What we hear is variations in firing interval, not the number of cylinders. A British twin and a Harley Sportster both have two cylinders but make very different and distinctive sounds. At idle, they shake their front wheels forward-and-back identically. When we hear the Sportster, our ears detect its two different firing intervals, of 405 and 315 degrees, which add up to the 720 degrees of the complete 4-stroke cycle. When a British twin passes by, its flatter, less textured sound has only the one firing interval–360-degrees–but two of those intervals again add up to the 720 of the four-stroke cycle.
That front wheel shaking? If in imagination we could swing a Sportster’s two cylinders together so both pistons came to top dead center together, we’d have a parallel twin and it would sound like a 50-year-old Triumph or Norton because the Sportster’s uneven 405/315 firing order would have become an even 360/360. But with only 45 degrees of V-angle between its cylinders, in vibration terms a Sportster is not so different from a parallel twin. Both of its pistons come to TDC fairly close together, so to moderate the shaking of those two pistons moving up and down, counterweights are placed on the crank at 180-degrees to its single crankpin.
Car engines balance roughly 50 percent of the piston shaking force. Why not 100 percent? Picture a single-cylinder engine. If we balance 100 percent of piston shaking force, then at TDC and BDC the counterweights and the piston will cancel each other’s shaking force. But what happens when the crankpin moves to 90 and 270 degrees? At 90 degrees, those counterweights are trying to drag the engine forward just as much as an unbalanced piston at TDC and BDC is trying to yank it up and down. In that case, all we’ve done is to change the direction of the shaking force from up and down to back and forth. We have not reduced it at all. So, this is why car engines are balanced at 50 percent of piston shaking force–because this reduces the up-and-down shaking by 50 percent, but creates a new forwards-and-back shaking force of equal amount. Peak load on main bearings has been reduced by 50 percent, which is good.
But a motorcycle is not a car. Car engines have rubber mounts and cars weigh six-to-eight times more than their engines do. Motorcycles weigh only two-to-three times what their engines do, allowing their engines to shake them more. Long ago, motorcycle builders discovered that riders more strongly feel up-and-down vibration than they do fore-and-aft vibration. So for this reason, both British twins (pistons move exactly together) and Harley V-twins (pistons move fairly closely together) are overbalanced, that is, their piston shaking forces are balanced at more like 65 percent than the usual car balance factor of 50 percent.
And that, ladies and gentlemen, brings us back to the characteristic front wheel shaking that British twins and Harleys share; it is their overbalanced heavy crank counterweights that create the forwards-and-back shaking that sets front wheels in motion in that charming and memorable way.
But Ducatis are V-twins, but their front wheels sit still at idle. And Ducatis–well, they may be V-twins, but they don’t sound quite like Harleys. The first thing is that Ducati twins have twice as much V-angle, or 90 degrees. This uniquely makes it possible to balance a 90-degree twin with counterweights of 100 percent of one piston’s shaking force. Here’s how it works. At the front cylinder’s TDC and BDC, the heavy crank counterweights cancel the front piston’s shaking force. But at 90 degrees and 270 degrees, those same heavy counterweights now cancel the TDC and BDC shaking force of the rear piston. In other words, both primary piston shaking forces are 100 percent balanced, leaving the engine smooth. So, there is no front wheel shaking at idle. And because the engine is no longer trying to yank and jiggle its way out of the chassis, every part can be made that much lighter.
Every choice you make has advantages and disadvantages. A parallel twin or a narrow-angle V-twin such as a Harley can be packaged close to the front wheel, keeping the tire on the ground and steering at high accelerations. Ducati’s original 90-degree twins had the front cylinder near-horizontal, pushing the heavy crankcase far back in the chassis and requiring a slower-steering, long, 60-inch wheelbase. To avoid this problem, Aprilia closed its RS1000’s cylinder V-angle to 60 degrees. This made the engine more compact, but now it needed two balancer shafts to achieve smooth operation. That small a V-angle left too little room for the intake system, so when Buell sourced their big V-twin, they gave it a 72-degree V-angle. More recently, Ducati have been rotating their 90-degree engine back more and more, so they no longer need the American-LaFrance-like 60-inch wheelbase of those 1970s bikes. There is no right answer–only better or worse compromises.
When Harley saw that some customers very much wanted to ride Harleys but wouldn’t accept its undiluted 1907-era shaking forces, the Motor Company built two balancer shafts into the bottom of the Big Twin’s crankcase. That’s an option; if you want the traditional (and, some will say, manly) vibration, you buy a different model. When I rode my 1965 parallel-twin, 180-degree-firing Yamaha TD1-B 250 racer, its bolted-solid engine at 10,000-rpm put my hands right to sleep while it was busily breaking its engine mounts. Honda’s first four-stroke parallel twins in the late 1950s and early ‘60s had 360-degree cranks like British twins, so that even firing order gave the usual flat sound. But on their sports engines the crankpins were set at 180-degrees, giving them a more interesting syncopated exhaust sound.
Another point of interest about the Harley V-twins is that they alone employ fork-and-blade connecting-rods, allowing both pistons to move in the same plane. Other famous engines employing such fork-and-blade rods are the Allison and the Rolls-Royce ‘Merlin’ aircraft V-12s of WW II. By contrast, the hallowed Vincent V-twins place their rods side-by-side on the crankpin as on any V-8 or V-6 auto engine, resulting in having to offset the cylinders by 1 1/4 inches.