Lovesong of the Flying Brick


Is it wrong to love an engine?

If you’re a BMW motorcycle guy, is it an even bigger sin if that engine isn’t a boxer twin?

Are Airhead engines sufficiently sentient to know if you’re ‘cheating’ on them?

“Dude”, you’re thinking, “You’ve got an awful lot of questions.”

“Got any answers?”

The only job of a motorcycle engine is to move the motorcycle its installed in down the road. Seems obvious enough, but how it completes that task is an ingredient that could be used to bake a baker’s dozen motorcycle books.

Now there are lots of guys that will tell you that way too much is never enough. These are guys that think that tire-shredding, burnout producing, ‘wheelies-in-all-five-gears’ power is the only kind of power.

I’m not one of those guys.

Don’t misunderstand me. There are lots of sporting applications where too much and then some is exactly what you want.

Prostock drag racing?


AMA superbike?


But for motorcycles operated on the street, especially for transportation, rather than sporting applications, its not so much how much power an engine can deliver, but the quality of how that power is delivered.

Hold on to the hate mail, peoples — I know this zip code doesn’t have a lot of neighbors.


Its funny how a single extended family of engineers can be so damned consistent.

I’ve ridden briefly in other people’s BMW automobiles, and my limited sample supports the notion of a ‘corporate standard torque curve’.

The CEO of a small company I worked for long ago had a very anonymous looking grey BMW sedan parked outside his office window. The Big Boss was a very conservative management type who had gotten to the corner office as the finance guy. One day, after I had seen him eyeballing my R90S out in the parking lot, he uncharacteristically asked me and one of my managers to lunch.

“So,” he asked coyly, “So you know about bikes. Do you know anything about cars?”

As we walked up to the car, he stuck his key in the trunk lock, and it was just as the lid started to rise I noticed the discrete “M5” badge against the grey paint.

Inside the trunk, it was like F1 Disneyland. There were color anodized aluminum bracing structures everywhere — all with holes of various radii cut into billet. The battery was in the rearmost portion of the right fender — there was a dry sump oil tank and color coded lines in the same position on the left. The strut towers were connected with more billet bracing, and I remember more color — mostly Gold — on the strut units themselves.

“Let’s go for a ride. Buckle your seat belt.”

Never let the countenance of someone who appears to be The Alpha Accountant throw you off the scent of a motor maniac. As the very junior member of this crew, I was in the back seat. Our offices were in a part of Northern Virginia that still could be called rural at that time, and as soon as we left the parking lot and the wheels were straight, The Boss took the accelerator smoothly to the floor.

God is not directly involved in the action of internal combiustion in this world.

But if he was, he would be paying particular attention to the straight six motor of that M5. I don’t recall any wheelspin, but I remember thinking that this was likely a lot like what the Space Shuttle felt like on the gas — I was pressed back into the seat with what felt like several Gs, and I couldn’t get away from thinking that this was the Hand of God himself felt like if he was into acceleration — omnipresent, omnipotent, and getting bigger the longer one engaged with it.

Our company’s Big Guy was customarily an all-bidnizz, humorless sort, but as we made the shift into third gear, and with it, hyperspace, he was in full kid-at-Christmas, laughing out loud, full being joy mode — a man completely transformed.

It was my first drink from the well of Big Bavarian torque.

After many years of trying to get it sorted, that R90S that bossman had been eyeing finally came 100% on the pipe, and I found it lived there , too.

A buddy of mine scored a leased car for a steal — a gorgeous jet black /7 series. It had an M series set of factory widened rear wheels that cost more than both my airheads combined.

Another lunchime ride, another Millenium Falcon experience.

These “Legendary German Engineers”, it seemed, had a thing for “Torque Directly from the Hand of God” that seemed to inhabit and animate their products across models and even across different vehicles. I have no question that their former Aircraft engines and their marine engines all tap into this when they’re in their happy zone and on the gas. Folks have also told me stuff about their European market performance diesels that seem to support this idea, and perhaps take it to another, incomprehensible level.


And that is what brings me to the K1200 longtitudinally oriented, transversely rotating, sideways mounted 4 cylinder engine — lovingly referred to as the last of the ‘Flying Brick’ motors.

BMW Motorcycles are famous for many things. They are also be infamous for a somewhat smaller number of things. One of those things is that the first version of a product that hits the market is never perfect. Another one of those things is that development of that intial concept continues — doggedly, methodically — until it has been brought as close to perfection as anyone could possibly expect.

