The Advantages of Flat Motorcycle Engines

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

There are a number of reasons for building flat engines, but foremost among them is balance. With two crankpins spaced 180 degrees apart, piston motions are always opposite each other so that piston shaking force is self-canceling.

That doesn’t mean there’s no vibration. Because the two crankpins have to be spaced apart (just under 2 inches in BMW flat twins) there is an oscillating force that acts around the vertical axis, caused by the fact that the acceleration/deceleration forces of the pistons can’t act along the same axis.

When budding designer Charles Lawrance designed his Model A flat-twin aircraft engine for the Penguin ground trainer right after World War I, he must have been going for simplicity. He gave it only a single crankpin, on which both con-rods pivoted. Instead of piston shaking forces canceling each other, as in a two-crankpin “boxer” design, they added together, making it into a legendary vibrator and parts breaker. You can see this dreadful little engine at the Teterboro air museum in New Jersey.

BMW’s latest and largest boxer, the R 18, has more vibration than smaller-displacement models because the crankpin spacing is increased because of a third main bearing between them.

BMW’s latest and largest boxer, the R 18, has more vibration than smaller-displacement models because the crankpin spacing is increased because of a third main bearing between them. (Jeff Allen/)

When BMW’s Max Friz designed that company’s R32 flat twin, which was produced from 1923, he gave it two crankpins at 180 degrees, so its vibration was very low—a real surprise in the days when most bike engines were hard-shaking 350 or 500 singles. All BMW flat twins have been built that way ever since. Friz did not originate the flat twin; Douglas in Britain had built such engines from 1907, and Karl Benz’s “Kontra” flat twin dates to 1897! We must conclude that the flat twin is a product of common sense.

A second reason for adopting flat or opposed-cylinder construction is that it can result in a very low CG, because the cylinders are beside the crankcase rather than on top of it. There was an era in US midget racing when souped-up VW flat fours were dominant, and their low CG was responsible for less weight transfer (overloading the outside tires) during cornering, as compared with taller upright inline-fours. This may have been important for the engineers who designed Honda’s Gold Wing, which began life as a flat four and later became an even smoother flat six. On a heavy bike, weight located up high makes low-speed handling (and just holding the thing up at rest!) less easy and confident.

For Friz, an aircraft engine designer, it was also important to provide good and equal cooling for the two cylinders. On conventional vertical singles the head and cylinder were in the wind shadow of the front tire and fork, and in the case of V-twins, the rear cylinder was often overheated by being “cooled” by the hot air streaming back off the front cylinder. This is why Guzzi disposed of the cylinders of its 500cc air-cooled V-twin at 120 degrees—each cylinder thereby received cooling air unheated by the other. Vincent engineer Phil Irving, whose  books Motorcycle Engineering and Tuning for Speed you must find and read, as they are undying classics, notes in his autobio that rear cylinder cooling on Vincent twins was insufficient for any serious racing application.

Therefore when Honda planned “Gold Wing” as a new-generation sportbike, the company understood that it had to be liquid-cooled, as otherwise the rear pair of cylinders would have run hot and suffered durability problems (it was Gold Wing buyers who saw its potential as a touring rig).

The flat-four-powered 1975 Honda Gold Wing GL1000.

The flat-four-powered 1975 Honda Gold Wing GL1000. (Honda/)

Then how do the many flat air-cooled engines—fours, sixes, and even eights—survive in gen-av aircraft? They do fine because cooling air is not circulated through a row of cylinders, front to back. Instead, fresh air is ducted through the fins of every cylinder separately from top to bottom. No cylinder is cooled by hot air from another.

One of the lesser-known flat engines is the 14,000-rpm 500cc flat four with which MV originally planned to turn the two-stroke tide in the mid-1970s. But that’s another story.

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