TEST & TECHNIK
Jan. 20, 2004, by Michael Pfeiffer, illustrations by BMW
In its last iterations the GS was a little tough and tenacious, and vibrated a bit too strongly from time to time and the competition had few more horsepower under the valve cover. That's too much to be overcome by double sparkplugs, for the boxer, at least in its 1993 guise, is at the end of its useful life. Soon its successor will appear, first as a GS, then in all the other replacement models of this successful series.
Still air-cooled as before, the new model is different in nearly every other aspect, even to the spacing of the main crank bearings, which have been moved closer together to make the crankshaft stiffer and reduce the cylinder offset. A;though the crankshaft has 2.5 mm more stroke, there is 2.2 pounds less steel spinning around in the coincidentally new crankcase. Even the crankcase is a hair over 3 pounds lighter, thanks to modern casting techniques and an altered shape.
To finally do away with annoying vibration BMW installed a counterbalancer. This is a first for the boxer motor. The shaft's placement is a stroke of genius; it rotates inside the hollow shaft for the camchains, and the crankshaft drives both shafts. The cam chain shaft is driven by a chain at 1/2 crank speed, and the counterbalancer shaft is driven in the opposite direction at crank speed (1/1) by a gearset at the front of the motor. Got it? The secret: the drivegear of the counterbalance shaft is made with an offsetting mass, and another corresponding balance weight is bolted to the other end of the shaft. These move in opposition to the crankshaft, with its connecting rods and pistons and should eliminate vibration for the most part. An elegant solution, and we tip our hats to such talented engineers.
All the parts that could improve power output have been redesigned, including lighter pistons, cylinder heads with better flow, and 2mm bigger intake and exhaust valves. With a displacement of 1170cc, the new bike puts out 100 HP at 7,000 rpm and a solid 115 Newtonmeters of torque at 5500 rpm, 15 HP and 17 Nm more than the R 1150 GS. The engine management system shows off all of the most current technology, including the world's first use of knock sensors in a production motorcycle. Sensors detect the first signs of detonation in the combustion chambers, and the engine management system retards the timing as necessary. This allows the motor to use low octane gasoline if necessary, in spite of the 11:1 compression ratio. There are two oxygen sensors (one for each cylinder) in the header pipes.
The cylinders are individually controlled, so each gets exactly the ideal mixture, and each side's ignition setting is constantly adjusted for maximum power output, triggering two sparkplugs per cylinder, like the old boxer motor from 2003.
The twin sparkplugs do not fire simultaneously; they are phased sequentially instead. The individual control of each cylinder permits the best possible fuel economy, as well as reduced emissions, which are further cleaned up by the catalytic converter in the collector. BMW saved an additional 6.6 pounds on the muffler, which has a restrictor valve that damps the noise at certain rpms.
Of course the R 1100 S riders may ask what all the fuss is about, since the R 1100 S is already making as much horsepower. They will soon experience the answer, because the new motor has a potential 120 horsepower on tap. It is not necessary to put out this much power for the GS, but BMW will really let the motor roar when a new sportbike is released.
Now back to the GS... The transmission is also completely new and weighs only 28.6 pounds. It has helical teeth on all the gears, the gears are engaged with individual sliding engagement rings, and the shift selector drum rotates on needle bearings, allowing precise shifting through the six gears.
The new drivetrain is spectacularly successful. The driveshaft spins in a massive single-sided swingarm. The rear drive is articulated at the bottom, and the Paralever
torque arm runs above the swingarm to a mounting point at the back of the transmission. The geometry is worked out to provide almost 100% elimination of any shaft drive reaction, and eliminates the need for the driveshaft to compensate for variable lengths of the drivetrain. The rear drive looks extremely compact, and the rear wheel is bolted to it with five small 10mm lugbolts.
BMW is also breaking new ground with the chassis. Lightness and stiffness in the most compact form possible required the use of a construction method practiced chiefly by Ducati: a tubular steel frame serves not only to support the Telelever in front, but also to carry the rider, passenger and luggage. Significantly, the steel frame now supports the rear swingarm, which eliminates a major load on the transmission housing.
Together the battery, alternator and starter save 4.4 pounds. The electrical system is multiplexed in current automotive fashion, with a single wire operating several independent circuits (CAN-Bus-Technik). Instead of each electrical device being connected independently through the wiring loom, a single cable bus and and several individual control modules fulfill the same function as the separate wires and relays did in the past. This saves weight and production expense.
Unbelievable, what is hidden in the new GS. The experience is convincing, and awakens anticipation of a whole range of new Boxer models. Well done, BMW!
For another interesting R1200GS article, click here