The twin-cylinder BMW Boxer, a mainstay of the business since 1921, is the flat engine with the greatest widespread recognition. The layout, which has a low center of gravity, a compact engine, and a generally sound balance between the moving elements, is also considered by other manufacturers.
Boxer Engines: Pros
- The center of gravity is low.
- Suitable for bikes with shaft drives.
- Six cylinders, well balanced.
Boxer Engines: Cons
- A wide engine may restrict lean angles and maneuverability.
- There could be torque steering.
- When pushed hard, it can feel underpowered compared to other engine types.
Boxer Motorcycle Engines: Design
The Honda Motorcycles favor flat-four and flat six in Gold Wing iterations. The GL1000 began with an all-four layout and later switched to a flat six for the GL1500 and the current GL1800. With the cylinders all moving in the same horizontal plane and the power strokes overlapping, it makes sense for a huge super tourer.
Only BMW has continued to use and develop the flat-twin design with the R series, despite Harley Davidson, Indian, Velocette, Brough, Douglas, and other motorcycle manufacturers dabbling with it at various points. The horizontally opposed cylinders were originally intended to be cooled by air. They also offered crash protection for riders and a place to warm your feet in cold weather.
The two-cylinder Boxer engine has an excellent, if not flawless, primary balance similar to a 90-degree V-Twin. Unlike a V-Twin engine, it also has regular firing intervals that result in very mild vibration.
Cooling Through Liquids
Due to the durability of the flat-twin boxer design, airheads, sometimes known as airheads, were produced from 1923 to 1995. And those versions are still in demand since they are dependable, easy to maintain, and repair. Many find new life after being transformed into contemporary café racers and customs.
Although they still employed air-cooling for the cylinders, BMW converted completely to partial oil cooling only in the mid-1990s, and the first water-cooled heads debuted in 2013.
GS Mountain Bikes
BMW still strongly identifies itself with the flat-twin boxer, making the straight-six K series, the inline-four S1000RR superbike, and smaller parallel twins. The dual sport GS series’ success is to blame for this.
The impact of the 2004 London to New York ride by Ewan McGregor and Charley Boorman in the Long Way Round TV series and book must be mentioned here. It made air-cooled BMW flat twins more widely known and popular on BMW R1150GS Adventures.
The high-capacity GS became the most popular bike in the BMW lineup for years and contributed to a boom in adventure bikes from many manufacturers. However, competing adventure tourers choose a V-twin engine, such as the Suzuki V-Strom 1000 and KTM 990 Adventure.
Why Do Few Bikes Use Boxer Engines?
There are some problems with twin cylinders. With a linear powerband and smooth power, it offers torque and is suitable for shaft-driven motorcycles.
The first drawback is clear since the protruding cylinders’ width makes maneuvering through traffic or confined off-road spaces difficult. Although BMW has strived to make the flat twin more durable, a variety of official and aftermarket accessories exist to protect the Boxer engine in a crash. However, it also puts several engine parts in danger of collision damage.
Since the engine architecture restricts the lean angle possible on some models, the other width issue typically affects BMWs that are more focused on the road. Despite its drawbacks, the BMW R1100S inspired the BMW Boxer Cup racing series. It included a shorter parallel torque arm that raised the motorcycle’s rear for better handling.
The modern BMW R nineT has a 1,170cc flat twin engine but also features riders competing in the bike’s Racer variant in the Boxer Cup racer series. Finally, the torque reaction that can happen when cornering is a limiting factor. Older machines’ propensity to lean slightly to one side during rapid acceleration and deceleration is more obvious.
Similar to Moto Guzzi V-Twins, you eventually get used to the fact that the real effect differs between motorcycle types, riders, and even specific bikes.
A flat-twin can feel underpowered compared to other bikes, especially when going quickly out of a corner on a track day. But a flat-twin BMW owner typically isn’t interested in track performance.