The BMW K series engines and motorcycles were first introduced to the market in 1983. The design was, in almost every way, revolutionary. There are tales of a test mule that used a Peugeot automobile engine mated to the driveline of a BMW boxer twin that was used to test the basic design concept. Once the basic configuration was found to be sound on the road, the design of a BMW engine and driveline could begin.

The starting point for the engine design was BMW’s Formula 2 4 cylinder racing engine, which had a reputation for bulletproof reliability when being run near redline for long periods of time. The Formula 2 engine — which displaced 2 liters — was scaled to 50 percent to acheive a displacement of 1000 ccs.

The K100 motorcycle’s 4 cylinder, water cooled engine was inline in the frame, with the engine laid over 90 degrees to the left, so that the cylinder heads of the engine were next to the rider’s left boot, and the crankshaft next to the rider’s right boot. This orientation, with the engine rotating transversely to the frame, unlike all contemporary inline fours, whose flywheels rotated inline with the wheels, maintained the roadholding and stability characteristics of BMW’s twins.

The engine, also in contrast to many of its contemporaries, was undersquare, with a longer stroke than its cylinder bore — this was a design necessity as every millmeter of bore added several millimeters to the overall length of the motorcycle, which was already long to begin with. This design imperative — make the engine short enough to fit in the motorcycle, even if it means an undersquare design — ended up creating the K-engine’s most significant feature — torque at every RPM.

Instead of a traditional frame, the engine was used as a stressed member, with steering head and rear subframes mounted directly to the motor. Power transmission was, as with BMW’s boxers, via a 5 speed transmission via driveshaft running in line with the engine crank down a swingarm on the right side of the motorcycle. Unique to the K-bike, though was that the swingarm was single sided – first via Monolever, and later via Paralever, which added an anti-torque reaction link, which controlled the rise and fall of the suspension under drive. Induction was via electronically managed fuel injection, which fed a very conventional 2 valve overhead camshaft cylinder head. Exhaust was via a one piece stainless steel exhaust system, with tuned headers and a catalytic converter.

Cars of this period were seeing such features coming into widespead use, but in motorcycles the combination of liquid cooling, fuel injection and catalytic exhaust was at the time completely unprecidented. The subsequent model year BMW introduced a three cylinder varient — the K75 — that used the same components and engine internal dimensions, with an added balance shaft, to acheive a 750 cc displacement.

Revolutionary advances almost always bring with them unforeseen consequences, and the K-motored motorcycles were no exception. The stressed member construction meant that any engine vibration was transmitted directly to the rider. The 3 cylinder K75s, with their balance shafts, became noteworthy as the smoother operating engine of the family.

The bike’s fuel injection system used an overdelivery and return line system where fuel that was not injected was returned from the injection rail to the fuel tank. In this process, unused fuel served as kind of a secondary coolant, with the fuel picking up more and more engine heat the longer the motorcycle was operated. Given the K-bike’s single wall aluminum fuel tank, filling it with hot fuel, combined with some less than optimum airflow coming from the bike’s cooling system radiators, made the bikes uncomfortably hot to ride, espcially in places like Texas and the American Southwest. It’s easy to understand how this might not have been noticable in the cool, damp German development test environment, but it didn’t make having one’s privates roasted if you were operating the bike in an American summer any more enjoyable.

The original K’s K-jetronic fuel injection used a mass airflow sensor — which placed a flap into the manifold airstream — to measure the velocity of air entering the engine. In practice, the sensor, expecially when dirty, became an impediment to air actually entering the engine, further slowing throttle response that was already not entirely thrilling.

17 years — which is the length of time between the introduction of the Flying Brick and the production of my K1200LT’s engine — is a virtual eternity in terms of engine development. And that eternity was more than sufficient to take the faults of the first K100 engines and correct them. The result, in my humble and completely biased opinion, is one of the most excellent motorcycle engines ever produced, and likely one of the most unappreciated.

BMW went after the faults of the orginal bikes and slayed them one by one. The 1988 introduction of the K1 — an oddball bike that was part top speed streamliner and part sport bike — introduced a more modern 4 valve cylinder head. Bosch components introduced a new FI system — the Motronic — to replace the K-Jetronic — and went to a more modern closed loop with hot wire air sensor system that ditched the mass airflow sensor flap. The combination produced an engine with markedly better throttle response and a really noticable power step when the flow impacts of the 4 valve heads kicked in at about 5500 rpm. All these improvements — as well as a new AntiLock Braking system — were then adapted from the K1 and shared with the rest of the K-bike line.

