Zündapp KS600, KS750 and BMW R75

In the 30’s, the Reihswehra was buying many machines from Zündapp company. Especially KS 500, KS 600 and K 800 found military application. Unfortunately, these motorcycles were produced for civilian use in the first place, and no improvements could change that fact.

In 1937 army’s command submitted conditions which should be met by the military vehicle:

- A motorcycle with a side car should carry three soldiers with their weapons, ammo and full gear, what amounted 500 kg in total.

- Three-wheel motorcycle should amount maximal speed of 95 km/h, keep a speed of 80 km/h on the highway, and drive with 4 km/h speed in a march column.

- Wheels had to be adjusted to off-road 4,5x16 tires.

- The ground clearance couldn’t be smaller than 150 mm and fender’s placing should assure easy non-skid chains installment.

Zündapp considered first if KS 600 couldn’t be modified in a way which would meet army’s expectations. But it came out quickly, that old construction (especially the frame, engine, gearbox and front suspension) would have to be modified too severely. Producers decided to work out a brand new construction and in 1939 they presented to OKH (Oberkommando des Heeres) two models of the prototype in order to carry out the test drive. With relation to KS 600, the capacity got increased to 700 ccm and cylinders got risen to make a ground clearance bigger. In mass production, engine cubic capacity was increased to 751 cm³.

At the same time the BMW company was ordered to construct a vehicle that would carry out mentioned conditions. During the tests Zündapp’s suggestion turned out to be much better than BMW’s. BMW was offered a license to produce Zündapps, but they refused. Otherwise Bmw would admit that they’re not good enough to match the competition. Nonetheless, OKH ordered the Munich company to introduce into production such characteristic for Zundapp KS 750 elements like rear wheel drive with differential, hydraulic brakes and wheel rims. Both companies reached an agreement (according to OKH recommendations) and produced identical parts, what perfectly rationalized spare parts management.

In 1940 works continued and next 7 prototyped were made. After intensive test drives in April OKH confirmed KS 750 as a completely useful vehicle for army’s needs.

In spring of 1941 mass production of Zündapp KS 750 began.

In Nuremberg 18 695 pieces of Zündapp ks 750 were produced in eight years.

In final version, Zündapp KS 750 received closed, oval-piped frame. It assured high stiffness even under a heavy load. Side-car’s frame was connected through screw-type breech with rear wheel drive. 70% of power was moved to the rear wheel of a motorcycle, and remaining 30% - to the side-car’s wheel. It gave better control on driving straight. Possibility of differential blocking made covering of terrain obstructions easier. Differential blocking is recommended only on low speed and off-road, when one of the wheels doesn’t come across enough resistance. On normal road and normal speed blocking limits possibility of proper steering. For the first time in a motorcycle hydraulic braking system was introduced. Here again both wheels got different power of braking. Rear wheel is equipped with 22 mm cylinder, and side-car’s wheel with 19 mm cylinder. Thanks to that distribution of braking power was even.

The engine.

751 ccm of capacity with 85 mm piston stroke. Heads and valve mechanism are almost the same like in KS 600. The head of KS 750 has got four cooling fins from below, while KS 600 has got three. Although you can’t use those two heads interchangeably. In KS 750 cylinder had been moved, so angles of holes in which the tappets are moving aren’t matching.

The modern battery ignition was given up and the magneto ignition was introduced instead. BOSCH (more stable and easier in exploitation) and NORIS magnetos were in use. Both of them were equipped with a centrifugal ignition acceleration regulator.

The carburetor Solex 30 BFRH was in use not only in KS 750. Outside there are no signs of a used type of jets, so one can’t be sure if given carburetor was installed in KS 750. The carburetor is equipped with intake mechanism, manually-controlled by a lever placed on a left engine’s cover.

In first two years of production Zündapp Werke made a few changes and improvements on users requests. Here are listed only the most important ones.

The suspension fork was lengthened for 10 mm to make a bigger clearance between a wind and a tire.
The way of a fuel tank fixing was changed, the lock of the tool box was removed to the other side. Rubber pad for knees on the frame were given up.
The most important change concerned the air filtering system. The wet air filter turned out to be malfunctioning and it didn’t secure the carburetor against freezing. New Neumann-Wirbelluftfilter filter with a heater was introduced. On army’s order Zündapp Werke exchanged all of wet filters which were in use on the house.
In 1943, in a search for ways to save strategic materials, producer changed some of them. Valve covers, instead of aluminum, were made from a sheet. Also rear wheel and side-car wheel drive lines covers and brake shoes were made from less expensive materials.

Side-car.

In KS 750 Zündapp introduced two types of side-cars. First, named BW 40, was original Zündapp’s construction. The wheel was springed by easy to regulate system of torsion shafts. The side-car also received a cushion on a torsion bar. Thanks to that, the side-car’s width got smaller, and the whole construction was easier to maneuver than BW 43 side-car.

BW 40 were produced only by Zündapp, and every frame got it’s own number. But the Nuremberg company was able to produce only limited number of side-cars. Rest of the frames were bought from the Sieib company and they were BW 43 type. BW 43 is an easier construction, not to use a word “more primitive”. The side wheel is springed by a torsion pipe and driving system cover working as an arm. The boat is hanged on two outside leaf springs.

The boat “W Krad B 2” was delivered by the Steib company. It’s an improved version of “W Krad B 1” boat, used before in Zündapps KS 500-600, K 800 and BMWs R 12 and R 71. B 2 can be recognized by its triangle reinforcements placed above the backrest of the seat and by thicker back tunnel for hanging the boat on a thick BW 40 frame. The seat is covered with black synthetic material and the backrest have special cutting for a holster. The tarpaulin was made of beige or blue-grey canvas and the colour was dependent on the motorcycle’s polish.