Early 6-speed transmissions

The same basic transmission was installed on the R1100S, K1200RS/LT/GT, R1200C, and all R1150 models (the second-generation Oilheads and K-bikes with Motronic 2.4 and the revised pushbutton switchgear).

Externally, the housings are different for the K1200 due to different engine mounting, and for the R1200C and R1100S because of the swingarm mounting.

Internally there are also some model-specific differences. The reverser mechanism on the K1200LT occupies the space where 5th gear usually lives, so there are only five speeds in all (the 4th gear ratio is changed to better bridge the gap). R1200C prior to the CL also came with only five speeds because BMW skipped 5th.

However, the overall design and shifting mechanism are common to all of these versions. This transmission design is unique in that it uses a dedicated gear just to drive the intermediate shaft, so the intermediate typically has seven gears in all (in other gearboxes, the input shaft drives the top gear). As on previous boxers since the /2 and possibly before, the drive gear is helical and all of the others are straight-cut.

This article only shows the six-speed versions.

Shifting

First let's take a brief look at the parts that do the shifting. The selector is basically the shift lever. When you rock the shifter up and down, it moves this back and forth. There's a ratcheting mechanism that turns the shift drum about 1/8 turn every time you shift.

The shift drum has a groove for each shift fork, so in each of its seven positions (including neutral) the shift forks are in different positions.

The three shift forks slide on rails and move according to their groove on the shift drum. Two of the forks are identified by arrows in the picture, and the third is on the other side of the drum.

Shafts and gears

At the bottom is the input shaft with the clutch spline on the left. This is where the power comes in, and it drives the intermediate shaft via the helical (angled-tooth) gear. The stuff on the rear part of the input shaft is a shock coupling which lets the gear twist to absorb driveline shock.

The gear pairs of the intermediate shaft (center) and output shaft (top) are in constant mesh. One gear in each pair freewheels on its shaft and is engaged by locking against a neighboring gear which cannot freewheel on its shaft.

On the intermediate shaft, for example, 4th and 3rd gears act as one part, splined so they can slide sideways on the shaft but not spin. They slide to the right to lock 5th gear to the shaft, and slide to the left to lock 6th gear to the shaft (shown in the diagram)...

This is about 25% of the full article, which is published in the January 2008 OTL in its entirety.