The terrible truth is that no matter how finely you add or subtract balance weights from your crankshaft using a high-tech balancing machine, a rotating counterweight can never cancel the shaking force of a piston moving back and forth in a straight line. The only way a single can be balanced is by generating an equal and opposite in-line shaking force.
Here’s why. Let’s start with a single-cylinder engine whose rotating parts—the crankpin, its bearing, and the big end of the connecting rod—have all been 100 percent balanced by rotating counterweights attached to the crankshaft at 180 degrees to the crankpin. All that’s left now is the straight-line shaking force generated by the piston, its rings, wristpin, and connecting rod small end. As we add counterweight to the crank, the up-and-down shaking force decreases, but we are also creating a new shaking force that acts forward and back. Adding counterweight to balance 100 percent of the “shaking parts” just moves the shaking from vertical to horizontal.
One way out of this is to fight fire with fire: Build a second engine of the same bore and stroke, having the same reciprocating mass but oriented in the opposite direction, with its crankpin phased so both pistons reach top and bottom dead centers together. Now the two shaking forces cancel each other. This is BMW’s flat twin.
Shucks. The result isn’t perfect because there is no simple way to place both cylinders on the same center line. The difficult way would be to have three crankpins, the center one at 180 degrees to the other two, one on either side of it. The con-rod on the center pin would control one piston, and the outer pair would drive the other piston. This could be made to cancel all shaking forces if the two outer con-rods weighed exactly the same as the single inner one. Also, the crankshaft would be seriously weakened by having three crankpins joined in this way.
So designers compromise. They have two crankpins at 180 degrees, located as close to each other as crankshaft strength will permit. The result, when BMW first did this in 1923, was an engine much smoother than the vigorously shaking singles of most of its contemporaries. Because the two cylinders are not on the same centerline but are separated by the width of one con-rod plus that of whatever crankshaft material joins the two crankpins together at 180 degrees, there is a force generated tending to twist the engine back and forth around a vertical axis. As long as the engine’s two pistons are small and light, this is unimportant. But when BMW began to make flat twins over 1,100cc displacement, piston mass became much greater, and this oscillation made itself felt. That ultimately caused the company to add a balance shaft that would cancel it.
Another way to cancel vibration is to combine two singles side by side as a parallel twin, phasing their two crankpins at 180 degrees to each other. Now when one piston is arriving at TDC, the other is arriving at BDC. Honda and Yamaha in the 1960s and ’70s built hundreds of thousands of parallel twins this way. Just as with the flat twin, this looks like most of the shaking forces of the two pistons should cancel each other. But no, they can’t, because the centerlines of the two cylinders have to be at least as far apart as two halves of one cylinder bore, plus two cylinder thicknesses. If the cylinders are air-cooled, we might also decide there needs to be some cooling fins between the two cylinders, spacing them even farther apart.
So now the two shaking forces are becoming widely separated. What results is called a “rocking couple.” As the right-hand piston decelerates toward BDC, it pushes the right-hand cylinder downward. But at the same time, the left piston is decelerating toward its TDC, yanking the left-hand cylinder upward. The leverage for all this yanking is one-half of the separation between the two cylinder centerlines. The result is a whacking great side-to-side rocking motion from the engine. Because shaking force is proportional to the square of speed, vibration increases rapidly with rpm. Yamaha’s parallel twin streetbikes, making peak power at maybe 7,500 rpm, produced only half as much rocking couple as did its 10,000 rpm roadrace version. That was too much for the racer’s chassis, whose engine mounts were, one by one, broken by the fatigue stress (at an average 8,500 rpm, a 10-minute race applied 85,000 stress cycles, so it didn’t take long).
We think this over and decide to join two of these rockin’ parallel twin engines together, end to end, to make a flat-crank (all crankpins in the same plane) in-line four. Excellent! The two rocking couples cancel, leaving this engine with no primary piston shaking force. The result, with Honda’s CB750 Four of 1969 and Kawasaki’s 903cc Z1 of 1973, was an engine that was very smooth for its time.
But alas, primary shaking force (occurring in step with the crankshaft) is not the only shaking force. Also varying the heights of the pistons is rod angularity, which changes twice per revolution, generating a secondary shaking force (twice per revolution) roughly one-quarter as great as the primary force. Again, as long as the pistons are light in relation to total engine mass, the resulting vibes aren’t too bad. But when Honda built its CBR1100XX Blackbird, secondary forces had outgrown human tolerance, and balance shafts had to be added to shush them. It was an exceptionally smooth engine.
Like a 180-degree parallel twin, a 120-degree in-line triple has a pronounced rocking couple, even though its center of mass sits still. That couple is canceled in modern triples by a balance shaft, but as with parallel twins, there’s another way: Join two triples end to end in mirror-image fashion. Now everything cancels, leaving us with an engine so smooth that BMW won’t leave it alone. If you equate smoothness with excellence, nothing beats an in-line six or a V-12.
Yet even today, there are riders who enjoy some vibration to remind them that there’s dynamic machinery downstairs. The focus groups get to decide just how much: Harley’s new Big Twins are canceling 75 percent of primary shaking force. If by the time of the next redesign the consumers have changed their minds, that percentage can be adjusted either up or down.
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