The Hidden Beauty Of Motorcycle Parts
Parts are more than just...parts
This article was originally published in the June-July 1998 issue of Cycle World’s Big Twin magazine.
It’s usual to think of engineers—people who design the basic parts of our material civilization—as nerds. Nerds are people with pocket protectors, people more interested in metal alloys than in Superbowl point spreads.
It’s odd, then, to think of nerds as a source of beauty, but they are. Machine parts are some of the most beautiful things created by our species.
How can this be?
A designer draws a part—a connecting rod, for example. When that part is manufactured and goes into service, it begins to establish a history. A few parts, used harder than most, may crack or prove unsatisfactory in some other way. As time passes and machines are used harder (as, historically, they always have been), more parts crack. Finally, the failure rate becomes unacceptable and the part has to be redesigned.
In this way, through a process of design, use, failure and redesign, the part comes closer to a shape that nature finds acceptable—a shape that actually fits the real flow of stress in the part, rather than cutting across it.
In nature, this process occurs naturally through evolution. For example, trees whose shape creates stress concentrations are broken by high wind. But the trees that remain have what we call graceful—even beautiful—shape, as the wide root system sweeps together to form the trunk, then tapers upward to the branches.
In the same fashion, a connecting rod evolves from less suitable to more suitable shape. The harder the machine is used (such as in racing), the faster this process operates. The final product has a graceful shape because that is what works. For some reason, the parts that please nature (that is, they don’t break immediately in a 96-inch fuelie engine) also please our eyes. Beauty is something more than wine-taster talk; it is a description of something that works.
In the very early days, engineers had only their experience and intuition to guide them. Later, direct methods of stress analysis such as brittle lacquer or photo-elasticity gave more guidance. Brittle lacquer was painted onto parts, which were then stressed. Where stress was concentrated, the lacquer cracked or flaked off. Today, we have mathematical methods to predict stress in even complex parts performing complicated motions. This is called “dynamic FEA” (Finite Element Analysis). Like anything man-made, it is not infallible, but it helps. The ultimate arbiter of this kind of beauty is actual engine running tests.
Beautiful Is As Beautiful Does
Beauty is everywhere, but revealing it can be hard work. Some engineers distrust a beautiful solution, assuming it must come more from the styling department than from first principles. Such an engineer may choose an ugly solution, assuming it must be free of styling-department contamination and therefore correct. The truth is, parts that carry heavy stress, and that have been through long development to enable them to endure that stress, are beautiful in both senses: They please the mathematics and they please the eye.
The same applies to flowing air. We know from the flowbench and from mathematics that air “likes” some port and cylinder-head shapes, and “dislikes” others. In general, the smoother and more organic the shape, the better the flow, while the more the ports resemble plumbing—with sharp edges, right-angle bends and sudden changes of section—the less flow we can get through them. The good shapes are, in the best possible sense, beautiful. We like the way they look, and they work.
Some people in the porting business rely only on their sense of beauty, and don’t use the flowbench at all. Keith Duckworth, the English designer of the original DFV F1 racing engine, liked to say that with his finger, he could tell more about an intake or exhaust port than most engineers could with all their fancy test apparatus. Duckworth’s sense of beauty was highly developed from experience, but we can all respond to a graceful shape.
People who spend their lives on the flowbench (and there are many) discover a beauty they can’t even talk about, save to another specialist. It has no language other than the numbers on the manometer scale, and the hours and days spent in achieving those numbers. This is far from being the only inexpressible beauty; we all know of others for which we have no words.
Some engine parts strike us as beautiful because they have perfect geometric shapes. A handful of con-rod rollers attracts the eye because they are perfect cylinders, gleaming like jewels. I like to roll them between my fingers, and I sometimes carry one in my pocket. The roller surfaces are extremely smooth; their average deviation from perfect is four one-millionths of an inch. To keep stress from concentrating at their ends, they are made ever-so-slightly barrel-shaped. Under operating stress, they subtly flatten like tires, to make an elliptical footprint.
Just under that shiny, ground surface, stress can rise close to 100,000 psi, repeated twice per revolution. If there are tiny defects in the surface or in the steel itself, this stress cycle will find and propagate them as cracks. Therefore, the material is made as perfect as humans can contrive, by melting it under vacuum, so that impurity substances oil away. This is “clean steel.”
The cage that keeps the rollers rolling accurately parallel to the crankpin also has to be a refined piece of work; any systematic out-of-parallel and the rollers will screw to one side, carrying the rod with them, causing friction burns against the whirling flywheels.
All of these super-precision parts are satisfying jewelry, much more accurately made, containing more accumulated human craftsmanship than anything actually worn for decoration. It’s true that the parts are made on automatic machines, but those machines are a distillation of all the skills of our fathers and grandfathers, machinists who made the machines that today make other machines.
Gears are severely beautiful. Each tooth has precisely the same shape, given to it, in the case of timing gears, by a final pass with a crush-formed grinding wheel. The tooth shapes must above all guarantee what is called “conjugate action”: For every degree that the drive gear turns, the driven gear must also turn exactly one degree—no more, no less. Any deviation from this imposes shock or vibratory loads on gears and shafts, leading certainly to noise, possibly to failure.
