Technical Information Pages for the Restoration of the BMW R75 and Zündapp KS 750.
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H - P Hommes
© 2006 h-p hommes
The assessement of the front forks for the Zündapp KS 750 and BMW R75 is always from todays point of view.
"The BMW already had a modern tele fork and the KS 750 had only a parallelogram fork".
Yes, this statement is correct! But as is said it is meant to suggest that the BMW R75 tele fork is the better one and the Zündapp KS 750 fork is an ancient non-functioning part.
The Zündapp KS 750 Front Fork
At that time it was the parallelogram fork that after decades of development found its non plus ultra in the solid totally capsuled Zündapp KS 750 fork. By the way this was not made by Zündapp but by Triumpf in Nürnberg and was then delivered to Zündapp. This fork is much more solid and is more elastic than the tele fork of the BMW R75. In videos of our off-road trips we had recorded a bumpy path which was travelled alternately by a KS and then a R75. At the KS you see the head lamp jumping up and down. It is attached to the fork but the handle bar and the drivers' hands remain fairly stable. At the R75 the head lamp, the handle bat and the drivers' hands jump extremley.
That this is not just my opinion also shows the letter and the therein made agreement between the Wehrmacht, Zündapp and BMW.
It says in the letter dated 14.08.1942:
"The GBK* desires without regard for the final changeover of production of the BMW company that the Zündapp KS 750 front fork should be built into ...". * Generalbevollmächtigter für Kraftfahrzeuge
But this did not happen because BMW had already a large number of front forks in production and the BMW R75 production should stop in June 1943 anyway. The BMW- company and the Zündapp - company should then produce together approx. 20.000 Zündapp KS 750 yearly. Since the production was constantly disturbed by shortages, bombings, etc. the start off production was constantly delayed until the end of 1944 when the final put off was declared.
Problems of the KS 750 - R75 fork.
Old springs often loose their suspension. The springs are easily worn out. During the restoration you take the length of a new spring and compare it with an old one. The difference is sometimes only a few millimeters and then you come to the decision to leave the old ones. You cannot check a spring this way. You would have to squeeze both springs with the same pressure and then measure the length. Only then the data of old and new look very difference.
The hinge of the KS 750 has a needle bearing. Here a strong wear occurs. if this creates too much play the fork dangles during the drive. Because the needles run directly on the hinge, the bearing surfache needs a large hardness. a subsequent grinding and larger needles do not help, because now the wear takes place even quicker due to the lack of hardness on the bearing surface.
You can just mount a new Gelenkarm eingesetzt werden. Dieser muss im richtigen Eingriff zur Stoßdämpferwelle stehen.
It is also important that the lower Gabelbolzen and the fork girder have not too much play in the bushing and that the shock absorber works.
Lift up the front of your KS, grab both footrests and move the fork backwards and forwards. So you can see where and how much play your fork has.
Most front forks, whether BMW or Zündapp, are very much neglected during the restoration. Only when the bikes are driven it is determined that a restoration of the front fork is urgently needed.
For the BMW R75 the play can be determined in the same manner as for the Zündapp.
When you can move the fork legs Gabelholme like on the left hand picture, then the bushings and the dip tube are already totally worn out and there is only a limited suspension.
It does not help to mount only new guide bushings
because the dip tubes are well worn inside on one side.
The fork legs Gabelholme jam and a suspension no longer exists.
Often the fork girders are bent too. Then only a complete exchange can help.
The BMW R75 fork was too weak for the heavy military bike.
The suspension travel is too short. Therefore the fork is much easier and cheaper to produce and to restore than the one of the Zündapp KS 750.
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