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Posted by Nordin W. Ibrahim

HOW HAVE TWINS returned from oblivion to center stage? Their era seemed to end with the British industry in the early 1970s, giving way to endless elaboration of the four-cylinder engine. Now, everyone has a Twin. Twins are chic, almost reaching the status of art. Twins win races. People want them. Why?

Two central causes: The first is that the four-cylinder performance envelope has expanded beyond the skill (and therefore the interest) of the average rider. No one wants to be humiliated by his motorcycle. One by one, older riders have abandoned their interest in this year's hot 600, 750 or unlimited Four, to seek other pleasures. Recently, that has often meant a Twin. Add to that the power of youth remembered-nostalgia for the BSA or Norton of young adulthood. Add also the proven taste of today's generation for the music, clothing and pursuits of their parents' youth. Twins fit.

The second is that Twins have burst through a social barrier that no increase of four-cylinder performance could pierce. Ducati and Harley-Davidson have created irresistible-legends into which even previously non-motorcyclists are pleased to step. By becoming accessories to the lives of corporate CE0s and the fashionable well-to-do, these Twins have done the impossible: found new buyers. Harley's legend centers on the lone rider, finding a new life of freedom on the open road. Ducati, its success driven ultimately by racing, wears with assurance the exotic Italian style and color of the kind always welcome at the Ritz. This bypasses the commodity approach of the Japanese companies, which seemingly work to blur all distinction between brands and often ignore the value of legend.

The Japanese, seeing Ducati's fantastic rise from unreliable curiosity to most- admired status, finally offered their own Twins - Suzuki's TL's, Honda's VTR and Yamaha's quarter-hearted 850s. They were dismayed to find that commodity Twins, even if red and genuinely potent, largely missed the point.

Never mind - that's marketing. How did Ducati put the sport-Twin where it is today? It was a glorious accident. In 1985, the prototype 851 was just another loud curiosity from the people who had brought us hard-starting, chrome-peeling sporty Singles and elite, heavy-steering, air-cooled twins. But the FIM was looking appraisingly at American Superbike racing, where Eraldo Feracci and his rider Dale Quarterley proved the track value of the new 851 design. When the FIM asked Superbike impresario Steve McLaughlin's advice on a combined Twins/Fours formula based on ratio of engine strokes. This has worked. Four cylinders 750s make peak power at about 14,800 rpm, the Ducati at about 10,800 - almost exactly the ratio of 1000/750. The rule-makers added a weight break for Twins, and World Superbike racing became a virtually unending success story for Ducati.'

Whatever you believe about the fairness of this, it allowed Ducati to earn a reputation that has put it where it is today. Happily for the Bologna Company, people were ready for an alternative to Fours, ready for a new paradigm in motorcycling, ready for motorcycles to be chic rather than adolescent. Arty, craftsman-like Twins clicked with the upwardly mobile. A man of 40 or 50 feels out of place on his son's 100-bhp 600cc Four, but can be socially comfortable on the (at least equally fast) Ducati 996.

Aside from today's association with art, style and status, Twins have special qualities simply as motorcycles. First comes the rational shape of a V-Twin - it fits a motorcycle chassis like no other engine, because both are basically two-dimensional. The result is a narrow, compact package. By contrast, the marriage of the inline-Four and the motorcycle puts two principles at right angles to each other.

 Next comes sound. A Twin has a deep, animal-musical voice that, to many people, is the essential motorcycle sound. The high shriek of a Four means power only to soulless technicians like myself. It's programmed into us by nature to perceive a roar as aggression, a scream as fear.

The variety in Twin sounds reveals their many cylinder arrangements. We can think of them all as V- engines with cylinder angles' ranging from 0 to 180 degrees. The classic British, Twins had a 0-degree V- angle - their cylinders were parallel. This gives an evenly spaced 360-degree firing interval and the resulting sound is a throaty drone. As the V-angle is increased, firing order becomes more irregular, introducing the interesting syncopation that so many enjoy. For example, 90-degree engines fire at 270 and 450 degree. Early Twins were built because it was an easy way to make a Twin out of a single, but as revs rose, vibration became a concern. When you give a V-Twin a 60-degree cylinder angle a secondary piston shaking forces become self-canceling as in Harley's VR Superbike engine, the new Aprillia RSV Mille or the late John Britten's V-1000. Increase V-angle to 90 degrees, and primary piston shaking forces self-cancel - so Ducati Motto Gus, Suzuki and Honda chose this angle for their sport twins. The familiar BMW Boxer Twins are "flat" with a 180-degree cylinder angle, and both cylinders come to TDC together thanks to two separate crankpin 180 degrees.

