BMW is known for its two opposed cylinder engine, enriching the program already for quite some time. Among its two-cylinder competitors it is definitely the best choice in terms of running culture and the center of gravity. Whether this principle should also be first choice for motorcycles might be subject to a dispute, e.g. with regards to the combination of possible inclined positions and the resulting change of the center of gravity.But, in any case, BMW elegantly transformed an outdated 2-valve engine with under head camshaft and limited numbers of revolution into a modern motorcycle drive.
How it works
Now, instead of cams, there are two chainwheels, thus the chain connection to the crankshaft still holds. Technically, it may no longer be regarded as camshaft. It transfers the rotation of the crankshaft to the camshaft in the cylinder heads. Since there is no difference in ratio to the camshaft, it is not necessary to put a large, disturbing chainwheel in between. The cam shafts do not operate the valves from above (outside), which would have enlarged the engine even more, but operate the valves through fork-like rocker arms with one in- and one outlet cam. This simplifies, in addition, the valve timing, still usual for motorcycles. The opposed cylinder engine is appreciated especially, because of its ideal compensating powers with regards to the moved masses. Because the pistons are not exactly level, nevertheless, some torque arises around the high-level axis. The perception is stronger with motorcycles, because of the direct connection with the frame. The former under head camshaft (green) has been turned into a hollow shaft and the second shaft (blue) was installed, carrying counterbalances at its ends, in order to reduce the torque. It is driven - a rare design - by a light 1:1-gearwheel in front by the crankshaft.
Scooter
Two-opposed-cylinder engine 
