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BMW R 1200ST

Not every highway is flat, not every road is straight. That’s why BMW has created the
R 1200 ST, a successor to the famed R 1150 RS and a clear step sportier than the
R 1200 RT with which is shares its basic engine and chassis. Your first clue is the rakish bodywork, shaped by the wind and defiant of convention. Twin stacked headlights lead the way with a blast of useful illumination. The pointed lower fairing helps channel cooling air to the engine-oil cooler. Like the R 1150 RS before it, the R 1200 ST has a mechanically adjustable windscreen to tailor the amount of wind protection to the rider’s liking.

Behind the fairing, the rider fits into a comfortable and sporty position thanks to adjustable clip-on handlebars set into an ideal ergonomic stance. The sporty ergonomics, reminiscent of the R 1150 RS’s, offer maximum control and comfort for high-speed and long-distance touring. The alloy handlebars adjust vertically along a 1-inch (25mm) range and, together with the height-adjustable seat, offer up all the tools a rider needs to create a personalized comfort zone.

As with the R 1200 RT, the R 1200 ST includes a dual-position adjustable rider’s saddle – 31.9 or 32.7 inches (810 or 830mm) – with an optional lower saddle – 30.7 or 31.5 inches (780 or 800mm).

Instruments of effortless travel

New for the R 1200 RT and R 1200 ST are stylish and legible instrument clusters that include an Info Flatscreen rider-information display. This module includes information on oil temperature, fuel consumption and tank level, plus gear selection.

Power for one, power for all: sharing the new boxer

Both the R 1200 RT and the R 1200 ST take advantage of new technologies launched with the R 1200 GS last year. Beneath the dramatically reshaped cylinder heads lies an amazing new engine. Although it retains the classic Boxer opposed-cylinder architecture, virtually everything is new compared to the previous-generation 1130cc Boxer family. Maximum power for both the R 1200 RT and the R 1200 ST is 110 hp, up from 100 hp in the R 1200 GS and a massive 16-percent jump from the engine in the R 1150 series.

This newfound power starts at the advanced cylinder heads. Compared to the
R 1200 GS, the RT/ST engine employs a higher compression ratio – 12.0:1 compared to the GS’s 11.0:1 – and revised camshafts for greater thermal efficiency and increased airflow through the cylinder heads. The four valves retain their high-cam activation system, which uses a single, lightweight camshaft high in each cylinder head to operate the valves through short rocker arms fitted with easy-to-maintain screw-type lash adjusters. Taking a page from high-performance aircraft engines, BMW engineers selected sodium-filled exhaust valves for the R 1200 engine because they promote more efficient heat transfer. Two spark plugs per cylinder are again used, as on the R 1200 GS, resulting in fast, efficient burning of the fuel-air mixture in the combustion chamber.

In addition to those changes, the new RT/ST engine configuration features a 500-rpm-higher redline – now 7500 rpm – to take advantage of the engine’s increased high-rpm power. A revised intake system works with a new, larger exhaust system to further improve engine efficiency and power output. It says something about the basic new Boxer design that a group of comparatively minor changes can bring about a 10-percent increase without loss of low-end or midrange torque.

Otherwise, the RT/ST engine is similar to the R 1200 GS’s, using aluminum cylinders — with treated bore for wear resistance without the weight of an iron or steel liner – to carry the same 101mm bore as the R 1150 engine. However, a 2.5mm-longer stroke (now 73mm) provides a total displacement of 1170cc with a desirably “oversquare” bore/stroke ratio to permit high engine speeds. Despite being larger, the new pistons are lighter than the R 1150’s. Under-piston oil jets — a racing technology designed to remove heat from the piston and combustion chamber by spraying engine oil at the bottoms of the pistons — are larger and flow more oil.

A new balncing act

Motorcyclists the world over marveled at the R 1200 GS’s impressive smoothness, the result of the first-ever use of a balance shaft in an opposed-twin motorcycle engine. Although the Boxer design has what’s known as perfect primary balance — the major reciprocating forces are directly opposed because as one piston reaches the top of the stoke, the opposite piston does as well, canceling out the primary forces inherent in reciprocating designs — there is some residual vibration caused from the connecting rods not being directly overlaid. In effect, the engine wants to oscillate around its center of mass, as viewed from above.

By employing a counterbalancing shaft, BMW has eliminated this kind of vibration, called rocking couple. As a result, the new engine is dramatically smoother, permitting not only elevated engine speeds with fewer stresses on components (including the rider) but it can be mounted to a lighter frame without sacrificing strength.

BMW’s engineers worked overtime to artfully locate the counterbalancer into an already efficiently packaged powerplant, and their solution is both inspired and effective. In the traditional R259 engine family, a gear-driven sub shaft is positioned below and parallel to the crankshaft plane that contains drive sprockets for the chain-driven camshafts as well as for the oil pump. In the new R 1200 RT/ST engine, this shaft is larger and houses, inside its inner diameter, a second shaft driven at crank speed — the sub, or countershaft, is driven at one-half crank speed — with one of the two necessary balancing weights integrated into the drive gear and the other bolted to the far end of the shaft outside of the oil bath to help reduce power loss. This design adds remarkably little weight, uses a minimum of engine power, makes the powerplant no larger yet dramatically improves rider comfort. And, just as important, the Boxer engine’s distinguishing opposed-twin rumble remains; this is a smooth yet charismatic engine.