K1200S - TECHNICAL BITS EXPLAINED
With the new K1200S come new technical ideas that we think are worthy of a bit of a closer look. There’s actually enough to make a whole article unto itself, so that it what we did – only hopefully written for the average person … with a degree in astrophysics. Well, maybe just an interest.
The over-riding requirements for this motor by BMW were to try and keep it compact, light and efficient. After some amount of research on different layouts and types, BMW came to the conclusion (somewhat reluctantly I suspect) that the across the frame inline four was the best format for this type of bike.
Okay, so an inline four cylinder with DOHC and four valves per cylinder is not exactly a revolutionary design, but there are some interesting ideas incorporated into the new K1200S motor:
LOW AND COMPACT
Unhappy with the high centre of gravity of the conventional inline four designs, BMW incorporated a 55-degree forward lean angle into the cylinders of the K1200S. This moves more of the load toward the front wheel and opens up space above for a large airbox, as well as giving freedom of frame design (no requirement for it to go around the motor).
By making the motor a dry sump design and slapping the alternator behind the cylinders instead of on the end of the crank, they have also managed to keep the overall width and height of the motor to a minimum. This enables a low mounting of the motor, while maintaining lean clearance (50 degrees) and aiding a low c of g.
Okay, that’s all fine and dandy, but what’s going on inside then?
In order to prevent those bar-buzzing secondary vibes that an inline four configuration inherently produces (and one that plagued the original four-cylinder K motors to such an extent that BMW ended up using extensive rubber mounting between the motor and frame), a set of twin balance shafts are used.
These are driven by gears off of the crankshaft, and rotate at twice engine speed (the required frequency to counter secondary vibration).
The cylinder head incorporates almost straight intake ports (for optimum air flow) and a relatively flat combustion chamber, enabling a very high compression ratio of 13:1 (see ‘MANAGED’ below for associated anti-knock control). According to the BMW literature this “clearly confirms the ideal geometry of the combustion chambers with an ideal combustion process and optimum efficiency”. Consider yourself told.
The valve-train uses little follower arms (rockers) that apply the force of the rotating cam in a direct line down the valve, as opposed to the more conventional cam to bucket system, which includes an undesirable side to side force as the cam comes on and off the bucket (BMW citing their Formula 1 experience as justification for this design). They also reckon it saves a bit of weight.
The clearance adjustment mechanism is worthy of note, as without buckets, conventional shims cannot be used. Instead, the end of the follower incorporates a cup into which a semi-spherical shim is located. These come in different sizes depending on the clearance required.
Drive to the cams is by a conventional chain and sprocket system, but the chain drives only one of the camshafts (exhaust), with a pair of gears – part way along – transmitting the drive to the inlet. BMW reckon that this achieves two things: more precise valve timing and a narrower cylinder head. Although any system that uses a chain will have some timing inaccuracies (as the chain wears it stretches and so as the tensioner takes up the slack it pulls the cams around a tad and retards the overall timing), minimizing the length of the chain will reduce that undesired effect somewhat.
By using a dry sump system (oil stored in a separate tank instead of in the sump) they not only keep overall engine dimensions down, but also ensure a steady oil feed to the motor under extreme conditions (oil-in-sump designs can theoretically gulp air into the pump, although the bike’s probably no longer shiny side up at this point or just well low on oil). However there is a price to pay in the form of an additional oil-pump (scavenge and feed) and extra oil lines to and from the oil tank (although BMW are using aluminium ones to try and reduce the associated weight penalty).
It’s probably a good time to mention that oil feed to the big-end bearings of the crank is delivered directly via drillings through the crank itself. Conventional supply is through crankcase drillings to the main bearings – which need a special ring grove to collect the oil – directing it into individual crank drillings and then from there on to the big-end bearings. According to BMW this means that not only are larger main bearings needed (increasing engine width) but the oil is also being pumped against the centrifugal forces of the rotating crank. As a result of this design, the 1200S’s oil pump can be run at a much lower pressure.
Since the vast majority of heat in an engine is developed at the very top of the cylinder (the combustion chamber), BMW have adapted the cooling system so that cooled coolant from the radiator is fed directly to this area. This is done by mounting the water pump on the cylinder head itself, with its drive coming from the intake camshaft.
Some of the coolant is directed to the cylinders, but their water jackets only extend partially down the sleeves – BMW stating that this allows for faster warm up time and subsequently, reduced wear of the cylinders and pistons. All in all, this system means that there is less plumbing and coolant (2 litres) required than a conventional system, saving weight.
However, unless I’m missing something, it also means the coolant flow no longer follows the natural flow of convection (i.e. hot liquid rising).
In order to fulfil their compact design-brief, BMW veered away from K-convention and not only fitted a multi-plate wet clutch (gasp) but also an integrated gearbox. The box is of the cassette-type, meaning that it can be replaced quickly (although that wouldn’t really apply in this case), but more importantly, assembled and tested before being fitted to the engine. Although this doesn't really effect the final product, it does give some savings in production time.
In order to keep it slick, the shafts and gearshift cylinder are mounted in roller bearings. In order to keep it compact, the input and output shafts are stacked (as per most other sportbikes).
Not willing to go too far from tradition, final drive is by shaft (the only sportbike to use one). Although a chain drive is a more efficient way of transferring power to the rear wheel, BMW point out that this is true only for a chain in good condition, whereas their shaft set-up suffers only a few percent of power loss and is maintenance free (albeit a tad heavier).
It’s a similar unit as used on the new R1200GS and is a sealed-for-life affair.
