Some brands are forever associated with a particular engine type, such as Harley-Davidson’s rumbling V-twins. In BMW’s case, an air-cooled boxer twin with its cylinders in front of the rider’s feet and shaft drive to the rear wheel has been the manufacturer’s distinctive signature for almost 100 years.
BMW introduced its first boxer twin in 1923, the R32 model, a 500cc side-valve with a car-type clutch, three-speed gearbox, shaft rear drive and a proper duplex frame. This was incredibly advanced for its time, when many machines still looked like modified bicycles and some still used belt drive.
BMW claimed many advantages for their layout: better cooling with each cylinder out in the breeze; lower centre of gravity for easier handling; and very smooth running, with most of the engine vibration self-cancelling. The potential drawbacks were limited cornering clearance with the machine banked over and engine torque reaction causing the bike to rock to the side when blipping the throttle; neither of these were any real concern outside the extremes of racing.
During the 1920s and 30s, the engine gained overhead valves and grew to as much as 750cc, with power up from the 8.5bhp of the original to 33bhp in the 1935 R17 model. This was also the year BMW introduced telescopic front forks, with rear plunger suspension following in 1938.
In 1955, BMW produced a new range of boxer twins with distinctive ‘Earles fork’ front suspension, a kind of swinging arm for the front wheel that made the bikes ideal for sidecar use. The most powerful derivative, the R69/S, delivered 42bhp from 600cc, making it fully competitive with the British parallel twins of the era.
Many consider that the machines introduced in 1955 and made until 1969 are the finest quality motorcycles ever produced by BMW, or any other manufacturer. They were certainly very expensive, both to buy and to manufacture, and are still highly prized in classic circles today. Their dwindling sales almost drove BMW out of the motorcycle business all together but they resolved to meet the market challenges, including competition from Japan, head-on by launching a new range of more modern, more cost-effective designs in 1969; the /5 series.
The cost cutting wasn’t all bad news. The /5 engines used long-lasting, car-type shell bearings for the crankshaft instead of the rollers of the older bikes, while the Earles fork was replaced with supremely comfortable long travel telescopic forks. As the /5 models evolved into the /6 series, BMW created a style icon in the R90/S model with its ground-breaking cockpit fairing and stunning smokey metallic paintwork.
The R100RS, which ousted the R90/S as range leader in 1976 took aerodynamics to a whole new level, with its sporty full fairing cocooning the rider and passenger in calm, dry air even at high speed through foul weather. Of the other /7 models, the R80RT (800cc with a barn-door sized touring fairing) became the mainstay of many police forces throughout the 1980s.
Production of the air-cooled boxers ended in the 1990s but despite diversifying into 3, 4 and even 6 cylinder bikes, boxer twins, now with oil-cooling, are still the basis of BMW’s adventure bike and retro ranges and look set to celebrate their centenary in 2023.
Have you had any adventures on a BMW boxer? Let us know in the comments.