The Rear Drive of the R 12
The BMW R 12 was the first BMW Model with a integrated drum brake into the rear drive. In 1935, far ahead of its time, this drive unit sets standards for numerous other models. The well-constructed housing did not have to be much improved compared to other components. Although the quality of the aluminum castings decreased over the years. Minor changes to the housing design makes it possible today to use the existing parts available in almost all years of construction.
On the pictures below you see one of these differences in case of the housing cover. Early Versions had a small sheet metal plate at the position of the input drive.
In the housing cover of the axle bolt was screwed. This bolt sits in a threaded bushing. These are left-hand threads, contrary to the direction of rotation of the wheel. The threads of the bushings are usually badly worn. It is recommended to replace this part, which is secured with two pins, as you can see on the pictures below.
A new threaded bushing needs to sit nice an tight. Very important that the pins that protect against movement when turning in and out of the axle bolt. New pins should sit nice and tight.
On the two fixture points of the Housing cover, the sidecar connection was placed. Solo machines got cover screws, as you can see on the picture below.
The strong housing was almost the same on every R 12.
The most important differences in the three produced main rear drive versions we know is about the gearing combination. The combination was depending on the usage of the motorcycle.
civil solo machines: 14/57
civil sidecar version: 12/57
military version (almost with sidecar): 11/57
We distinguish in the 3 gear combinations, as you can see in the picture below.
(left to right: 11/57 ; 12/57 ; 14/57)
All these three Versions where fitting in one and the same housing. Differences in the distances were compensated by spacer discs. Each cardan was fitted individually.
The original parts were typo-marked in the majority of cases.
In this case, input shaft and ring gear always formed a pair. The combination of using different spare parts put together, some of which have already worn differently, is not recommended. In general, the gears form a unit. As can be seen in the picture below, the parts already formed an inseparable unit from the factory on.
Due to differences in the size tolerance, space problems in the housing may occur in some cases. That can be a Problem if a 14/57 gear set is to be installed in a housing that was originally delivered in 11/57.
The set screws have a M8 thread on the housing side and a M8x1 thread on the side for the nuts. Seal under the shaft flange was made of cork . The Seal for the shaft was originally made of leather. Today we can buy advanced replicas, which are made out of rubber which you can see below.
On the outside of the housing the gearing combination mark was stamped. But even that was not in all cases and the Layout was changing.
Drive shaft presses against the pinion. It is recommended to place a thin rubber between the screw head and the shaft and avoid damaging the pinion.
The pinion has two bearings. The main bearing as it was original is not available today. Alternatively, an axial deep groove ball bearing DIN 3304 can be used.
The second bearing is a special needle roller bearing that runs on the surface of the pinion. If the wear is not high only the needles can be replaced. Replicas are still available.
As you can see on the picture below, also replacement rings are an option. Nice and clean run is absolutely necessary!
The ring gear is mounted on a drive flange and attached by Ribe 8G or later Ribe 10K Screws. The two bearings on this part are also special and not available today. Original measurements are 85x45x15 mm. Today it is a option to use a DIN 6209 bearing, that has the measurements 85x45x19 and grind down 2,0 mm on each side with a surface grinder. Bearings that are produced on this way are available.
The Screws are fixed by a wire.
It is important to adjust the contact pattern of the teeth by correctly inserting the spacers on the end faces of the ball bearings from the drive flange.