K1200S - TECHNICAL BITS EXPLAINED

GOOD BALANCE

Twin balance shafts are gear driven.

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).

GOOD HEAD

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.

Chain (lhs) drives the exhaust cam, which in turn drives the inlet cam by direct gearing (counter-rotating).

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.

LUBED

Oil pumps and crank feed.

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 (scavengeand 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.

COOLED

Pump is on the head!

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).

GEARED

Cassette style gearbox can be fitted as a complete assembly.

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.

MANAGED

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

Huge exhaust at least sounds good.

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.