BMW K 100 and Yamaha XJ 900 N in comparison

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BMW K 100 and Yamaha XJ 900 N

Classic naked bikes on tour

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Finally spring! Our barn find, the BMW K 100, is drawn back out into nature after decades of deep sleep. She is accompanied on her first outing by a Yamaha XJ 900 N. The two strong nudes have more in common than it seems.

D.he most important motorcycle for BMW at the beginning of the 80s was – no, not the R 80 G / S, this rather fortuitous hit from the boxer kit. But the BMW K 100. The decision to build the four-cylinder at the beginning of 1979 was not only a signal for the departure into the future, it was also THE long-awaited commitment of the BMW superiors to the motorcycle. A courageous one at a time when the Japanese manufacturers were relegating the venerable boxer concept to a niche product for wealthy traditionalists in terms of performance and quantities.

With these thoughts in mind, I push our K 100 into the glaring sunlight. The red metallic paint now shines almost as it did before November 5, 1990, the beginning of the more than two decades of Sleeping Beauty. A push of a button, and the four-cylinder comes to life, inhaling wonderfully fragrant spring air for the first time in a good 21 years. As a matter of course, as if it had never existed, the long period of silent infirmity in a dark hall. I am relieved because the effort for resuscitation was ultimately manageable.

BMW with brittle charm

With steady idle, the four-cylinder warms up, grumbles gently from the exhaust, overlaid by the whistling of the helical gear set to the output shaft, the hum of the fuel pump in the tank and the ticking of the injection valves. A soundscape that is as typical as it is complex, which Mockers once compared to a vacuum cleaner. At the debut there was also malice for the appearance of the K 100, which took getting used to, which did not necessarily correspond to the current ideal of beauty. In particular, there was blasphemy about the lamp mask, the radiator trim with the BMW kidney grille or the coarse footrest plates, which are more reminiscent of agricultural machinery. And, of course, about the long stainless steel exhaust, which to top it all had to hide under a black panel. It looked like it had been retrofitted, as if BMW had realized too late that the square stovepipe was not a well-rounded thing from an aesthetic point of view.

Today, the design of the BMW K 100 is still polarizing, even though the waves have long since smoothed out and have even given way to silent admiration. Its strictly functional, no-frills design needs no explanation, and gives the naked K a very independent style, which even many critics of the time attest to its character and above all a remarkable timelessness. No, the BMW really doesn’t look old. Maybe that’s why the K 100 is not really on the radar of the classic scene. The “flying brick” will celebrate its 30th birthday next year. The naked Bavarian definitely deserves a little more attention. Even if, with its brittle charm, it still appeals more to sober pragmatists than to aesthetic lovers of classic elegance.

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Yamaha XJ 900 N.

Yamaha with classic lines

They rave about the undisguised Yamaha XJ 900. With her air-cooled, shiny silver in-line four-cylinder, lots of chrome, slender lines and gently flowing shapes, the Japanese woman, who in a direct comparison seems almost delicate, represents a delightful contrast to the imposing German. But only for the eye, because in terms of both weight and performance The two were on the same wavelength of the targeted target group. Back then, in 1985, the K 100 and the XJ 900 N were the only two undisguised four-cylinder touring bikes in the Big Bike class that sent their torque to the rear wheel via a low-maintenance cardan drive. However, little of this can be felt on the XJ 900 N..

Even when accelerating strongly, the rear only lifts out of the springs, thanks to the 47 centimeter long swing arm, the righting moment of the cardan is kept within limits, even when driving at a brisk pace. The five-speed gearbox, which is as smooth as butter and precisely shiftable, shows fine manners, and its gear ratio also perfectly matches the performance characteristics of the air-cooled four-cylinder engine. The narrow two-valve engine is a striking gem that flatters the eye from every angle with its classic dohc layout, central timing chain, filigree cooling fins and the finely brushed housing cover. In terms of driving dynamics, the engine does not show any nakedness either, boldly hangs in at 1500 tours and increases its torque pleasantly evenly and powerfully on the way through the speed range up to the performance zenith of 98 hp at 9000 tours. He is a sovereign drive with a gentle nature that you can confidently recommend to less experienced bikers. The only drawback: the significantly increasing vibrations over 6500 revolutions.

