Motorcycling's Dynamic Duo

For more than a century, twin-cylinder engines--especially V-twins--have been the backbone of motorcycling. Here's why.

As the 19th century gave way to the 20th, personal transportation as we know it seemed as plausible as the rocket belts Popular Science promised us in the 1950s. Turn-of-the-century transportation had serious shortcomings. Horses were expensive and, well, demanding. Trains offered relatively rapid transit, but only if their routes and schedules suited yours. Walking was economical, but impractical for long distances.

A transportation revolution was just starting to get traction, though, with the safety bicycle, invented in the 1880s, as the first real breakthrough. The first practical four-stroke internal combustion engine was the next big advance, and combining it with the safety bicycle created a versatile, truly personal vehicle, one with seemingly limitless possibilities.

Entrepreneurs and inventors couldn't resist the opportunity, and in garages and blacksmith shops, bicycle stores and barns, they went to work with a vengeance, mixing the ingredients in every conceivable permutation. And while the results of their labors might have been little more than bicycles with small, crudely attached single-cylinder engines, they were enthusiastically received by a public to whom every development was fresh and new in a way jaded, sated 21st-century consumers might find impossible to comprehend.

Almost as soon as single-cylinder-powered bicycles appeared, people clamored for better, faster and even more versatile vehicles. Now, it doesn't exactly take a 12th-degree Mensa black belt to figure out you could roughly double that power just by fitting another cylinder to a single's existing crankcases, reaping the benefits of economical production with minimal weight gain. What's more, the new tandem twins had narrow Vee angles, allowing them to fit neatly into the triangle formed by the top tube and downtubes of a bicycle frame. That location placed the crankshaft just ahead of where the pedal crank would be, creating a convenient connection to the chain or belt driving the rear wheel.

With almost double the horsepower of their parent singles, the new twins were fast; faster than the cars of the time. Indeed, they were fast enough that their owners demanded better chassis than bicycles could provide, and more comfort. Chasing profits, manufacturers responded. From the turn of the century up until the beginning of World War I, motorcycles slowly evolved from minimalist transportation tools to symbols of speed and adventure, style and sport--almost entirely because of the V-twin powerplant.

Although the V-twin's dual virtues of easy packaging and good power lifted motorcycles from the mundane to the adventurous, the character of the twin's power delivery won more friends. By spacing cylinders unequally about the crankshaft's 360-degree circle, power events occur unevenly. A 45-degree V-twin such as Harley-Davidson's Sportster or Big Twin puts alternating 405- and 315-degree pauses between power pulses. As a result, the rider feels each one at low revs, providing an exquisitely tactile sensation of the combustion process, and--especially when traction is marginal--a sense of control. At higher revs the spacing actually increases traction by giving the rear tire a brief timeout between power pulses. In dirttrack racing, that phenomenon lets the rear tire dig in for a better drive onto the straightaway, helping explain why Harley-Davidsons have been virtually unbeatable on dirt ovals.

More subjective are riders' responses to the V-twin's sound and vibration. Some feel the twin's loping exhaust note sounds more restful than a single's; this is also true when compared to a multi, which can feel hyper. Similar qualities can also be ascribed to a V-twin's vibration style. Compared with a single of equal displacement, a V-twin's shaking is more subdued thanks to smaller, lighter internals. And although a multi's vibrations tend to be lower in amplitude, their higher frequency (versus a V-twin's high-amplitude/low- frequency vibration signature) can create a buzzy feel some find tiresome and annoying.

Of course, the tandem V-twin is not the only way to arrange a pair of pistons and cylinders. There's the opposed-twin, which Harley-Davidson tried in 1919. The same configuration has powered generations of BMWs, starting in the early '20s and continuing today. Some might say with tongue lodged firmly in cheek that the opposed-twin is merely a 180-degree V-twin.

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