The BMW R28 – the last single from Munich 

During the early nineteen-sixties BMW was struggling to survive in a marketplace where motorcycle sales had fallen to their lowest point since 1948. Since the bankruptcy of 1959 the company had been kept afloat by sales of the Isetta ‘Bubble Car’ and the 700-Series small car, but popularity of these motorcycle engined saviours was now falling rapidly. The Company’s survival depended upon the success of the ‘Neu Classe’ car range, led by the launch of the 1500 saloon in 1961, into which all spare funds and effort were being diverted.

A potential lifeline for the struggling motorcycle division came in the form of a Government specification calling for a multi-purpose on/off road motorcycle to equip both the Bundeswehr (armed forces) and frontier guards. Strict standards were laid down for weight, speed, fuel consumption, carrying capacity, and cross-country performance.

While the well-proven 250 cc engine and gearbox of the R27 were available, it must have been obvious to BMW that in terms of off-road performance they would be at a disadvantage to competitors who would base their designs on lightweight two-stroke engines employing chain final drive. A successful military contract would transform the flagging health of the entire motorcycle division, but there was no spare money to develop a new lightweight engine for the military model.

BMW were no newcomers to off-road motorcycling, their flat twins (which shared their frame and forks with the R27 single) having enjoyed many successes in the International Six Days Trial. They had however been ridden by experts, rather than average soldiers.

After field tests were made using an R27 it became obvious that the Earles forks would not be suitable. After shortening their pivoted arms problems were still encountered, and it was decided that the limited development funding would have to be used to provide an entirely new frame and forks if they were to meet the specified weight, carrying capacity, and cross-country capability.

The result was the R28, a new machine whose duplex frame was similar in layout to that used by BSA on their A7 and A10 series twins of the time. The generously proportioned telescopic fork was the first used by a production BMW since 1955, and like the swinging arm it pivoted in taper-roller bearings. The full-width hubs from the R27 provided excellent 160 mm drum brakes, around which were eighteen-inch rims carrying cross-country tyres. The final drive casing was as used on the twins of the period, but with a lower (4.5:1) gear ratio than was used by the R27, which restricted the maximum speed to 70 mph.

The engine and gearbox were those used by the R27, sales of which were now falling rapidly. These were attached to the frame by their usual rubber mountings, with a high-level exhaust pipe and a sturdy sump guard.

The 250 cc single used a bore and stroke of 68 mm, and with an compression ratio of 8.2:1 provided a power output of 18 bhp at 7,400 rpm. The main difference was the use of a rubber-mounted carburettor on an inlet pipe almost 250 mm long, this extended inlet tract helping to increase low-speed torque. The waterproofed carburettor was therefore mounted in line with the nose of the dualseat, alongside the large air filter and plenum chamber, which also enclosed the battery. 

Electrical equipment was very good, with a six-volt 90 watt alternator mounted on the front of the crankshaft, coil ignition, a 12 ampere-hour battery, and trafficators. Starting was of course by means of the usual BMW kickstart, operating sideways from the left side of the gearbox.

The combination of a new suspension layout, ‘sports’ blade-type mudguards, braced high-level handlebars, and a dualseat had produced a BMW which was not only of orthodox appearance, but also looked thoroughly modern. A large fuel tank of around four gallons, frames for quickly detachable panniers, and the standard BMW headlamp unit completed the picture. Optional equipment included a centre stand, pillion footrests, leather panniers, a holder for despatches, and a mounting for a flashing beacon.

At 155 kg the R28 weighed slightly less than the R27, but could carry 50 kg more, and boasted a ground clearance of almost seven inches while carrying a 75 kg rider. It remained, however, heavy for a 250 cc machine – for example, the BSA C15, a typical 15 bhp 250 cc single of the period, weighed only 125 kg.

Evaluation trials were held during 1964, and it was found that the R28 had no difficulty in meeting the specified requirements, in many cases exceeding them by considerable margins, in all but one area – weight. As was surmised at the beginning, the BMW was at a severe weight disadvantage when compared to machines using lightweight two-stroke engines, and this was never more apparent than in mud and rock-strewn cross-country conditions. Worse still, any attempt to pick up a fallen machine required far more effort than was the case with a typical two-stroke. 

The result was that the prized contract was awarded to the Sachs-manufactured Hercules two-stroke, which while it was by no means such a good all-rounder as the R28 possessed sufficient performance combined with far lower weight and manufacturing costs.

The R28 was a modern go-anywhere motorcycle, which combined excellent performance both on and off the road with a fuel consumption of almost 88 mpg at 50 mph. The 1964 specification sheet shows that these virtues had not escaped BMW’s marketing men, who obviously had ambitions for its sale to civilian motorcyclists who wished to combine road and cross-country riding. In 1964 there were no production trail bikes, and a unique machine of modern appearance could have transformed the fortunes of BMW, but this was not to be.

If the R28 had survived it would have anticipated the trail bike fashion by almost ten years. A larger engine could have been developed, perhaps based on a barrel and head from the R90, which was available in 1969. Such a 450 cc trail bike, refined by its use of rubber engine mounts, would have been very popular. Perhaps we should view the R28 as being the right machine at the wrong time.

The R28 left a considerable legacy in the form of its cycle parts, the forks being fitted to the ‘US’ range of large flat twins destined for the American market. More importantly, some of the frames were soon seen carrying the flat twin engines of the period, proving that this new frame was capable of harnessing more power. These are shown in several books, usually described as ‘Design exercises.’ (See Roy Bacon’s book ‘BMW Motorcycles’ pages 47 and 50.) These hybrids, using R69S engines, gained several victories in the German Cross-Country Championship of the nineteen-sixties in the hands of Sebastian Nachtmann and Kurt Tweesmann.

Before long a new range of flat twin engines was being developed, designed to be accommodated in the R28 frame, these ‘Five’ Series models saving the motorcycle division in its darkest hour. This prompted BMW to produce an R75G/S prototype in 1970, followed by several other prototypes up to the lightweight GS800 models, developed by Hubert Sheck during the late ‘seventies. These projects eventually led to original Paris-Dakar models, after which the Company marketed its first on/off road production machine in the form of the R80G/S of 1981.

The R28 should therefore not be remembered as a failed military motorcycle, but as the concept which made possible those victories in the German Cross-Country Championship and the Paris-Dakar wins twenty years later. It was the first GS.

Article and photos by Mike Fishwick - first published in the BMW Club Journal