In this respect, however, the crew on the BMW has to suffer far more. In contrast to the disguised sisters RS and RT, the row four of the naked K was screwed rigidly to the bridge frame without rubber bearings. The result: across the entire speed range, it tingles sometimes in the hands, sometimes in the soles of the feet or, most importantly, in varying degrees of intensity. The vibrations are most unpleasant in the area of ​​the recommended motorway speed. So, off the train, into the lush green, yellow and white of spring, which you can enjoy much more intensely on the winding country roads of the Swabian Forest.

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BMW K 100.

The BMW is a case for Lange

And the world looks a lot more relaxed on the BMW. Especially for tall guys who take a casual, comfortable, slightly bent forward position with a comfortable knee angle on the high bench. The Yamaha integrates its pilot better, but forces tall ones to kneel at a sharp angle. And in a direct comparison, it is also noticeable that the long-stroke BMW four-wheel drive has more flavor than the Yamaha, it pulls up smoothly in fifth gear of the longer-geared transmission from idle speed. But he runs out of breath faster on the top, speed orgies are not his thing. Which is why you automatically do without hectic gearshift work in the reasonably precise transmission that operates over long distances. And prefer to surf the powerful torque wave. The relaxed character is underscored by the successful coordination of the injection, which does not implement gas commands particularly spontaneously, but is extremely gentle and jerk-free. All in all, a drive that is made for fast touring. And thus exactly met the requirements of the specifications at the time. Nevertheless, it is surprising how close the BMW and the Yamaha are when it comes to engines. Much closer than the concept, performance specifications and appearance would suggest. Even if the single-sided swing arm of the BMW, which is much shorter at 40 centimeters, does not prevent the up and down of the cardan shaft during load changes quite as effectively as the longer counterpart of the undisguised XJ.

Time for a short break, accompanied acoustically by the crackling and cracking of the cooling BMW stainless steel exhaust. So far our K has done very well, everything works perfectly. Surprisingly, even the moisture-sensitive Motometer instruments, which are absolutely airtight and have withstood the previous high-pressure wash without water penetration. The visual inspection only shows harmless oil mist on the underside of the engine, in the area of ​​the oil pump. The only annoyance: the black exhaust cover, which does what it usually does with a K 100 – it rattles.

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Yamaha XJ 900 N and BMW K 100.

Both surprisingly handy, but the BMW drives more stable

We go on, the streets become more angular and narrower, the asphalt more wrinkled. Now the chassis is required. The BMW rushes forward as if it wanted to shake the last bit of tiredness out of the beams. With its low center of gravity and the handy chassis geometry, it steers in much more jaggedly than the massive exterior suggests. The Yamaha, with its narrower tires, bends in a similarly agile manner, and the big bikes can be driven surprisingly effortlessly over winding stretches. If need be, you can rely on effective brakes with both, whereby the Brembo fixed calipers of the K 100 bite the discs a little more powerfully and more easily.

The Munich resident continues to enjoy advantages on bumpy slopes and drives more calmly than the XJ, which sometimes loses composure astonishingly early on such terrain. So it’s better to slow down, otherwise the beauty will turn into a beast, bucking and rocking to show the limits of its underdamped suspension. In addition, the fork has hardly any reserves with its standard springs that are far too soft and lock up even when the brakes are lightly applied. A well-known ailment of Yamaha, which today can be cured quite easily and, above all, very effectively with progressive fork springs, thicker oil and accessory shock absorbers.

With its stiff bridge frame, solid fork bridges, torsion-resistant single-sided swing arm and long-stroke spring elements, the BMW can cope better with transverse joints and frost cracks. However, the slack damped fork also gives little feel for the road.

A small update in the form of contemporary spring elements could be used by both, after all, hikers don’t go on tour with flip-flops either. But nothing more is needed to strengthen the distinctive all-round talents of the Yamaha and the BMW. Great, because these undisguised large four-cylinder cardan calibers are still an attractive alternative to the two-cylinder guild for many wind faces. The sun is slowly disappearing on the horizon, a glorious spring day with two powerful motorcycles is coming to an end. What remains is the realization that conceptually and visually such different cardan-naked bikes are much closer than expected. So close that it is almost a question of taste whether you tend towards the Yamaha XJ 900 N or the BMW K 100.

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Good-bye! The pilots clearly enjoyed the trip together.

Conclusion: Our opinions

Stefan Gluck about the BMW K 100
Well, I’ve never really liked the K 100 in terms of style. It only becomes apparent when driving. Beefy engine, decent chassis, that’s still fun today. The K 100 is a bike like a building society loan agreement: a little uncool, but very serious. You can build on this stone.

