A primer on the design and engineering of the beloved V-twin motorcycle cruiser engine. From the April 2005 issue of Motorcycle Cruiser magazine. By Marc Cook.
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.
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.)
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?
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.
Smoothing the Savage Beast
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.)
| Though bigger is normally supposed to be stronger, the 1130cc Harley-Davidson V-Rod engine makes enough power to beat all other OE V-twins. |
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 Vulcan 2000 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?"