Disclaimer:
This is an excerpt of a document written by Bruno Saillard .
The complete original document (in French) can be found at: http://ecsm.dyndns.org/~afmb/nouveau/img/membres/articles/K100LT.html
The credit to the author must be included in every reproduction of this document.

Crazy Frog.

Owner since May 1987 of a first K100LT, I propose to brighten the long winter evenings of my friends by trying to resume the history of the K100, K100RS, K100RT and K100LT. I do not take into account the K100 RS/1 although this is an excellent machine but the 16 valves engine is too different from the K100 range.

  BMW motorcycle riders must admit that the car technology had made the birth of the K series possible. More precisely, engineers had tested their original idea with a Peugeot 104 engine ( on the first K prototype!

 In 1977, Josef Fritzenweger opened the way for the new concept. This represented a blow to the ideology of boxer engine worshipers but it was for BMW an urgent need to avoid a commercial marginalization like Moto Guzzi with their V twins. BMW had to change their design. Although they were resolutely decided to continue the production of the boxer, they could not take the risk of moving away from the technological innovations that benefited the range of BMW automobiles. The aim was to produce a motorcycle with the most modern techniques, while respecting the broad principles of BMW motorcycles (shaft drive and cylinders in a flat configuration…).

The idea of Josef Fritzenwenger was to have the 4-cylinder engine positioned lengthwise which offers the advantages of an in line engine with the good sides of the boxers. The center of gravity is low, the shaft drive is reliable and the accessibility of the engine is excellent. In addition, a 4-cylinder can easily obtain more power. It is quite economical to build, the noise is manageable and it is so rigid that it may be an element of the motorcycle frame.

For the very first prototype, BMW had selected a foreign engine to test the concept. They used a Peugeot 104 (http://en.wikipedia.org/wiki/Peugeot_104) because it is set in the car in an almost flat position, it is cooled by water and its attachment points were close to the idea of Fritzenwegger. Besides, it was an aluminum engine therefore relatively small. The experiment succeeded and a BMW gear box was connected to the engine. The car on 2 wheels demonstrated that it was feasible and the prototype was destroyed. There are no official pictures of it.

Encouraged by these results, the research team began the task of building an engine adapted to the bike and borrowed the technology from the existing BMW automobiles. The dream of making a versatile engine for motorcycle and car was the basic objective. The first trials brought disillusion, because the engine proved to be far too heavy and too long. The history of K100 suffered its first impasse.

On the 1st of January 1979, Wolfgang Aurich, Karl H Gerlingen and Eberhardt Sarfert were appointed as head of BMW. They were asked to help to save the motorcycle production and had to think about a new range of models.

Martin Probst, father of the Volkswagen Golf engine and father of the BMW formula II (which was covered with glory), was appointed director of the engine department - Motorcycle development.

The concept of the future engine was set on February 20, 1979: Compact Drive System, which will be immediately patented. Besides, the K series hold numerous patents to avoid probable Japanese copies.

After the conception on the drawing board, the second phase was the construction of a prototype of the engine. This achievement did not take place without difficulty. The engine was assembled with a temporary exhaust, a manual adjustable timing and a fuel injection system. On August 18 1980, the engine started for the first time. Then they developed the cooling and lubrication system. Two dozen points of measurement were integrated in the cylinders and head to feed the Electronic Fuel Injection with indications about the engine temperature. The cooling system and lubricating system were quickly optimized. The gas combustion was studied and the ideal point of ignition was developed. Several came shafts were tested and the exhaust was developed. Engineers have chosen the electronic injection and it was entrusted to Bosch. In fact no manufacturers were able to supply carburetors small enough to fit this limited space. In addition, BMW had already mastered the electronic fuel injection control. The first tests were encouraging and a power of 90 horsepower was obtained.

The severe endurance test on the bench (10,000 hours) revealed that the basic construction was strong. The tests of the rubber bumpers on the output shaft were problematic. With the heat of the engine oil, the material disintegrated. Seals also caused more research than anticipated. The chemical composition of each seal had to be studied separately. The engine was tested for 500 hours and monitored by a computer. It is common place today but it was not in 1980. The speed and load was constantly varied. The oil was heated to extreme temperatures and thermal shocks were caused by injecting ice cold water into the cooling system. This represented 50,000 km in the worst conditions. After these tests, the K589 prototype was ready for the first road tests.