Uli Holzwarth about the Yamaha XJ 900 N
For me, the XJ 900 N is a naked bike straight out of a picture book, it cuts a fine figure from every perspective. Great also the cardan and the reliability. It is easy to change that she is on shaky legs. Then the XJ drives as it looks: harmonious all round.

BMW K 100

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BMW K 100.

BMW K 100 (Series I: 1983-1986)

Price 1983: 12,490 marks

Data (type K 100)

engine:
Water-cooled four-cylinder four-stroke in-line engine, installed lengthways, two overhead camshafts, two valves per cylinder, actuated via bucket tappets, displacement 987 cm³, output 66 kW (90 PS) at 8000 rpm, max. Torque 86 Nm at 6000 rpm Power transmission: Single-disc dry clutch, five-speed gearbox, cardan drive Chassis: Bridge frame made of tubular steel, telescopic fork, Ø 41.4 mm, single-sided swing arm, one spring strut, tires 100 / 90 V 18 at the front, 130/90 V 17 at the rear, Double disc brake at the front, Ø 285 mm, fixed calipers, disc brake at the rear

Measurements and weight: 
Wheelbase 1516 mm,
Weight with a full tank 243 kg
Vmax: 219 km / h

technology

In 1983, the technology and concept of the K 100 were radically new. Although Ariel had already tried a longitudinally installed, lying four-cylinder prototype with a cardan shaft in England 20 years earlier, BMW was the first manufacturer to bring this unusual configuration into series production. The Munich-based company justified the courageous decision in favor of the “Compact Drive” baptized drive concept with the advantages that cannot be dismissed out of hand. The aim was to transfer the positive properties of the boxer to the new generation of vehicles. These included a low center of gravity, easy-care cardan drive and the ease of maintenance and repair of the four-cylinder engine thanks to its good accessibility. With the Compact Drive System, for example, the crankshaft drive with crankshaft, connecting rod or piston can be reached without having to dismantle the engine; the cylinder head with its innards is freely accessible. Another advantage of the special arrangement of the drive unit was the independent appearance of the new four-cylinder series, desired by the BMW executives.

It was clear early on that the mixture preparation should be carried out by an electronic injection system, since the engineers in the automotive division already had enough experience with the precisely working Bosch L-Jetronic with overrun fuel cutoff. As spectacular as the entire drive train with the single-sided swing arm made of cast light metal, mounted in the gearbox housing, looked, the interior of the long-stroke engine (bore 67, stroke 70 millimeters) was just as conventional. Two overhead camshafts that each actuated two valves via bucket tappets – that had long been the Japanese standard. But not the stiff bridge frame, which received the drive unit at five points and integrated it in a load-bearing manner. The cardan shaft’s shock absorber was also new: two concentric tubes, separated by a pressed-in rubber bushing, allow brief slip in the event of heavy load peaks.

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BMW engine: Long-stroke engine with a wide usable speed range: the robust and durable, horizontal four-cylinder.

Checkpoints

Mechanically, the K 100 is one of the most solid constructions, mileage of 200,000 kilometers and more are more the rule than the exception with good care. However, this does not mean that you can negotiate an all-round carefree package with one K. Because the unusual concept also shows weaknesses in detail. The teething troubles of the early years of construction are legendary, which despite goodwill and model updates could not always be cured. Even today, steamed up instruments are a typical annoyance, as are electronic problems caused by loose plug connections, leaking brake cylinders or torn exhaust covers. In the beginning, the main stand broke more often, as did the retaining tabs on the side covers due to incorrect handling. Sometimes it also pushes the lubricant out of the engine and fork seals. Loss of coolant in the combined oil / water pump is also known.

market

The naked K 100 is offered relatively rarely, but this is not reflected in collector’s prices. On the contrary, most prospective buyers do not see the original version as a future classic, but as an opportunity to buy a BMW. The price range starts at around 1000 euros for battered long-distance machines and, even for top models with a manageable mileage and history, usually barely reaches the 3000 euros mark.

Specialists / forums

If you have problems with the K 100, any BMW dealer can usually help. A lot of helpful information can also be found in the absolutely recommendable and extremely competent BMW-K forum at www.flyingbrick.de.

history

1987: The second series of the K 100 with the flat plug-in seat, the K 75 tank, more chrome and high handlebars made cruisers until 1989. The radiator cover is also new (15,800 marks)

Yamaha XJ 900 N

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Yamaha XJ 900 N.