Engineers kept revisiting the engine’s displacement and internal dimensions, and continued to eke increases out of the tiny spaces with which they had to work — using methods like thinner cylinder walls and higher speed coolant pumps to shrink the dimensions of the cooling jackets until the original 67mm bore and 70mm stroke of the K100 had been maxed out to the 70.5mm x 75mm of the K1200 series. I remember reading an analysis of the K12 engine in a motorcycle magazine — it might have been Kevin Cameron — who observed that the long 75mm stroke, combined with the bike’s 8750 rpm redline, produced a piston speed that was the highest one that had ever been measured in any production street engine, car or bike.


The K12 series engineers also made several additional changes that took the Bricks from quirky machines to fully developed, optimized products. As appealing as the frameless design had been, it was simply not sufficient for the increased power and suspension loads made possible by modern radial tires. The K12 motorcycles adopted a cast aluminium frame that was rigid enough to do anything that was asked of it — and with it the amount of vibration that reached the rider was reduced to effectively zero. That frame was combined with modern full wrap bodywork that engineered both the flow of heated air from the radiators away from the rider, as well as allowed for an insulated shroud around the fuel tank that also kept cockpit temperatures comfortable.


BMW’s engineers, in short, spent a decade and a half completly developing the K-series engine, which simultaneously creating motorcycles that made the best of that powerplant while shielding the rider from their negative personality traits.

Which brings me back to that engine.

An old racer I knew once told me, “It doesn’t make any sense to make more power than you can get to the ground.”

And its exactly that thinking that defines how the K12 LTs behave on the road.

At any sensible engine speed, opening the throttle produces as much torque and acceleration as is possible to use outside of a short, police officer induced trip to jail. A total of 90 some foot pounds isn’t huge, in an ultimate sense, but it is more than sufficient to move these bike’s significant mass in a pretty authoritative manner. The evenness of the torque delivery is also striking — at no point in the curve does it produce less than about 72 foot pounds. These characteristics, when combined with the smoothness of the engine and frame unit, makes the bike sneaky fast — there’s very little sensation of speed, but there definitely is speed.

I posted a ride report to the Internet BMW Riders after my initial test ride. Reading it again today its clear that all of this was gobsmackingly obvious after about five miles in the saddle. Being a bit of a geek, the combination of a very pronounced intake shriek and the sensation of being launched made me think of Darth Vader’s TIE Fighter from the first Star Wars movie. I spoke of being seduced… the Dark Side. Apparently, a lot of folks understood exactly what I meant.

Overall, it is that sense of effortless fitness for purpose, refinement and huge thrust that marks the whole K12 experience. The length and mass of the engine and its placement forward in the frame loads the front tire to a degree that makes the bikes extremely sure footed. The evenness of power delivery and careful selection of flywheel mass create a power delivery character that is tractible in every way — one is completely in control of what is happening at the rear contact patch — and the bike makes complete use of all the power it puts out.

I’ve ridden K12s in LT and RS configurations back to back, and I’m not going to tell you that the RS isn’t fun, because it sure as hell is. There is more power and more torque, but it is located further up the powerband. The rider can go from things happnening fast to what-just-happened-there-crazy-fast, but you have to have the skills and discipline to keep your revs up to enjoy the whoosh. The LT trades some of the top end for a flatter curve, and for more thrust off the bottom. For a Sunday sport rider, the RS is a treat — for everything else — the torque tuning of the LT engine is more flexible and more versatile. There is no wrong gear on a backroad — its just twist and go whereever you happen to be.

The cruise behavior of the engine is a revelation, as well. There is a designed in ‘sweet spot’ at the engine’s torque peak just under 4000 rpm. At this rpm there is almost no vibration, making tank-to-tank riding possible, yet as everywhere else, a turn of the throttle makes everyone else on your road select relative reverse, and brings the horizon up stat. If you’re in more of a hurry than that, a smart downshift gets one instantly to the very top of the engine’s output, and then one gets a dose of the motor’s signature intake howl and had better have a long stretch of open pavement firmly in view, because the here/there transition will occur with Sci-fi abruptness.

Cruising at 5000 rpm in top really brings out the engine’s inner racing monster… engine sound is full-on racetrack, throttle response improves, and counterintuitively, the fuel economy improves as the 4 valve head hits its flow happy zone. Problem is, if you’re going to do this a lot, you’re going to go through a lot of tires, and you should make sure that your bail money is stashed at home where someone can easily find it, because any subsequent conversation with the constable is going to be somewhat unpleasant.

The Flying Bricks possess one other quality, and that is a nearly impossible degree of mechanical robustness and stability. In the more than 100,000 miles I’ve had these bikes — I know I am but a mere K-bike lightweight, a no0b — I’ve been under the engine covers a few times and it always strikes me how the engine internals look like they’ve factory new — having never been run. All of the casting faces are clean — there’s no sign of any sludge, carbon or staining anywhere in sight — hard plated surfaces are shiny, with no wear patterns on the visble valve train components, cams, buckets. I’ve seen that kind of mechanically halo-producing ‘eternal life design’ a few places before — ususally in old Mercedes engines that have already been through more than three-quarters of a million miles and are found on the workbench to not need anything.

