Unmuffled motorcycle engines make a racket, and no less a person than Valentino Rossi likes to have mufflers on his race bike because he finds the noise distracting. Still, if you’re not so close that it’s giving you a headache, there are both music and information in the sound. Harley and Ducati fanciers are in love with the syncopated sounds of those engines, while for others the flat drone of a 360-degree-firing British twin spells excitement. 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. Iconic American car racer Dan Gurney recently announced his version of a big twin, with a whopping five-inch bore! Since he spent his life in racing, he was unwilling to build an engine that couldn’t rev without tearing itself out of its mounts. Therefore his Merry Men designed it with two contra-rotating crankshafts, geared together, one ahead of the other. Each of the two cranks carries counterweights that balance 100 percent of one piston’s weight. What happens at 90 and 270 degrees? The front crank’s counterweights are yanking the engine forward and the rear crank’s counterweights are yanking it backward, so they cancel, leaving the engine smooth. Like Ducati, it will be able to rev to the skies above without destructive vibration. Revs are another aspect of the sound differences between Harley and Ducati. Production Harleys pull from 1200 to 5000 revs, slowly enough to clearly hear the syncopation of their irregular firing order. The Ducati has an even more irregular firing order, but we hear it less clearly because Ducatis are often turning higher revs. It’s like the difference between a single gunshot, which makes a loud bang that reverberates, and the buzz of an MG42 machine gun at 1200 rounds per minute. Now we come to flat twins as used by BMW, Ural, and once upon a time, Douglas. The cylinders point in opposite directions with 180 degrees between them, and the 180-degree crankpins cause the two pistons to always move opposite to each other and come to TDC simultaneously. Thus, their primary shaking forces are self-canceling. But if you look down on a BMW twin from above you will see that its two cylinders are not in the same plane. There has to be enough cylinder offset to allow the two connecting-rods to pass each other and for their crankpins to be somehow joined together. This cylinder offset causes flat twins to twist back and forth slightly around a vertical axis. When BMW enlarged its biggest flat twins’ pistons to bucket size in the name of sport, they canceled the growing buzz with a balance shaft. Because flat twins fire every 360 degrees they sound like 360-degree parallel twins. 
Back in the teen years of the 20th century a man named Charles Lawrance built a flat twin with just one crankpin, so its pistons always moved in the same direction. For this reason, their shaking forces added instead of canceling each other, creating a super-vibrator. Why did he do it? Maybe he was seeking simplicity. In recent times yet another kind of twin is being built–one with one or more crank-driven oscillating weights under the crankshaft, moving opposite to the pistons to cancel both primary and secondary shaking forces. The BMW F800 and the British Maxsym engines employ such systems. Some makers of parallel twins, wanting the more exotic sound of a V-twin, have phased their two crankpins at 270 degrees instead of the usual 360. This gives the same firing interval–and the very same sound–as a Ducati V-twin. Another way to build V-twins is with Honda’s “offset dual-pin crankshaft.” Normally, the only way to balance a single crankpin V-twin is by giving it a 90-degree V-angle, as described above. But Honda found that by using two crankpins instead of one, and by offsetting them by the correct angle, a self-balancing V-twin could be built with any cylinder V-angle–without need of any balance shaft. To get this effect in a given engine, we subtract twice the V-angle from 180-degrees to find the correct crankpin offset angle. Thus, in a 90-degree V-twin (Ducati, Guzzi) this is 180 – (2 X 90) = 0 (no crankpin offset, which is what Ducatis and Guzzis have). If we want to balance a 45-deg V-twin, the numbers become 180 – (2 X 45) = 90-degree crankpin offset. Carrying this out requires a rather wide crankcase, as there has to be some way of connecting two crankpins spaced 90 degrees apart at its center. This takes the form of a 3rd flywheel disc. Normally, the strength and durability of a forged crank is reckoned in terms of the “overlap” between crankpins and mainshafts–something you can easily see in a V8 car crank. But in the case of two crankpins joining a central flywheel disc they are too far apart to overlap at all. The result is a degree of weakness. Production machines with this type of crank give satisfactory reliability in their designed rpm range, but when they are subjected to greater stress (at sustained higher revs, for instance), cracking can result. For example, Honda’s RS750 dirt-track engine has dual offset crankpins, and was reliable on the dirt-track duty cycle (only a few seconds of high revs twice each lap). But when re-purposed by the Commonwealth Team as a twins road race engine, the crank had to be changed every 150 miles to avoid distressing incident. But let’s not feel bad; Chris Carr and others have revealed that Harley factory dirt track XR750s get a new crankshaft for every national. In their case, it is the accelerated accumulation of fatigue damage in a roller big-end bearing originally designed for 7500-rpm, today being run to 10,000 or a bit above. Because load increases as the square of rpm, that is an 85 percent increase in load, making the shortened bearing life understandable. Or, think back to the SKF big-ends on Vincent V-twins. Because these are uncaged needle rollers, their normal design rpm ceiling was 5300. Sorry, folks, this story swelled up past a joke, and there’s still lots to say about twins. Another day, maybe. 
Gadget Reviews: mamaktalk.com
Car Reviews: automoview.com
Entertainment News: 38now.com
Today's Promotions: freepromotoday.com