Remember the super-high 13:1 compression ratio? Well, although high-compression provides for a greater power production for a given amount of fuel, it also puts the engine perilously close to blowing itself up if the fuel’s octane is not sufficient to cope with the heat generated (high compression = high heat = fuel detonating before the plug can fire = bad. Very bad).
In order to protect itself from this possibility, the K1200S uses the anti-knock control first seen in the R1200GS. Sensors in the cylinder head check for any signs of knock developing and counter it by retarding the ignition timing, thus reducing the combustion temperatures.
This system also allows for regular fuel to be used, although the resulting ignition retardation will result in a modest drop in power and slight increase in fuel consumption.
IN AND OUT
Thanks to the forward angle of the cylinders, a large (10 litre) airbox can be installed above the motor. There’s also space for two direct feeds from the left and right of the headlight (where air pressure is the greatest), giving effective ram-air at higher speeds. This in turn supplies a greater charge of fresh air to the motor, boosting engine torque and subsequently power.
Once it’s all burnt up, the charge exits the motor into a rather humongous (9.5 litre volume) exhaust containing a catalytic converter – yet it manages to tip the scales at a respectable 10.4 Kg. Oh, and they reckon that it’s tuned for a sporty sound – which I can confirm, ‘cause I rode it.
BMW are probably best known for their forays into the world of weird and wonderful suspension designs. Coming up with the excellent Paralever front-end back in 1993, they’re not afraid to think outside the box when it comes to separating suspension and steering forces in the quest for a comfortable and controlled ride.
The K1200S sees two new ideas in the suspension world: the Duolever front-end (based on the Hossak design) and Electronic Suspension Adjustment (a choice of different damping and preload rates available at the press of a button on the handlebar).
In typical BMW style, the company introduces the new suspension system with the statement that the preceding Paralever provides “superior function and comfort features and is the absolute optimum for the flat-twin machines in BMW’s Boxer series”. However, for the sportier new K1200S they needed “an even better solution with perfect kinematics”.
This was found in the Hossack design (invented by Norman Hossack back in the 80’s, but until the K1200S, unused in a production machine), although BMW have labelled their version the Duolever.
The Duolever uses two wishbones (although in reality they’re actually rings) mounted at their ‘bases’ to the front of the frame with their fronts mounted to a large fork casting that holds the front wheel. The bases can pivot up and down within the frame, while the fronts are mounted in ball joints to allow for suspension and steering movement. The front shock absorber is attached to the lower wishbone at its base and to the frame at its top. Okay, look at the picture and locate these bits. Done? Good.
Steering is provided by the two triangular bits that sit in front of the upper wishbone (look, locate). One end connects the fork casting and the other to the steering shaft that is turned by normal operation of the handlebars. At both these connecting points, and indeed where the two parts join, are pivot joints that allow the assembly to scissor together during suspension compression without interfering with steering actions (imagine suspension and steering movement in the picture. Done? Good.).
As the front wheel hits a bump, the fork casting moves up and pivots the two wishbones, the lower compressing the shock between it and the frame. The action of the two (almost) parallel wishbones means that as the suspension compresses, the front wheel follows an almost straight up and down path, meaning that the bike’s castor and wheelbase hardly change during use (an ideal for suspension travel).
BMW reckon that this set-up provides consistent spring response (due to low friction) and a stiffer front-end to counter braking forces (thanks to the spacing of the wishbones and close mounting of the lower wishbone to the wheel axis). The design also allows the main casting can be contoured to give maximum width where there is maximum bending force (to give strength where it's needed).
There’s also inherent anti-dive when braking, with subsequent forces causing virtually no compression of the spring. However, some wheel loading does cause a “slight dive effect”, although BMW are quick to counter that by saying it results in “useful feedback on how hard the rider is applying the brakes”.
The whole kit weighs just 13.7 Kg – about 10% less than a Telelever set-up.
That stands for Electronic Suspension Adjustment, and controls the amount of spring preload and damping rates front and rear, via a button on the left handlebar – a world first.
To adjust suspension, the rider must press and hold the ESA button until its current setting is displayed on the LCD read-out on the main instrument panel. With the bike at a standstill, the rider can then adjust the preload by selecting either solo, solo with luggage or rider with passenger and luggage (all illustrated by helmet and bag symbols; i.e. one helmet = solo, etc).
Adjustment is by an electric motor, although this can only be done with the bike at a standstill for safety reasons. Besides, you should know which one you need before you start and it shouldn’t change … unless you’re the wheelying type and your passenger is caught unawares!
Damping rates (compression and rebound at the rear, but only compression at the front) can also be changed, with the options of Comfort, Normal or Sports modes (damping getting progressively harder respectively). The actually adjustment is taken care of by small stepper motors in the suspension units which either restrict or increase oil flow depending on the mode selected.
The damping adjustment can be done with the bike in motion – which makes sense as road conditions are constantly changing. Well, unless you live on the Prairies …
It appears that ESA will be an option (Canadian cost still to be determined, although in Germany they’re asking the equivalent of C$1200.00), with a non-ESA bike coming with the usual slot adjustments for damping and a big knob for preload adjustment on the back.
The K1200S uses the Single Wire System originally found on the R1200GS – a single wire being used to send info to each electronic control unit, with each unit only taking the info that is specifically intended for its use. It also incorporates a self-diagnostic system that can decide when the next service is due, depending on how the bike has been ridden. I.e. if you ride at redline in every gear, expect the service light to come on a tad early.
There are also no conventional fuses on the bike – the main computer opting to switch off any faulty component that would normally melt a fuse. The information is then stored for diagnostic purposes. Each time the bike is subsequently turned on, the system rechecks the suspect component, actually reactivating it if no further errors are found.