From BMW Tech
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The BMW K75 is a BMW motorcycle that was produced from 1985–1995. The K75 was a successful entry-level Sport touring motorcycle. At the time of its introduction, the K75 was BMW's cheapest motorcycle. It was intended be a Sport bike offering a claimed acceleration of 0–60 mph in 4.6 seconds and a top speed of 120 mph (193 km/h).
Engine and transmission
All K75 models share the same drivetrain. They are powered by a 740 cc Straight engine, Bosch fuel injected, engine. The US EPA specific engine produce 68 hp (51 kW) while all others produce 75 hp (56 kW). It utilizes a five-speed transmission and a Driveshaft final drive. The engine used by the K75 is often described as "bulletproof," indicating that it is quite reliable. This engine has an advantage over the K100's larger four-cylinder because it has inherently small secondary imbalance, and perfect primary balance via counter rotating Balance shaft, as opposed to the K100's inline four cylinder engine, a type noted for its secondary imbalance vibration.
Various models of the K75 were produced.
- K75, with no fairing
- K75T, US only model with a windscreen, touring bags, engine crash bars, and rear top case
- K75C, with a small handlebar mounted 'cockpit' fairing
- K75S, with sports fixed fairing and lower bars
- K75RT, with full fairing for 'road touring'
S and RT versions have a rear disc brake and 17 inch rear wheels whereas the others have a single leading shoe drum brake and 18 inch rear wheels. A stiffer, "anti-dive" front suspension was added to the S model. The RT version has a windshield that can be raised or lowered. Some taller riders complain of wind buffeting with the smaller S model stock windscreen.
Possibly the greatest strength of the K75 lies in its engine, which is a liquid-cooled, inline, fuel injected three cylinder design rotated 90 degrees. It is not unusual for this engine to go more than 200,000 miles without significant repair. One example, "Old Smokey," has accumulated more than 340,000 miles. The engine produces very low vibration at the handlebars, an accomplishment which few manufactures have achieved in the two decades following this design. The bike was also one of the first motorcycles to be equipped with a catalytic converter and stainless steel exhaust. The engine was designed to be less vulnerable to damage should the cycle fall over.
Background to K75 launch and design
The K-bike lineup, including the K75 and K100, were not just new models; these designs were radical departures from almost every aspect of previous ones. The K-bikes introduced new technology and refinement for a premium brand. BMW and Harley-Davidson were the only major manufactures that did not offer liquid-cooled engines. Competing brands, notably of Japanese manufacture, were touting the superiority of their liquid-cooled engines and had introduced low maintenance shaft-drive technology into a growing number of their models.
Two unexpected surprises occurred:
- The motorcycle industry was caught by surprise by an unprecedented multi-year decline in motorcycles sales, and BMW was faced with a rebellion of existing R-bike owners who threatened to abandon the brand altogether if the air-cooled boxer motor was eliminated. R-bike owners had become accustomed to easy maintenance and a tried-and-true design which had remained fundamentally unchanged for decades...
- The K models required less frequent maintenance intervals than the older R models but were much more time consuming and difficult to work on. Spare parts for the K bike are expensive, often twice as much as R-bike parts. As a result, the K-bikes failed to replace the older air-cooled R-bikes or Boxers which were updated with oil-cooled cylinder heads. As a result, R-bikes make up about eighty-five percent of the motorcycles that BMW produces.
The K-bikes brought unprecedented refinements in the motorcycle world including: computer-controlled fuel injection, catalytic converter, all stainless steel exhaust, rust-free aluminum fuel tank, anti-lock brakes or ABS, mono-shock absorber, rear wheel attached by a mono-arm, tool-free adjustable headlight, 460 watt alternator, cigarette lighter accessory plug-in, self-cancelling signal lights. It would take more than two decades for other brands to catch-up. The new engine of the K75 produced some of the lowest vibration of any motorcycle ever made. It was superior to the K100 which was designed a few years earlier. These amenities came at a price: US$10,000 for a K75S with fitted options in 1992.
Twenty years after launch the K75 has shown few weaknesses. The engines offer reliability and low maintenance over hundreds of thousands of miles. Some common problems are listed below:
- Speedometer — The speedometer was designed for use in airplanes by MotoMeter. Moisture can get trapped and cause the lens to fog. BMW acknowledged the problem by adding Gore-Tex vents to all models, 1987 and later, to reduce fogging. The lenses can also get small cracks if exposed to high temperatures.
- Radiator Fan — Radiator fans are known to seize after about 60,000 miles.
- Transmission — False neutrals between gears when shifting are another well-known problem that has affected many older R-bikes. Excessive movement know as play or slop can develop as a result of a loose grub screw or set screw inside the transmission.
- Engine Smoke at Startup — Occurs when the bike is left on the side-stand, even for a short period of time. This was caused by un-pinned rings in early models of the K-75. Post 1989 models have the rings pinned with the split on the top so that oil does not leak out when the bike is on the side stand.
- Rotors Crack — Some units developed cracks in-between the vent holes in the front rotors and warping in the rear rotor. BMW changed the rear rotor to a thicker solid rotor and hardened the front rotors which required different calipers.
- Drive Shaft Spline Problems — Drive shafts require proper lubrication, if neglected the splines will wear and the drive shaft will be destroyed. The cost of replacing a set of drive splines is often prohibitively high compared to the value of the motorcycle. BMW claims to have had the shafts nickel-plated from approximately 1988 on, but proper lubrication is still the best way to prevent spline wear. Many k75's have over 100,000 miles on the original set of splines.