankshaft with 120° between the throws and was counterbalanced by adding balance weights to the water pump accessory shaft, which ran at engine speed as is correct for a 120° straight-three engine. To increase the smaller engine's power, the K75's engine was given longer valve timing, the compression ratio was increased to 11.0:1 from the K100's 10.2:1,, the combustion chambers were redesigned, the intake manifold was shortened, and the exhaust system was retuned. The front engine mounts on the K75 frame are placed further back than in the K100 frame and the downtubes are at a different angle; otherwise the frames are identical. The K75 had the same wheelbase, seat height, and steering geometry as the K100. According to BMW, 80 percent of the K75's parts are interchangeable with the K100's.
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 two-piston Brembo callipers onto undrilled discs. Two different fork 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).
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, aluminium fuel tank, adjustable headlight, high capacity 460 watt alternator, Hella accessory plug-in, self-cancelling signal lights.
The same team would later develop a three-cylinder version of the engine for the 740 cc K75, and an improved four-valve-per-cylinder head for the aerodynamic K1. In later models, the standard swingarm was replaced with a Paralever just as on the K1. The LE Jetronic fuel injection system with separate ignition management was replaced by Motronic engine management with the introduction of four-valve cylinder heads with the 1990 K1 and the 1991 K100RS.
Anti-lock brakes (ABS) were developed for K100 and K75 motorcycles and were installed on later models, which were among the first production motorcycles with this feature.
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, with its balance shaft, 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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