Merkblätter zur Restaurierung der überschweren Kräder BMW R75 und Zündapp KS 750.


Herausgeber:  ERSATZTEIL- DIENST   Hans-Peter HOMMES       D-41748   VIERSEN   Tiefenstraße 10       Tel. 02162 - 8100933

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Rear Wheel Drive


H - P Hommes


© 2006 h-p hommes




Rear Wheel Drive


The rear wheel drive has the function of a lockable differential with the difference that 70% of the power is allocated to the rear wheel and 30% to the sidecar wheel.

Zündapp and BMW each produced their own version of the sidecar drive.

From the outside both drives only differ by the vent bolt on the housing of the Zündapp drive and the casted spare part number -  BMW 275 1 86 052 0 and Zündapp 490 k 11.

Different are  the ratios of the crown wheels,

The Zündapp crown wheel and bevel pinion set has a much stronger gearing than the BMW R 75 set. As the BMW crown wheel and bevel pinion set has more teeth engaged, it makes up for it and both sets can be regarded as equivalent.


BMW R75             12/33 this represents a ratio of 2,75:1.

Zündapp KS 750    8/21  this represents a ratio of  2,62:1.        2,75/2,62 makes a difference of 1,04:1,00.


They are regarded as interchangeable and can be mounted in the BMW R75 as well as in the Zündapp KS 750, because there is only a small difference of 4%.

Today we produce the 12/33 only, because in the final velocity  this one has a better ratio than the Zündapp gear set.

Badly reproduced crown wheel and bevel pinion sets are often the cause for a total breakdown of the rear wheel drive.

Some manufacturers have especially their problems with the warp-free hardening .

Over hardening and full hardening of the crown wheels and bevel pinions do not only lead to a deformation during the production but also to the destruction of complete gear teeth during driving operations.

 As a result the rear wheel drive can be destroyed .

When the backside of the crown wheel was grinded again after hardening you should be suspicious.

Was this surface warped after hardening?

Is this the reason why it was grinded again?

When the lower surface was warped then the gearing is warped too.

You can only see this because of the different tooth image  when you assemble it.

If the gap is different at different points of the diameter of the crown wheel then it is not a good crown wheel set.  


The satellite gears consist of two double and two single gears.

The double gears have a marked tooth.

When mounting the gears the markings must show to the center.


Oil ?

The manual and the lubrication chart indicate that you have to fill in 0,4 liter transmission oil.


I use SAE 80 transmission oil.

When filling in the oil flows very viscously into the housing. 

 It is recommended to lift the bike at the back and to turn the wheel slowly while filling in the oil.

This way the oil is transported into the housing.



Installation into the bike frame.  

I found out that the contact surfaces are damaged on many rear wheel drives.

It may be that the drive was not correctly attached or that somebody has reworked the contact surface.


When you mount this rear wheel drive into your frame it can happen that the direction to the transmission is not correct.

When the direction is incorrect the bearing of the bevel pinion will be damaged in the long run and for BMW also the cardan joint and the main shaft.

Most often this is only noticed when everything is assembled and you cannot turn the rear wheel drive. 

You should be able to turn it by hand. So check first.

BMW frames are nearly always warped without exception and must be adjusted. Zündapp frames are warped less frequently unless they have accident marks  that suggest frame damage.

It makes always sense to check the frame before painting it.

You can see the instructions for adjusting the crown wheel and bevel pinion on the following pages


Instruction page




Wir bieten an:

Ersatzteile + Instandsetzungsarbeiten
für Motor, Getriebe etc. der
Zündapp KS 750 und 600

Wir bieten an:

Ersatzteile + Instandsetzungsarbeiten . für
BMW R75 und Zündapp


H-P Hommes Ersatzteil-Dienst
Ersatzteile + Instandsetzungen
repairing + restoring

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