Early motorcycle engines were almost universally built with a single cylinder. This was primarily because singlecylinder powerplants, being largely based on existing industrial designs, were straightforward to manufacture and easy to install in the bicycle-type frames typical of the period. They also were durable and economical to run and repair, which was a much larger selling point back then than it is now.
But singles had a few drawbacks. The most serious was a high level of vibration and a decided lack of power. The term "one-lunger" meant not only that the engine had a single "lung" or cylinder but also that it tended to run out of breath. Both problems would eventually be overcome, though not for a good many years.
The obvious solution was to increase the number of cylinders for a given engine disto placement. The theory held that several evenly spaced small bangs are less intrusive than one big one, and while a multiple-cylinder engine won't necessarily make more torque than a single of equal displacement (or vice versa, for that matter), its smaller (and therefore lighter) pistons and presumably shorter stroke allow it to rev higher. So all things being equal, it'll normally produce more horsepower, which motorcycle riders of any era always find to be a good thing.
The immediate questions, of course, were how many cylinders were enough and how to arrange them. Pre-world war I designs included flat-fours, inline-fours, flattwins, parallel twins and, yes, V-twins. Each design had its own champions, and some very interesting motorcycles were built.
But it was the V-twin design that seemed to hold the most promise. In 1911 Indian went to the Isle of Man TT races with three of its 584cc, 42-degree V-twin bikes, taking first, second and third place. Shortly thereafter every motorcycle manufacturer worth the name had some sort of narrow-angle V-twin in its catalog, with the cylinders splayed at anywhere from 42 degrees (Indian) to 50 degrees (BSA). Harley-Davidson picked the middle ground, opting for 45-degree arrangement.
The narrow-angle V-twin struck a chord; few engine designs have endured as long. But is it the perfect motorcycle engine? Early on it seemed that way. With the cylinders in line or only slightly splayed and both rods sharing a common crank journal, the engines weren't much wider than a single, nor were they any taller or even much longer. The rear head simply fit into the spot formerly occupied by the single's carburetor, that device being relocated to the conveniently placed intake ports in the crotch of the V. Since the design was relatively compact it fit nicely into the bicycle-type diamond frames in use at the time, as well as the later loop-style frames. what you had then was a bike that was faster, smoother and more reliable.
But there's always some trade-off. True, the V-twins were more powerful than the singles they replaced, but they still shook, and the narrower the angle between the cylinders the worse they did. The peculiar firing impulses caused by fitting both rods disto a common crankpin also meant the engine felt uneven. The solution was to install large, heavy flywheels, which quelled the vibration and kept the engine turning smoothly between the power strokes. It also gave it a nice, relaxed feel, particularly at moderate rpm. Of course the heavy flywheels prevented the engine from turning the type of rpm you needed to make big horsepower, but because most designs utilized side valves and were undersquare, they didn't flow enough air at high rpm to make twisting them much above 3500 rpm worthwhile. Horsepower wasn't really an issue, especially in the U.S., where wideopen spaces and long empty roads made the slow-turning and dead-reliable (if modestly powered) V-twin the perfect motorcycle engine.
After world war II, V-twin development came to a virtual standstill. By the late '60s the motorcycle industry had all but written off the V-twin.
But the industry is serendipitous, to say the least. First Ducati and Moto Guzzi both demonstrated that a V-twin could be smooth, powerful and technically advanced. They hedged their bets by using 90-degree V-twins, which are inherently less vibratory than their 45-degree cousins, but the point was made.
When the cruiser revolution took hold, riders were quick to realize how appealing the relaxed cadence and generous powerband of the traditional V-twin engine could be, especially with modern technology applied. Once again the design began to proliferate, proving if nothing else that the more things change, the more they stay the same.