A 90-degree V is attractive for balance, but the wide V takes up a lot of fore-aft room on a motorcycle. On some V-twin cruiser models Honda has adopted the staggered crankpins pioneered by Moto Guzzi on its prewar 120-degree, 500cc roadracers. While staggered pins combine natural primary balance with less than 90-degree V-angles on street machines, modern racing rpm has a history of breaking such stagger-pin cranks.

A parallel Twin is for balance considerations, the same as a Single because it is two pistons move as one. No simple scheme can balance this because straight-line up-and-down motion cannot be balanced by a crank counterweight whirling around in a circle. An unbalanced Single or parallel-Twin jumps up and down along its cylinder axis because of the inertia force generated by the piston's starting and stopping. Let us call this force 100 percent.

lf we add counterweight to the crank, we can reduce this up-and-down force-but only at the cost of creating a new forward and backwards imbalance force at right angles to cylinder and crank axes. If we balance out 100 percent of the up-and-down, we get a forward and back shaking equal in force to the old 100 percent up-and down force. The obvious compromise is to balance only 50 percent of the shaking force. Auto engines are usually balanced this way, because it cuts main-bearing peak loads in half.

When parallel-Twins like those of Triumph, BSA and, Norton were balanced at 50 percent, they shook horribly. Up and down shaking excited the frame rails into sympathetic vibration and in any case, humans feel up-and-down vibration as more unpleasant than forward and back. Therefore, practical British engineers just increased the balance factor until it felt tolerable, ending up as high as 75-85 percent, It is the resulting large forward-and-backward vibration that drives the front wheel of these classic machines into their characteristic, whipping motion at idle-something that is, in my mind, inseparable from the essence of Triumph Motorcycles.

Vibration becomes a signature of an experience. Vertical-Twins shake, but 90-degree V-engines do not. BMW flat-Twins are

smooth, with both primary and secondary shaking forces being self-canceling. Yet because their two cylinders can't be located in the same plane, there remains a small rocking force, tending to twist the engine back and forth around a vertical axis. This is "BMW Buzz," which never goes away. This is no more an annoyance to BMW owners than front wheel whip is an annoyance to Brit-Twin riders. These are comforting signals that tell them they are at home on their favorite machine. Of course, too much vibration over long miles is annoying, as any owner of the Evo Softail can tell you. Which is why Harley's new "Beta" Twin Cam motor is so appealing. The idea of balance shafts opens many possibilities, for combining two counter-rotating imbalances on-separate shafts produces a simple back-and-forth shaking force that can be used to cancel piston shaking forces. Milwaukee did not invent the concept, of course. Aprilia's 60-degree RSV Mille engine has such a pair of primary balancers, While Honda CBR 1100XX engine carries a pair of secondary balance shaft to remove the -Four's annoying, high frequency secondary imbalance. Some people sneer at balance shafts for their extra complexity, but the chassis weight they save results in a net weight saving overall and they eliminate a major source of rider fatigue.

Because big engines can maintain high road speed low rpm, a large-caliber Twin seems to lope along with an attractive lack of effort at highway speeds. Vincent builder Big Sid Biberman likens the cadenced beat of such engines to the comfortable assurance that a cantering horse gives you hear and feel the pulsating action and know there is plenty more where that is coming from. This is, he asserts more animal than machine.

Turning the throttle on a Twin yields another appreciation. On a four-cylinder bike, with its small flywheel mass, turning the twistgrip is like firing a thruster on a space vehicle you feel the push, but it appears to come from nowhere. When you roll off, it disappears. On a twin, the flywheel mass is an energy memory, remembering the motion when you roll off, modulating the spin-up when you roll on. Flywheel mass must be large enough that the energy in the spinning crank at idle is enough to push one piston through compression to remain running. The bigger the displacement of one cylinder, the greater the flywheel mass must be, and vice versa.

Riders like the wide torque of a Twin, and Superbike racing has shown that Twins usually have advantages in acceleration off turns. While -Japanese Fours have large valve and carb sizes, chosen to hit impressive horsepower numbers, Twins deliver their peak torque at lower rpm by virtue of proportionately smaller valves and ports. This is not so much a matter of design as of geometry; as you scale, up a given cylinder, its valve area becomes smaller in proportion to its displacement. Ducatis competing in World Superbike often lack the top speed of the Fours, but more than make up for it in bottom and mid-range acceleration. Ordinary street riders like this characteristic just as, much as racers need it - it feels lively and responsive with less need to row the gearbox to get the desired performance.

In one sense, Twins are just the novelty of the moment. In another, they are an enduring and natural way to build motorcycles - just ask Harley-Davidson. Balance or imbalance, smooth or unsmooth, Twins have character, those who own the come to associate these qualities with the riding experience.