Well, Duh-They're Like Harleys
It's impossible to detach the Harley experience from the V-twin experience, isn't it? Virtually every mention of what it's like to ride Big American Iron includes some reference to the familiar potato-potato sound, to the thumping of two big pistons as the heartbeat of the beast. This strain of character comes from many things, not just the fact that the motorcycle is motivated by a loping, low-revving V-twin; it's the heft of the controls, the sturdy clank of the transmission moving from gear to gear. It comes from the Harley's ability to glide away from a stop with little throttle and the feeling of bottomless torque. Such sensations can be had from other engine configurations-a modern, 955cc Triumph Speed Triple has that utterly at-ease feeling, and no one would dare say a bike like Suzuki's Hayabusa is a slug off the line.
But they're different, very different.
Fury, But Sound, Too
When Harley took steps to trademark the sound of its engines, it embraced a way to protect against others using single-crankpin, 45-degree V-twins. And yet there's much more to an engine's exhaust note, or signature, than the spread between the cylinders, and, indeed, all current cruiser engines evoke the same emotional response from their loping cadences. (In yet another example of defining the breed, Harley's potato-potato note has come to symbolize freedom only because the idea is associated with the brand that makes that particular noise.)
V-twins make lots of distinctive sounds, and perhaps at the core of our appreciation for them is the idea that we can actually hear the combustion process in slow motion. Bang, bang...pause...bang, bang. It's a more natural-seeming rhythm than the regular, symmetrical buzz of a four-cylinder, and closer to a heartbeat than the chainsaw razz of an inline-four. Harleys sound the way they do because of the narrow V angle, which places the firing of the cylinders close together with a long pause. Again, think of: boom, boom...pause... boom, boom...pause.
As you'll see later, not all cruiser engines are built the way Harleys are, so their exhaust notes are different, too. But design orthodoxy has largely taken over, so current cruisers sound more similar than unalike, with a pleasing rumble that speaks of raw torque and untrammeled roads.
Good (And Bad) Vibrations
Unquestionably, what most riders perceive as the soul of the V-twin emanates from shudders and quakes from the engine room. Why do V-twins sound and feel as they do?
It's simple, actually. Modern V-twin engines are big mommas. If Honda's VTX 1300 is considered entry level, and Kawasaki made big hay about its Vulcan line getting a 1600cc iteration, you know you're talking about some seriously big bits and pieces. Inside your cruiser's engine are not the jewel-like pistons, anorexic connecting rods and nonexistent flywheels you'd find on a 600cc sportbike; no sir, these are big pistons moving a neighborhood away along a lengthy stroke. (Are we clear on this? Bore is the width or diameter of the piston inside each cylinder. Stroke is how far up and down that cylinder the piston travels.)
In fact, engine vibration comes from two principal sources: the inherent imbalances in the reciprocating parts (meaning they go back and forth or up and down inside the engine) and pulses descending from the combustion event itself-the ignition of fuel and air in the cylinder head. In other words, there's a kind of shake or vibration that is part of that engine's very design, present regardless of the power produced, and yet another that's the result of laying down the torque-much of this is the throb you feel as the bike launches away from the stoplight.
Inherent vibration is often associated with the V-twin mystique, but it's an unfortunate side effect of the most popular designs. Harley placed its two cylinders 45 degrees apart on a common crankcase mainly so the engine would fit into a bicycle-style frame originally intended for a single-cylinder powerplant. Unfortunately, a 45-degree Vee angle has a prominent vibration "signature."
Consider what's happening inside. Imagine you have a single-cylinder engine. The piston starts at the top of the cylinder (a place called Top Dead Center, or TDC) and travels downward; it moves slowly at first and travels the fastest midstroke. It has to stop suddenly and reverse direction at the bottom of the stroke, called Bottom Dead Center (BDC). This very motion creates a force; the piston wants to keep going, so the crankshaft it's connected to (and the crankcase that the shaft is rotating in) must absorb this acceleration. Typically, engine-makers add weight to the crankshaft. Called counterweights, they attempt to cancel out some of these imbalances. But this only goes so far. Vibration caused by the piston moving up and down is known as a primary force or imbalance because it occurs at the same frequency as crank rotation. There are other imbalances created in the engine we'll touch on later, but the primary imbalance is the one you notice most.
Adding a second cylinder to a single and creating a 45-degree angle between the two builds an engine with, it must be said, undesirable vibration characteristics. Although the primary imbalance is not as great as in a single, it's still very strong-this is why traditional Harley engines bolted solidly to their frames shake like a wet dog. One thing the H-D configuration has going for it is that vibration is isolated to the vertical plane because the parts of the connecting rods attached to the crank straddle one another and are not placed side-by-side. This is why Harley cylinders are directly in line, and other engines' cylinders are offset side to side. The Harley's big primary imbalance makes it relatively easy to counter by rubber mounting-you can locate the engine firmly left to right and allow it to shake its little heart out up and down-or in the case of the new Twin Cam 88B engines, by use of an internal counterbalancer (more on that later, too). The Harley engine also has a bit of what's known as secondary vibration-imbalances that occur at twice the engine speed-acting in the same plane as the primary imbalance. Think of the primary as the throb and the secondary as the sizzle.