The R5 in all its original glory
Continuation of chassis and engine design within the hallowed halls of BMW took on a new pace in the mid-to-late 1930s. Motorcycling was experiencing a rising tide of popularity. For this and affordability reasons, competition from other manufacturers and countries pushed BMW to revaluate its range of bikes and implement new and improved machines rather than simply revise existing models. The 1936 BMW R5 and 1937 R51 became two such benchmark machines.
The implementation of new technology wasn’t aimed squarely at motorcycles themselves. BMW placed new ideas, funds and trust in advanced machinery to assist in manufacture, i.e., machine tools and construction processes. This was best seen with the introduction of the R5 in 1936, where new design and manufacturing processes were heaped upon this newcomer.
BMW served up the R5 as an entirely new model; design-wise, it was a technically advanced, beautiful machine and powerful to boot – valid reasons why the R5 is considered by classic BMW riding fans as the very best of pre-war BMW Boxer motorcycles.
The R5 appeared with a new frame. The German company had learned that although the pressed steel, braced and riveted “Star-concept” frame was originally cheap to produce and strong enough to support sidecar use, it was time consuming to assemble. But instead of returning to brazed frame joints, BMW embraced fully the latest techniques in electric arc welding. For this reason the R5 frame was a return to tubular steel but with oval cross-section.
Although the R5 featured the latest hydraulically damped telescopic front fork, the rear was still a fixed, rigid assembly, using the tires and seat mounting as the only form of suspension. Another benefit of the new R5 frame design was the reduction in weight compared to previous models with pressed steel frame construction. The R5 weighed in at a ready to ride 364 pounds.
As far as the engine goes, the R5 retained the classic 494cc and square 68 x 68mm bore/stroke dimensions, but featured two chain-driven camshafts above the crankshaft, resulting in shorter pushrods actuating the overhead valves and raising the rpm ceiling. A healthy compression ratio of 6.7:1 and an Amal 5/423 carburetor, complete with an ear-like air filter for each cylinder, led to a peak power output increase to 24HP at 5,500 rpm. All the mechanicals, including the new four-speed foot-operated gearbox – an auxiliary manual gear change was incorporated – were housed in a tunnel-type, one-piece crankcase; this design stood the test of time by being used for a further 45-plus years.
Quite simply, the R5 was a sweet handling performer backed up with an engine that further delivered rider enjoyment. The 200mm drum rear brake wasn’t the most effective unit available, but it was a marked improvement on BMW’s old and simple Cardan drive shaft brake. Top speed was a claimed 86 mph (140 km/h), which made this OHV “sports” machine a true speedster even though it was “only a 500.” The fact that it was priced at 1,550 RM also helped it to achieve a production figure of 2,652 units in the years 1936–37.
Refinement of the R5 engine continued and it appeared elsewhere in BMW’s model range with varying capacities. Much of the updates were direct feedback from BMW’s racing program with the 500 Kompressor racing model. A switch to spring-supported vertical slider suspension proved its worth when the 500RS took the checkered flag many times throughout 1937, the most notable in the blue riband Grand Prix series. From this point it was always in the cards that eventually rear suspension of some type would see the light of day on road-going machinery.
Given that long-term success appeared to be assured, R5 production was cut short. But it was replaced by a machine of perhaps greater standing that would see a long and industrious build life – and quite possibly safeguard the future of BMW following WWII.
The mists of time must have shifted to and fro since 1938, the introductory year of the R51 and the replacement for R5. Many people argue the R51 was the first road-going machine to feature a full sprung chassis, but others will argue for the R61. If it was, then it was only by a few months. But the truth is BMW rolled out a series of new machines in 1938, including the R66 and R71, and all featured sprung chassis with hydraulically damped telescopic forks and spring-supported vertical slider suspension, more commonly known as “plunger” rear suspension.
Unusually, BMW was not the first manufacturer to feature rear suspension. Early competition models had leaf spring-style suspension and the 1912/13 Pope had a pair of rear spring plungers. The BMW system did not have a pivoting swingarm as is known today, but retained the fixed rear frame. Actual movement of the rear axle, including the Cardan shaft box and drum brake, was accomplished with it moving up and down two vertical posts (removable for service work) mounted to the back of the fixed rear frame. Springs positioned on the posts, either side (top and bottom) of the rear axle shaft, controlled the rate and distance in which the rear axle could move up and down on the two fixed posts. This was a simple design in itself, but one that could be fine tuned with different spring rates, as was often done to tune the rear suspension to deal with the variation in racing arena surfaces on- and off-road.
One area in which the road-biased R51 achieved fame was with police forces. Thanks to the R51’s fully suspended chassis, it was able to cover a variety of road surfaces at a healthy pace. The perfect chassis of the R51 also proved to be an exemplary starting point for racing machines, and the factory produced a run of R51SS racing units and a limited edition Rennsport R51RS.
The future looked bright for BMW. Sales of the R51 exceeded 3,700 units within two years and the next generation of technically brilliant BMW motorcycles was on the horizon. But so was an event in history that would put paid to BMW’s immediate plans for the future. R51 production was halted due to the outbreak of WWII and the next large capacity model, the R51/2, did not appear until 1950.