In autumn 1981 the first two models took the road under a coarse camouflage. They were equipped with two modified flat-twin exhausts and an electronic injection was temporary adjusted. They made 60,000 km each. In spring 1982, the 2 prototypes were modified with all the necessary adjustments resulting of the road tests.

 Three new prototypes were again tested by engineers, mechanics and pilots at the test track of Fiat (Nardo in southern Italy). From morning till night, the pilots ran around the 12 km track. Each motorcycle rode 30,000 km. It turned out that the development of Bosch EFI still left to be improved at high speed. The engines were covered with oil because the seals were not adequate. That is why the BMW team went for a new set of test at Nardo in autumn 1982. The tests at high speed allowed making many changes.

On May 2 1983, the first 25 bikes came out of the Berlin factory. These models had to make a pre-250,000 km each before the K100 was finally presented to the public.

In September 1983, an international presentation occurred on the Cote d'Azur(French Riviera). The K100 were first loaned to journalists of major motorcycle medias for a quick test and then they were given to BMW dealers present at the event.

The first models were officially released to the public immediately after the 1983 Paris motorcycle show as it was sixty years earlier for the R32.

After decades of slow changes, BMW had developed a motorcycle radically different from what existed in the BMW (as well as the competition) range. The responses from the press were quite positive but the usual BMW clients were particularly shocked.

The year 1985 saw the emergence of a K100 touring series equipped with windshield and higher handlebars.

The K100 was manufactured from 1983 to 1991. BMW produced 12,871 K100 base model, 34,804 K100RS, 22,335 K100 RT, and 14,899 K100LT. The only difference on the K100RS is a different ratio of the differential.

The first models were not designed to run with unleaded gas. This modification started only in January 85 for K100 with frame number 0007291, November 1984 for the K100 RS (frame number 081107) and frame number 0024999 for the K100RT.

Owners of K100 bikes have had many complains for this bike while stressing the reliability of the engine. Here is an anthology and I let you assess the relevance of the accusations.

The owners of 83-84 models have suffered many set back but have always been supported by BMW. This had rarely led to the immobilisation of the vehicle.

-The front edges of the seat tear very quickly.

-The saddle is too high for people shorter than 1.70cm which requires removing 4 inches of foam. With the exception of LT saddle, it is slippery hard and uncomfortable. The seat of the LT turns into a bathtub after heavy rain.

-Unlike the former Krausers sold with the flat twins, the bags are full of nooks and not waterproof. In addition, the luggage is optional on all models except the LT.

-The luggage key (before the 1989 saddle bag model) is so small that you need a flashlight to find the locks at night!

-The 20 litres top-box is much too small and the lid tends to open on its own if it is not locked.

The hinges were placed in the back of the case and if not closed properly, the lid was catching the wind.

-The 30 litres top box of the 1988 k100LT leads to the instability of the bike at high speed. It also cracks quickly (point of attachment).

-The lids covering the radio and glove boxes fly very easily if they are not locked.

-With the exception of last LT, you need more than one key as locks are all different.

-With the fairing (RT/LT), it is almost impossible to access the steering lock

-The rubber covering the handles stays in your hand when taking the bike off the centre stand (motorcycle to the ground).

-The side stand folds automatically and a distraction can lead you to drop the bike on the ground. Furthermore, this stand hits the exhaust when it folds up

-The centre stand can be affected by internal corrosion and the motorcycle will crash on the ground on its own. This is a well known problem for the models built before 1988 (The rubber plugs at the end of the tubs prevents the water entering the centre stand)

-The wind pressure is unsustainable over 160 km/h for the K100 basic leading to the obligation to buy a fairing.

-The fairing of the RS is only efficient for people shorter than 1.80 m. It creates a back draft in your neck and in the face of the passenger.

-The RT/LT fairing acts as a pressure cooker in the summer because hot air is pushed to your legs.

-The windshield equipped with a deflector protects only riders whose size is between 1.70 and 1.80 m. smaller riders do not see anything through the deflector and experience buffering.

To cut short the perpetual criticism, in 1989 BMW has installed, a windshield that could be cut to accommodate the size of the rider.