Today, such tooth curves are derived by math analysis, but practical engineers knew the right tooth forms for hundreds of years before such analysis existed. The wooden gear teeth in water- and windmills naturally wore to such correct profiles because any points of non-conjugate form created local high pressure, and so wore away more rapidly. This led millwrights to study and reproduce these shapes in the new gears they made. This is another example of how nature modifies our engineering work.
Like connecting-rod rollers, gear teeth live with intense, repeating surface stress, and this requires hardness. The core of each tooth must carry the bending stress without the brittleness of extreme hardness. The teeth therefore are surface-hardened, while their cores are heat-treated for toughness.
The Concert Hall
Holding a cylinder head in your hands is like walking into an empty theater or concert hall: There’s almost no clue to the action and drama that take place there. It’s just a silent space.
Ever think about combustion as you ride? When I think about combustion, I think of those speeded-up movies of cloud motion. You see the air writhe, twist, shred and tumble, and this is just what goes on inside your cylinder head during combustion. The piston’s suction stroke has drawn in fuel-air mixture at high speed—hundreds of feet per second. In the cylinder, this intake jet continues as swirling motion. Then the piston rises on compression. As it nears the head, the squish areas—regions in which the piston comes very close to the head at TDC—squirt charge into the open region under the valves. As the swirling mixture is crowded into this smaller space, it encounters the edges of valve pockets and other shapes. The regular swirling motion dissolves into random turbulence like that of the speeding clouds in the movie. This is truly a tempest in a teapot.
Then the spark comes, igniting a piece of this action. In an instant, the flame is whirled away, shredded, wrinkled. A gasoline-air mixture burns at the rate of a few inches per second in still air, but this turbulence makes the flame front so huge, and does it so quickly, that it doesn’t matter: The flame eats up the whole charge in two or three thousandths of a second.
I tried to get the people at Sandia National Labs, whose specialization is the study of turbulent flames, to give us a picture of this, modeled on a computer, but they didn’t call me back! So, I have to mix together in my mind the speeding, whirling clouds and the most thunderous of red-orange sunsets (the real flames are blue-white, but I’m doing a little imagineering here). The turbulent flame transforms the chemical energy of the fuel into heat, and the heat becomes pressure. This pressure—the beating impacts of uncounted trillions of heat-excited molecules against your pistons—is what sends you down the road.
A roller tappet follows the rotating cam lobe, translating the cam’s rotary motion into a carefully designed schedule of straight-line motion. There is tremendous pressure between the roller and cam lobe—too much for unhardened steel to tolerate. So, these are hard parts; the steel in them has been semi-finished, then heat-treated until they are too hard to machine. Then they have been finished by grinding.
Most automobiles use flat tappets—plain cylinders that ride against the cam lobes without rollers. Rollers, which are used in Harley engines, reduce friction, and the hydraulic clearance-adjuster in a tappet compensates for the expansion of the tall tower of aluminum that is the cylinder and head. In World War II, hundreds of thousands of roller tappets, looking just like this one, operated valves in American radial aircraft engines.
The lifting of a valve has to be performed gradually. When you push-start a car, the push-car noses up to its bumper, makes gentle contact, and then pushes. In this way, nothing gets dented. The same has to happen in the valve train. The acceleration builds up gradually, applying load to the pushrod, rocker arm and valve like a firm shove, not like a hammer blow. This allows the valve-train parts to last a long time, and to follow the cam profile accurately. Whole books have been written on this one point—exactly how best to schedule the acceleration applied to a valve train.
When the valve closes, it has to decelerate as it nears the seat, just as a jetliner flares to minimum descent rate in the seconds before touchdown. The parts are strong, but they can’t stand impact. The forces are large, but the motions are mechanical grace.
Crudely put, an engine valve is like a manhole cover on a stick: The stick fits closely in the valve guide, and the rocker arm bears on the tip of the stick—the valve stem. This pushes the valve off its seat, exposing area through which intake charge or exhaust gas can flow.
There is another group of specialists who devote themselves to finding the best possible shape for the valve, its seat, and the region of port and combustion chamber near that seat. The internal-combustion engine would be nowhere were it not for these people, who are able to interest themselves in the smallest details of the reality they choose.
Exhaust valves constantly cycle: hot-cold-hot-cold. When the valve opens, hot exhaust gas flowing at the speed of sound violently heats its underside and exposed stem. The surface metal temperature rises sharply. Under prolonged full-throttle running, exhaust valves operate at red heat or hotter. When the valve closes again, this heat begins to flow out of the valve and into the much cooler valve-seat ring, so the valve cools.
Anyone who has hung around engine shops much has seen truck or car valves that are cupped, or with cracks in them, or with pie-shaped sections eroded out. This heating and cooling is tough duty, and it calls for special materials. Many exhaust valves are made of so-called austenitic stainless, but there is a series of even tougher materials, related to jet-engine turbine alloys. These contain nickel and cobalt, and they resist erosion superbly.
Yet when you hold such a valve in your hand, it looks like any other—innocent, shiny metal, saying nothing. Its secrets, its years of hard-won technology, are all inside it, invisible. This beauty is concealed in metallurgists’ phase diagrams—and in the durability of red-hot valves.
It is typical of the beauty of mechanical evolution. Of the beauty of effective solutions to engineering challenges. Of the beauty of something that works.