SOME HISTORY

After the second world war, The German motorcycle manufacturers weren't allowed to build engines with more than 250cc displacement. The BMW plant was heavily bombed, so they were rather late with their first postwar bike, the R24. Not the famous boxer engine, but a very modest single. Nonetheless it was a huge succes, and allowed BMW to make enough profit for their first postwar boxer, after the restrictions were relaxed. This bike the R51/2 made from 1950 - 1951 was nearly the same as the prewar R51. The bike had a twin tube frame with plunger rear suspension and a telescopic frontfork. It had an engine with two camshafts, driven by a chain, wich allowed short pushrods. In 1951 the new boxers were introduced, the R51/3 and the R67, and the R68, the first BMW 100 mph runner, followed in 1952. These had a new engine in virtually the same old frame. This engine is of interest to us, because it is the same as used in the '/2' series until 1969, with detail modifications of course. This engine featured one camshaft, driven by helical cut gears. A magneto on front of the camshaft, a DC dynamo on front of the crankshaft and the oilpump on an extra gear under the crankshaft. All shafts had very big ball bearings. The big ends ran on needle bearings, and the small end had to do with a bronse bush. This layout was retained until 1969, with the excepton of the rear crankshaft bearing on the bigger, more powerfull machines, which had a swival joint bearing to cope with crankshaft flexing. The cylinder was cast iron. The piston and cylinder head were made from light metal. At the back of the engine a large flywheel was attached to the crankshaft, wich incorporated the car type single plate dry clutch.

In 1955 was the introduction of the R50 and the R69. They had the new frame but the old engines. The new frame was a sensation in these days. It was very comfortable and had good road holding properties. At the back was a swing arm, at last universally regarded as the optimum configuration for the rear end of a bike. The mainframe still had the same style as the old plunger frame, with neat loops next to the rear wheel and rather strange extensions to hold the rear suspension units. The front was very exotic. Here was a swing arm , according to the Earless patent. This was a very hot item, mainly used on racers, and on some fancy street bikes. But these latter weren't always a succes, because of their flimsy layout. BMW did it better. They made a very strong fork from conical tube and used conical roller bearings to carry the swing arm. They used the same bearings in the rear swingarm. Also the gearbox was changed in 1955. The previous models had a two-shaft gearbox, but these newer boxes had tree shafts. The consequence was a change of turning direction in the cardan. This was dealed with by changing the position of the pignon wheel in the rearwheel.

The introduction of the sportsmodels R50S and R69S, started some big difficulties. The engines had a lot of reliability problems, like broken crankshafts, cylinderhead failures and even teared off cylinders. The crankshaft was too flexible for these kind of power outputs. The R50S was discontinued in 1962 after only 1634 units produced. The R69S got a vibration damper on the crankshaft in front of the dynamo. These engines are recognized by the extra bump on the front cover. The R69S also got a hydraulic steering damper to cope with the high speed instability. At the very end, since 1967, the US models were available with a tele scopic frontfork, as develloped for the /5 BMW's. This model was only available in the USA, and German customers were rather sauer about this. They had to wait until the introduction of the /5 range.

The development of the single cylinder models was a bit different. After the R24 without rear suspension, came the R25 with a plunger frame, like the R51/3, but smaller and cheaper manufactured. Although the R25 and its succesors the R25/2 and /3 sold in huge quantities, the factory didn't spend as much development effort in these bikes as in the boxers. There were some thermical problems that were dealt with in the cheapest way, by painting the cylinderhead black. Later the heads had much larger cooling fins, and thus the engine was used in the R26. The engine was quite different from the boxer engines. It also had a longitudinal crankshaft, but the camshaft was driven by a chain. It also hadn't a magneto, but it had to do with coil ignition. The frame of the R26 and R27 strongly resembles the frame of the R50 etc. In fact it has completely the same Earless frontfork, of course with different shoch absorbers and springs. The differences are at the backside. The reinforcement bars near the swingarm bearings are different, and the suspension units are more slanted. The 'cardan' of the single cylinder models also hasn't a cardan coupling, instead it still has the Hardy coupling. This is a disk of woven fabric, which allows some up and down movement. The R27 has the very special rubber engine mountings. BMW had investigated this subject intensily. Because the R26 had quite a lot of power, the vibrations were very strong. This was not only very uncomfortable, but it also inflicted the reliability. These rubber mountings lessened the degree of vibration to a very acceptable amount, although the power output was raised from 15 to 18 hp.

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