In detail: Yamaha XJ 900 N (1985-1986)

Price 1985: 10,278 marks

Data (type 58L)

engine:
Air-cooled four-cylinder four-stroke in-line engine, two overhead camshafts, two valves per cylinder, actuated via bucket tappets, displacement 891 cm³, output 72 kW (98 PS) at 9000 / min, max. Torque 81 Nm at 7000 rpm. Power transmission: multi-disc oil bath clutch, five-speed gearbox, cardan drive

landing gear:
Double loop frame made of tubular steel, telescopic fork front, Ø 38 mm, two-arm swing arm, two spring struts, tires 100/90 V 18 front, 120/90 V 18 rear, double disc brake front, Ø 267 mm, fixed calipers, disc brake rear

Measurements and weight: 
Wheelbase 1480 mm,
Weight with a full tank 242 kg
Vmax: 217 km / h

technology

When it debuted in 1983, the XJ 900 represented the classic Japanese standard with its air-cooled inline four-cylinder, two overhead camshafts and two valves per cylinder, combined with a low-reaction cardan. The latter came from the XJ 650 Turbo, while the four-cylinder was the drilled out version of the XJ 750 Seca. And that in turn was based on the 650, which had been built since 1980. With the 97 hp XJ 900, however, this modular system reached its limits, it initially had to make do with 853 cc. The engine housing of the 750 series was nothing more than that. At least that’s how Yamaha justified the little label fraud at the time. It hasn’t been an issue since the 1985 model year. With 1.5 millimeters more bore (now 68.5 millimeters) with a stroke of still 60.5 millimeters, the XJ 900 with 891 cm³ now lives up to its name. Other changes included larger carburettors, an adapted exhaust system and reinforced piston pins.

The increase in displacement gave the XJ not only more power, but above all more torque over the entire speed range. And not just on paper: the XJ 900 from 1985 with the specified 98 hp developed noticeably more steam than earlier models, which hardly ever came close to their rated output. At the special request of the German Yamaha importer, the disguised XJ 900 F was given a naked sister this year, the XJ 900 N. Practical: For the model only offered in Germany, the handle reached into the parts shelf of the XJ 650, the headlights, Cockpit and handlebar donated. In addition, the undisguised 900 had to do without the already useless anti-dive on the fork, and the shock absorbers and adjustable damping were still rationalized away.

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Typical XJ 900: robust engine, slimline thanks to the alternator positioned behind the cylinder bank.

Checkpoints

There was a good reason that the XJ 900 models were offered until 1994: the outstanding reliability. In fact, the XJ 900, regardless of whether it was an F or N, was one of the most problem-free motorcycles on the market, usually easily covering more than 100,000 kilometers. There are practically no serious mechanical weaknesses. If an XJ twitches, it is mostly due to a maintenance backlog or lack of care. Nevertheless, of course, an XJ 900 also has a few problem areas. Rusty exhaust systems, for example. Or steering head bearings and struts that wear out early. And vibration damage to the trim parts. On the F models, the instrument glasses also tear more often. Most of the weaknesses can, however, be remedied relatively easily. Except for the rare goal well-known rust perforations on the frame beams. Usually the left one in the area of ​​the side stand mount is affected. So take a closer look here too!

market

While the F is offered in all price and kilometer categories, the naked 900 is one of the rarities on the market. An XJ 900 N is therefore traded much higher, whereby the condition has a more decisive influence on the price than the mileage. Top specimens with less than 50,000 kilometers can still bring over 3000 euros.

Specialists

If the XJ 900 causes few problems, then there is no need for specialists. Especially since every Yamaha mechanic should still be able to master the XJ mechanics in their sleep.

Forums

Tea currently most extensive German pages about the XJ 900 F / N can be found on the Internet at www.xj-forum.de.

history

1983: Launch of the XJ 900 with 853 cm³, 97 hp, handlebar-mounted cockpit fairing and fork with anti-dive. Conversion campaign to frame-mounted fairing due to chassis weaknesses (9458 marks)

1987: Last technically important revision: The front internally ventilated brake discs are replaced by the solid discs and brake calipers of the XJ 600. Price: 11,200 marks

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