The K12 engines look like that with no miles, and they look like that with 100,000.

My current ride has just over 85,000 miles on it. I do let professionals work on this valve train. Their worksheets indicate only 2 of the 24 valve adjustment buckets have needed to be swapped in that time.

My K engines have not used a a single atom of oil.

I don’t know anyone who has worn one of these engines out. Good men have tried. The three people that have tried the hardest to wear flying bricks out have had their bikes killed violently at that hands of others at very advanced ages. Paul Glaves had a K75 that had well over 300,000 miles on the clock when it was struck and totalled by a car. Another mileage big dog from the DC area — Don Arthur — had a LT that was just coming up on 300K when it too was struck by another automobilist. Paul Mihalka had traded in a K75 to a local dealer that used it for a service loaner — it was well beyond 300K when a customer totalled it.

How far might these bikes have gone? Its like the Tootsie Pop and their “How Many Licks?”

Reality keeps biting so that we never get to find out.


Don’t misunderstand me, these bikes are far from perfect to live with.

They’re kind of tough on tires.

There are certain maintenence procedures which, due to some of the ‘semi-planned’ nature of the K12’s engine and frame packaging, are a little challenging.

Removing and replacing the fuel tank — even with some high tech pressurized bronze fuel line disconnects installed — is harder than it should be.

Changing the front shock is fun.

Getting to the upper bolt of the rear shock is also fun.

Changing the crankcase breather hose — which is a rubber part which is exposed to enough heat to make it fail repeatedly — is still more fun.

Did I mention the factory shop manual recommends the use of a shop crane to assist in lifing the frame off the motor for that job?

I changed the ABS Controller on one once.

I can say without fear of ridicule, that it made me cry.

More than once.


But that stuff is complely forgotten the minute you roll the throttle open.

The clouds part. The Hand of God emerges. He places his hand on the small of your back.

And deftly shoves your ass straight out your left ear.

And does it again, every time you ask, for a half million miles or until you lose interest, whichever comes first.

I’ll freely admit that the seeming ease with which it does this somehow strips the experience of some emotional component.

A full throttle run through the gears on my R90S is ecstatic, soul stirring — every sensory input, every power stroke colludes to make you feel deeply: “I am going hella fast.”

The same experience on the K is all Joe Friday: “Just doin’ my job, ma’am”.

And that’s how, I think, that what is objectively a magnificent motorcycle engine gets given universally short shrift by riders and moto-scribes.

By makeing whatever you ask of it just seem too damn easy.



2 thoughts on “Lovesong of the Flying Brick

  1. Wow! Great writing! You captured the appeal of these superb, yet under appreciated, machines perfectly. Having owned five Klassic K bikes over the past three decades, I’m here to tell you that the “flying brick” love song ain’t no joke. Bulletproof reliable and all-day smooth, they just work… and work… and work. Currently on a 2004 K1200GT with ~75,000 sMiles on the odometer (just getting broken in). For me, the siren song of the brick started with a 1985 K100RS and grew even stronger when I moved to a 1988 RS/SE (ABS Special), then a couple of K1 4-valve beauties (paralever rules!) and ultimately the crescendo of the final edition RS/GT model. All told, well over a half million trouble-free miles on these machines (but who’s counting) and still having a blast every time I twist the throttle and bring that excellent Kompact in-line 4 to life. Other than routine maintenance (oil, filters, fluids, etc.) there’s very little wrenching required. I’ve had virtually no problems with any of these beemers aside from a single fuel pump failure on the ’88 RS, which was probably my own fault for letting her sit too long with old fuel in the tank. Since 1991, I *might* have used 1 quart (total) between oil changes on ALL 5 of my K bikes. Only minor complaint with the K1200GT is that its linked “whizzy” brakes tend to quickly eat through rear brake pads, plus it wears out the tires sooner than I’d like. Be that as it may, I haven’t found anything better for the sort of diverse riding/touring that I like to do. In fact, the end of brick engine production is probably the main reason I haven’t yet “upgraded” to a newer beemer. Many great motorcycles to choose from these days, but none sing to me quite like the bricks do. The “slant” K’s are nice and I drool over the K1600GT, but at the end of the day I’m quite content to ride into the sunset on my fabulous flying brick.

  2. Pingback: 9090 | Rolling Physics Problem

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