-Tall riders hit their knees on the edge of the fairing

-Rainy weather produces other inconveniences. Indeed the rear wheel waters copiously the backs and legs of the rider/passenger. The remedy is to extend the rear fender by installing a mudguard extension.

-The brackets of the side covers easily break.

-The foams handle grips of the K 100 LT quickly become sponges when riding under rain.

-Some series in 1989-1990 had brake discs problems. They were warping or prematurely wearing off.

-Brake pads squeak.

-At the exception of the 1988 with ABS, brakes are not at par with what you can expect for a modern motorcycle. The efficiency could be improved by the replacement of the brake line with stainless steel shielded ones. This prevent the expansion of the rubber hoses.

-Another criticism: The command for the flashers is not to the Japanese standard! The colors chosen are ugly and age very badly.

-The parking light position is situated just before the position "on". Thus it takes two notches to reach the position "off". The parking light often stays on if you are not careful.

The original battery of the first K100 (20Ah), is too low because it requires at least 25 to 30Ah to start the bike.

-The original head light is not powerful enough. It is advisable to quickly replace the bulb for an 80 or 100 watts. The replacement of the bulb is really hard with the RT/LT fairing.

-With the exception of the K100 LT ABS, there is no gas gauge or water temperature gauge.

The aluminum gas tank, is much too small (22 litres) and allows only 200 km of autonomy (highway riding) before the red light warns you that its time to fill up (280 km on regular roads respecting the speed limits).

The first gas caps were not waterproof and sometimes, gas vapours will dissolve the paint around the cap.

-The gas filter in the tank is not strong enough and with the gas pressure the filtering element will break and clog. It is imperative to change this filter every 15000 km.

-The square exhaust (4 sides as 4 cylinders) is a masterpiece of bad taste. Since 1986, there is a plate to hide this ugly muffler but the screws on the plate breaks with the vibration and easily rust. The solution is to weld the nuts. You must nevertheless stress that elbows exhaust and muffler are therefore indestructible (stainless steel).

-The rear shock is of very poor quality except for the Nivomat of the LT. This Nivomat remains functional for a long time if you ride solo. When riding in duo, it will have to be quickly replaced. To obtain a comfort up to the reputation of the Bavarian brand, you must replace the rear shock with an Ohlins or a White Power.

-The riding impression is different because of the handlebar is mounted flexible. With the exception of a few K100RS and K100LT in limited edition with a fork stabilizer, the handling is not the best in curves or at speed above 200 km/h. mounting a Téléfix system will resolve the problem.

-Unlike the Japanese competition, it is impossible to shift on the fly. Although powerful (90hp), the engine is linear and without much personality.

-The wheezing due to the electronic injection is particularly unpleasant. With this injection, the K100 runs poorly in the high altitude. This is due to the depletion of oxygen (Some models were equipped with an altitude plug but it never worked correctly).

-There are vibrations on the footrests between 4000 and 4500 rpm but they fade with time.

-The K100 vibrates more than their counterparts because of different mounting of the engine.

-Finally this bike requires specialized tools due to the use of electronic components. You can no longer tinker and the maintenance budget at the dealer is much higher than for a classic flat. In fact, even the tool to remove the oil filter is a specialized tool.

-The electronics doesn’t require adjustments and the valve keep their adjustment for a very long time.

-The majority of K100 do not consume oil. Until August 1988, K100 parked on the side stand were puffing a cloud of blue smoke when the engine started. The engine oil had the tendency to seep into the combustion chambers. On the 1988 models, BMW has developed pistons and rings with a different design to resolve this inconvenience.
 

-The oil  level sight glass (so convenient) tends to darken with time.

-Problems with the circuit board braking in the instrument cluster had required the replacement of the complete unit. Some bikes had to have the cluster replaced more than once and unless you have the service record of the bike, you cannot trust the mileage showing on the odometer.

-In wet weather, the needle on the tachometer (the speedometer) remains at zero, just give it a gentle tap and the needle will work again. This problem is due to a loose connection. The latest models have a connection reinforced by a screw. The instrument cluster has the tendency to fog up with cold weather. BMW tried to rectify the problem by the adding 2 breathers covered by a piece of Gortex. It never worked!