The traditional 45-degree...
The traditional 45-degree design popularized by Harley has been widely imitated.
Thumb the button and the starter engages with a clank and labors to put a few rpm into your cruiser's gleaming V-twin motor. Whirr, whirr... cough...thump, thump, thump. It's alive.
You're the prototypical enthusiast, the kind of motorcyclist who contemplates a fabulous vista thinking, "That'd make for a good ride." So the sound of this big-inch cruiser's engine coming up to temperature in your driveway is truly music to the ears. Yet it's not just the distinctive rumpa of the exhaust or the shimmering whiz of the primary gear--your ardor rises from an appreciation of the whole aesthetic. Today's best cruisers are powerful and reliable, yet also soulful in ways that high-strung, racing-oriented bikes could never be. Seeing that large V-twin quavering in the frame reminds you every mile of why you bought this bike: attitude. Laid-back, but ready to go when you are.
Cruisers, perhaps more than any other subsegment, live or die on the emotional strength of their engines. (And by this we are not describing the fortitude needed to take on Dr. Phil.) Again, this is not a numbers game; pure horsepower, even a tasty power-to-weight ratio, means a lot less here than personality, soul and charisma. And yet your infatuation with the modern V-twin engine is not without technical merit--regardless of what the green-leather boys have to say--because today's tailor-made V-twin cruiser engines are extraordinarily good at what they're intended to do. And that's no mean feat.
Harley's bread-and-butter...
Harley's bread-and-butter (and knife-and-fork) V-twin has evolved significantly over the last century but remains the standard for cruiser V-twin style.
Dark Ages
In the beginning, there was the single-cylinder motorcycle engine, and it was good. Early motorcycles, like all vehicles in the late 19th century, were crude--essentially bicycles with unreliable, oil-dripping, flame-belching internal-combustion powerplants often scarcely stronger than a man. In other words, they were a perfect match for America's highway system, which wasn't much of a system at all. But everything improves, and eventually the need for more power (certainly more reliability) than a single-cylinder engine could provide won out on the desire for utmost mechanical simplicity. After all, a single is about as simple as it gets, but running it hard may result in a less reliable engine than a more complex design that's loafing along. If you're already making one cylinder, it's not too much of a stretch to imagine plonking another on a single crankshaft--bingo, double the displacement and, so the theory goes, double the power.
In fact, done right there are some great advantages to the V-twin; for instance, one cam can actuate valves in two different cylinders or a single carb could share two intake manifolds. Strictly speaking, you wouldn't necessarily be doubling the parts count for a useful boost in (still, at this point, theoretical) power.
Indian built the first V-twin in 1903, the same year as the first Harley-Davidson (a single), but the layout appeared to have so many intrinsic benefits that it quickly spread. Harley's first V-twin, which debuted in '09, was not a success--providing a fantastic engineering lesson that simply doubling the cylinders was not enough. Harley didn't offer the V-twin in '10, but rebounded with the Model 7D in '11 that clearly formed the nucleus of what Harley was to become. Say what you want about Harley-Davidson, but the lads learned from their mistakes in the early days.
From these humble beginnings--really just an engineering expedient to getting more power by using two cylinders on a common crankcase--the V-twin engine propelled Harley-Davidson and Indian into a prominence bowed but unbroken by the Great Depression. That Harley survives today has to do with luck and business acumen as much as engineering talent--many would argue that Indian was technologically more advanced, and that motorcycles from all parts of the globe could do anything a Harley could do, only better and for less cash.
Harley's fortunes ebbed and flowed throughout the 20th century, and we rejoin the plot as Japanese manufacturers, circa the late '70s, notice that the "classic" Harley-Davidson--basically a touring bike still powered by what at the time was a terribly outdated and woefully unreliable air-cooled V-twin engine--was moving well enough from showrooms to warrant a closer look. (More than that, Harley had survived the "bad boy" image of the '60s and, as a manufacturer somewhat less than a cultural icon, was on the verge of cashing in on the outlaw profile.)
Americans like the look of...
Americans like the look of pushrods, but they can also help reduce V-twin engine height where that is an issue, such as with the Kawasaki Vulcan 2000 engine.
Fresh Wind From the East
Soon, Japanese customs arrived, blatantly but often inexpertly styled along Harley lines--meaning a "teardrop" tank, a stepped saddle, chrome fenders, silly buck-horn handlebars and even a vestigial sissy bar. Yet there was a critical distinction: Japan Inc. didn't make a suitable, big-inch V-twin--and anyway, it was considered unseemly to so boldly go where Harley had been before--so these cruiser caricatures were powered by whatever was around--big, air-cooled four-cylinder engines, vertical twins, thumpers, what have you. At the height of the insanity--the late '70s and early '80s--Japanese manufacturers tried everything, including an across-the-frame, liquid-cooled, pushrod, four-valve V-twin. (Honda, we all thought at the time, would design something just to prove it could, not because it was the best configuration for the job.)
And yet all the newcomers were missing a critical ingredient--the V-twin engine configuration that Harley had defined and that in turn absolutely defined Harley, and by extension any concept of an "American" motorcycle. Yamaha broke into the category with the Virago 750 in '81, offering a handsome and capable motorcycle that was more like a Harley (OK, a small, forward-thinking Harley) than any of the Japanese bikes before, and it was an instant success. Thus emboldened, the Asian manufacturers wasted no time bringing V-twin engines to the blossoming "custom" market. By '83, Honda had two Shadows (a 500 and a 750), plus the hairy V4 V65 Magna (and a smaller version whose descendent is still made), and Yamaha had a bigger, 920cc Virago. Two years later, the Kawasaki Vulcan 750 debuted. We'll ignore Suzuki's ungainly Maduras of '85 and instead trumpet the arrival of the Intruder in '86. Thus, by the mid-'80s, each of the Big Four had at least one V-twin cruiser, and such nonendemic brands as Ducati and Moto Guzzi were trying their hands at American-styled bikes.
Today, metric cruisers are sales darlings, and every one that sells worth a damn has a V-twin engine. Why do you suppose that is?