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.

Yet while the parallel-twin is relatively inexpensive to build and compact compared with other engine configurations, it has shortcomings. The V-twin's pleasant character is largely absent in its parallel cousin. Rather than residing in the plane of the bike's centerline, inTurner's design vibration rocks the engine from side to side. Commonly known as a rocking couple, these forces stem from the spacing of the crankpins relative to the crankshaft's centerline.

Norton was the first manufacturer to effectively squelch rocking-couple vibration, isolating the engine, transmission and rear wheel from the rest of the motorcycle with rubber mounts. Other manufacturers have subsequently used balance shafts to quell the parallel-twin's inevitable quaking. And while the solution yields good results, such engines tend to be bland compared with multicylinder engines and more advanced V-twins. All of which proves that while it is entirely possible to obtain perfect dynamic balance, it can be done without achieving perfect smoothness--or even the desirable kind of character.

Still, after more than a century of development, the V-twin still offers the greatest adaptability to the widest variety of roles. For example, the overwhelming majority of customs/cruisers have kept the classic V-twin architecture, just as they have since the first fender was bobbed. In fact, according to Motorcyclist's 2004 Motorcycle Buyer's Guide, 94 percent of cruisers and touring bikes are powered by twins, and 86 percent of those are V-twins. Likewise, more than half the '04 sportbikes, sport-tourers and standard bikes for sale in the U.S. are twins, and some three-quarters of those V-twin-powered. Although displacement, power and torque have skyrocketed during the last century, V-twins have made the most of their inherently relaxed, satisfying cadence as cruisers and tourers.

In sportbikes, V-twin engines have become true fire-breathing monsters that dominated World Superbike racing for 15 years. (Shifting rules have kept the V-twin from dominance in AMA Superbike racing, however.) Success on the world stage just demonstrates that V-twin Superbikes (and their street-legal counterparts) are some of the best-performing motorcycles ever built. What's more, the nature of their power remains linear and controllable. Sporting V-twin power has almost always been friendly. Contemporary examples show it's now an exceedingly muscular friend.

But twins have the power to be more than motorcycles. Indeed, twins have put entire corporations on the comeback trail. For example, V-twins are responsible for Harley-Davidson's extraordinary success. V-twins carried Ducati from its low point as a Cagiva subsidiary to unprecedented racing success as an independent and financially stable company. Opposed-twins have sustained BMW's motorcycle group since the '20s. Triumph's legacy as the originator of the four-stroke parallel-twin was celebrated by a new '01 Bonneville.

Such performance--both on and off the bike--is what makes twins in general, and V-twins in particular, so extraordinary. From its simple, innovative beginnings as cheap transportation, the V-twin has become an icon. No other engine configuration is as light or compact, yields such a short, rigid crankshaft or bolts so elegantly and intelligently into a motorcycle's chassis.

Whereas the march of technology seems to have contributed to smaller, more tightly defined niches, it has also expanded the V-twin's capability and utility. Having become more powerful, smoother and more sophisticated, it has adapted to a broader range of duties, even as motorcycling has become more specialized.

Plainly, the V-twin has stood the test of time. With any luck, it will continue to do so for another century--or two.

Harley-Davidson Iron Head
BMW R1150
Suzuki SV650
Aprilla Mille
Triumph Bonneville
1911 Harley-Davidson F-Head