Grace In Motion
Grace in motion
TDC
Kevin Cameron
THE MOST BASIC PISTON ENGINE-THE Single-is an ungraceful beast. Its whole structure leaps up and down jerkily in reaction to the startings and stoppings of its piston. It twists around its crankshaft in response to the rotary thumpings of its widely spaced torque pulses. Any chassis to carry this vibrator had best be rugged and heavy.
The Ducati Supermono Single is a “virtual V-Twin.” Its second, pistonless con-rod generates a balancing force that counters the simple piston shaking. This cancels primary imbalance and leaves only the double-time secondary force, which is about onequarter the amplitude. This is civilized.
Animal and human locomotion provide interesting parallels. As you walk, one leg swings forward and the other back, but because your legs are offsetone on the right, one on the left-a torque is created. This would twist your upper body, but to counter this, you create a varying counter-torque by swinging your arms in opposite phase. It all cancels out.
BMW’s flat-Twin, too, has an offset problem; if the cylinders were directly opposite each other, both primary and secondary piston-shaking forces would cancel because they would occur exactly opposite each other and in perfect antiphase. There is no easy way to do this, so the cylinder axes have to be offset, one slightly ahead of the other, by enough to find clearance for two crankpins 180 degrees apart. A rocking force results, in proportion to the offset, oscillating the engine back and forth around a vertical axis. Because the Boxer Twin has no arms to swing, its rider must tolerate the resulting slight buzz.
Another mechanical arm-swinger is the pendulous counterweighting used in large radial aircraft engines. When a cylinder fires, the crankshaft normally speeds up slightly, then slows again as the propeller takes power from it, then speeds up again with the next firing. This speed variation is torsional vibration, which if transmitted through the reduction gears to the propeller, can break the gears or even the propeller blades themselves. Therefore, the designer makes the crank counterweights as massive pendulums, each capable of storing quite a lot of energy. Instead of the crank speeding up when a cylinder fires, the pendulums absorb and momentarily store the energy pulse, giving it back to the crankshaft between firings as they swing the other way, to prevent the propeller load from slowing it. The result is a torsionally smoother output-and unbroken propellers. You can walk without swinging your arms, but it is a jerky, ungraceful motion.
Builders of four-stroke V-Twins have it easy with regard to offset, because both con-rods are usually on the same crankpin, side-by-side, as in Ducatis. Offset is just the thickness of a rod big-end-too small to need balancing. But if the engine is a two-stroke, each cylinder needs a separate crankcase (except for the Swissauto 500cc VFour, which has two cylinders per crankcase), and this greatly increases the offset. This hefty offset is what compels Yamaha to put a balance shaft in its current TZ250 V-Twins.
Horses deal with leg offset by striding in such a way that their feet barely miss each other. If right and left tracks were highly offset, the animal would have to sway from side to side, as elephants do, to remain stable. The narrower the track, the smaller the sway.
If you walk beside a horse, you will see its head dart forward and back, twice for every stride. To support itself at a walk, a horse cannot pick up two legs exactly at once-that would be unstable. So there is a slight “leg-lag” that creates a forward-and-back cyclic force as the legs start and stop slightly out-of-phase with each other. This is balanced by the head motion.
Each cylinder of a 90-degree V-Twin balances the other, and they share counterweights, but if the Vee angle is reduced, a primary shaking force appears, growing as the Vee angle diminishes. The equivalent of the horse’s head motion must be supplied by some device to balance out this force. In the Harley -Davidson VR1000 Superbike engine, the “horse’s head” is a small crankspeed balance shaft in the crankcase.
As a human strides along, the body rises and falls as each leg swings; the leg holds us highest when at 90 degrees to the ground, but we are less tall as the leg angles forward or back. This is completely analogous to the secondary vibration of engines, created as the conrod angles forward and back, modulating the piston’s height up and down, twice per revolution. But watch a cat stalking, and you will see none of this; the cat’s “connecting rods” are jointed, and to be smoothly unnoticeable, the cat so bends the joints to maintain a slinking constant height.
When a cat moves in this sinuous way, we perceive it as graceful, feline. What we are seeing is good dynamic balance, avoidance of shock-producing motion; energy economy. Grace-if that’s a good term-seems to please us wherever we find it, especially in certain members of our own species.
Ultimately, powerful engines adopt a solution similar to nature’s, which is to react one force against another. A trotting horse moves two legs forward and two backward simultaneously, and in a four-cylinder engine, the primary up-and-down forces all cancel in the same way, leaving some buzzy secondary vibes to remain as a seat-of-thepants tachometer. In a Six, everything smoothly cancels to zero, leaving the engine creamy-smooth. In a V-12, because the power impulses are spaced only half as far apart, even the power delivery is smooth. Listen to the starter turn a V-12; because the compression events blend so smoothly one into the next, the starter turns the engine at constant speed, in startling contrast to the weo-weo sound of a V-Eight’s starter motor. All that primary and secondary yanking and buzzing adds up to zerothe smoothness for which V-12s are legendary. To preserve the simplicity for which motorcycles are likewise noted, their engines must have fewer cylinders and more vibrations.
