Historically, Harley has tried to manage the primary imbalance by fitting massive flywheels, which have the knock-an effect of smoothing power delivery-at the cost of making the engines "lazy" or unwilling to accelerate quickly, plus the penalty of the added weight of the assembly.
If primary imbalance is a bad thing, and the 45-degree V-twin has lots of it, is there a good V-twin configuration? You bet: The 90-degree Vee, like Ducati and Moto Guzzi use, has inherently counteracting primary forces and minor secondary imbalances acting in a plane perpendicular to the centerline of the Vee as viewed from the side. In other words, this design is naturally quite smooth for a twin-cylinder engine. (You want really smooth? Try an inline-six or a V12.)
If it's so good, why doesn't everyone use a 90? Well, it's difficult to fit into a frame gracefully and, let's face it, just doesn't look the cruiser part. Because even the most advanced metric cruisers embrace a profile made of the necessity of housing a 45-degree V-twin, the narrow Vee angle is considered the only way to go. Ah, yes, but how do you make it smooth?
Two methods prevail. One is to split the crankshaft where the connecting rods meet it, in effect offsetting the pistons to make the engine act more like a 90-degree Vee. Honda's Shadows, for example, set the cylinders 52 degrees apart and use offset crankpins. This makes the pistons rise and fall in their respective cylinders nearly 90 degrees apart. There's still a bit of imbalance leftover, entirely manageable by simple rubber mounts or leaving them unvarnished for the rider to enjoy.
The other method employs counterbalancers. Resembling tiny crankshafts without the rods, counterbalancers are merely weights rotating on a shaft timed to exert a force opposite to the inherent, undesirable vibration of the engine. Counterbalancers have the minor disadvantages of adding weight to the engine and sapping a bit of horsepower, but otherwise they're technology superbly well suited to making a shaker into a smoothie.
So you've got your paint-mixing, bar-shimmying engine that can't help it because of the movement of the internal parts, and then you've got the kind of vibration we all like-that stammering, rubber-hammer jab you notice on acceleration. This is a result of the combustion of fuel and air in the cylinder setting all those carefully chosen balance characteristics, well...out of balance. (After all, there's no corresponding bump when the piston reaches Bottom Dead Center.) What's more, some of the vibration you feel-the very heartbeat of the machine-is the result of torsional stresses in the engine. Every time a cylinder lights off, the crankshaft, which seems massively strong sitting on your desk (doesn't everyone have a stroker Harley crank on his desk?), actually twists a bit under the load. Then, almost as quickly, it springs back. Because four-cylinder engines have twice as many combustion events, you'll never feel this thumpa-thumpa from a multi. (And yes, by that we mean more than two.)
Honda has leveraged the power-pulse vibration to the max with the VTX 1800. There are myriad vibration dampers in the driveline to help take the worst shocks out of those massive pistons rising and falling, but the system is cleverly designed to remove the harshest jabs while accentuating the vibration that riders perceive as desirable. More technology for the good of motorcycling-kind.
It's About The Power, Baby
Our love of the V-twin engine extends beyond its distinctive texture because it's a wonderful real-world powerplant. Let's step back once more to theory-don't worry, we'll be quick about it-to remember that an internal-combustion engine is little more than an air pump. Consume more air-and put correspondingly more fuel into it-and you get more power. There are several ways to improve power: As suggested, allow the engine to take in more air (and, just as importantly, expel more air) and you'll produce more work. At the same time, if you increase engine speed, it can do a given amount of work over a shorter period of time, which gives you more horsepower. Finally, you can take that fuel/air mixture and squeeze it harder, making the combustion event-it's not an explosion but a very rapid, controlled burn of the mixture-more powerful. By "squeeze" we mean to increase the compression ratio-which is the difference in cylinder volume between TDC and BDC. The higher the compression ratio, the greater the BANG...up to a point. It turns out that for the ideal cruiser engine, bigger is often better, and once the decision is made to hit a certain displacement target, the remaining choices come prepackaged, like mechanical Lunchables.
Historically, V-twin engines have been undersquare-or long-stroke-engines; their bores measured less than the length of the stroke. (Square is when the bore and stroke are equal; oversquare means the bore is bigger than the stroke is long.) A long-stroke engine is inherently good at making low-end torque, and, as though to give something back, inherently limited in the ways it might exploit anything but low-rpm power. For starters, a long-stroke engine, for any given rpm, makes its pistons move along the bore faster and sustain harsher starts and stops at the extremes of piston stroke. (In fact, the dynamic loads on the bottom end go up with the square of the engine speed.)
For a given overall displacement, a long-stroke engine will have a smaller bore, which makes less room for the valves. The smaller the valves, the less air the engine can pump. But there's a countering effect. A long-stroke engine with smallish valves may turn asthmatic at elevated revs, but it breathes very well and very efficiently at low speeds. Conversely, engines designed for high-rpm power often have large valves, camshaft timing that opens those valves wide and zippo for low-end responsiveness; until the engine is really spinning, those big valves and tall cam lobes are dancing the wrong steps.
In theory, a low-revving engine doesn't need more than the minimum number of valves-one each for the intake and exhaust. Yet the majority of Japanese engines feature a three- or four-valve design. In large part this is because that's what the engineers are familiar with, but it's also true that a good four-valve head can be more efficient than a two-valver even at low revs. It's all in the design.
Harley is once more the archetype. The engines are large-though bikes such as the VTX and the Warrior have raised the bar on max cubes-with small valves, modest compression ratios and an overall design that makes the best use of the low-speed bias. Who needs overhead cams when the max engine speed is less than 6000 rpm? (Apparently not Harley...or Moto Guzzi or Yamaha.) Who needs massive valves when the piston speeds similarly keep a lid on rpm? Honda's taunt that the VTX has an exhaust valve the same size as a P-51 Mustang V12 Merlin fails to point out that the airplane has two such in each cylinder while the VTX has but one.
It would be both uncouth and inaccurate to say Japanese cruiser-makers simply took the Harley concept and modernized it. Once the industry cottoned to the idea that customers wanted V-twins (and, basically, V-twins only), they sat down and did the best they could. True, early versions could be awkward-looking, but engineers eventually figured out that American riders want visual simplicity and a ton of low-end grunt. Through evolution, trial and error, sales successes and failures, manufacturers have found themselves with handsome, genuinely powerful engines that somehow manage to combine charisma and civility, answering the question of whether the V-twin is an ideal cruiser powerplant with a pugnacious, "Just how big would you like it, Jack?"