The Development of the BMWR75
Already at the beginning of the early 1930's many European Armies decided to increase the mobility of some parts of the infantry with bikes with sidecars. At the introduction of the universal conscription in 1935 a Biker Private Troop was put up. Its task was to get three fully equipped soldiers on each bike into position at the front line in very short time.
Until the first years of the Second World War the bikes available for the military were civilian bikes, painted in military grey and equipped for military use. Because of so many different types a lot of bikes fell out for lack of spare parts after a very short time. An operation off the beaten tracks under aggravating geographical and climatic circumstances was only possible to a limited extent.
Already in November 1937 the OKH (Oberkommando Heer) was discussing with the industry (BMW and Zündapp) the subjects
The following conditions for the new to be constructed bike with sidecar were established :
1. The tyres of the Volkswagens (tyre size 4,50 x 16) should be used.
2. The mudguards had to be dimensioned in such a way that non-skid chains could be fixed on the front and rear wheel.
3. The tank content for one way had to be for 350 km.
4. The max. speed fully loaded was established at 95 km/h, the permanent speed on Autobahn at 80 km/h .
5. A minimum ground clearance of 150 mm is needed .
6. The bike has to have a loading capacity of 500 kg, which was split 250 kg (driver with sidecar passenger and saddle bags) for the bike and the remaining 250 kg for the sidecar.
That is the reason why this drive system was an absolute must for BMW. Furthermore the telefork that had been developed some years ago could be used. The advantage compared to the front wheel leaf springing is a very quick and soft springing on the ground. The frame should be made of a pipe chassis that can be dismantled .
In the spring of 1941 a powerful looking "workhorse" came up. Besides very easy steering abilities this one had all the well know BMW handling characteristics. As a result of the complete fixation of all movable parts including the telescope fork and the propeller shaft drive the bike remains tough and very reliable under all driving conditions. Even crawling along in convoy for hours, driving through water or on steep slopes or driving fast on Autobahns have no influence on the operational safety .
At first view the technical specialities stand out that were realised during the construction with regard to the use for military, colonial and official services as well as for forestry.
A two cylinder boxer engine with 750 ccm cubic capacity is the energy source. The valve control, the camshaft and the hemispherical combustion chamber were not laid out for peak performance but for high permanent performance (26 PS at 4400 U/min). The maximum torque of 5 mkg is reached at 3600 U/min. The Aluminium cylinder head is strongly and extensively ribbed, so that the cooling effect is big enough even at marching speed in great heat. All bearings are roller bearings. Advantage: low frictional resistance and higher life span. The drive of the camshaft and the oil pump comes from the crankshaft via cogwheels. The clutch is a dry single disc clutch, that has the same dimensions as a medium size vehicle. The two carburetor types Graetzin 24/1 and 24/2 have a common wet air filter on the transmission housing. An ignition magnet is built in so that a defect of the battery cannot bring the bike to a standstill. The ignition magnets were alternatively the Noris type ZG 2 a or the Bosch FJ 2R 134.
The cogwheel transmission has four gears and two reverse gears. A cross-country reduction gear can be put underneath the first three gears. If you put the fourth gear a dog at the gear lever the reduction gear lever is in a neutral position. The energy flow from the transmission to the final drive is then interrupted. The street gears are changed by foot or hand. Using the foot the hand gear lever is used at the same time as a shift indicator. The reverse gear and the reduction gear can only be changed by hand. To minimise hard punches from the transmission to the final drive a universal joint with rubber buffers at the transmission exit is built in.
The Rear Wheel Sidecar Drive
Both wheels are driven by a lockable differential unit ( 70 % rear wheel, 30 % sidecar wheel )
For the first time hydraulic brakes are used for heavy bikes with sidecar (sidecar rear wheel). The front wheel has a mechanical inside shoe brake, that is controled by a hand lever via cable. The brake drum diameter corresponds with 250 mm to that of a medium size vehicle. When the sidecar is taken off a double valve closes the brake line automatically.
The chassis is an easy to take apart unit with construction of special strength. For any faults it is no longer necessary to change the complete chassis but only parts.
A motorised unit of 10 to 12 bikes only needs three chassis for spares. It was very unlikely that the same chassis parts would be defect in all bikes.
The telescopic fork corresponded to the proven version of the solo bikes. It was only a stronger version corresponding to the higher load (high empty weight of the bike and cross country operation). By using double effective oil pressure shock absorbers the fork gets a very soft springing. It was only filled with normal motor oil. This made the supply at the front much easier.
Strong tubes welded to a rectangle build the sidecar chassis. The sidecar wheel is fixed at a control arm where the drive is situated. The springing results through a torsion spring which is situated together with the drive shaft in the back of the sidecar chassis transverse tube. The sidecar boat is fixed at the back with two leaf springs and is positioned on rubber in the front.
The early versions of the tank have a tool box on top. It contains 24 l , whereof 3 l reserve. The consumption on the road is between 6 and 7 l/100 km, cross country up to 10 l/100 km. These are figures based on today's experience.
The exchangeable wheels have a low-bed rim and thick end spokes. The life span of the wheels 4,50 x 16 is compared to the normal wheel dimension 3 – 5 times longer, because the specific load is in spite of the high payload smaller the drive friction is split on two wheels.