Where does invention come from? A common idea is that there’s a room somewhere full of wacky Gyro Gearloose figures who build spaceships out of junk. In fact, invention usually occurs during attempts to solve specific problems.
At the end of World War II, tire companies, needing to know more about the structure of polymers, invested millions in X-ray diffraction studies and patented their way to new materials revealed by that work. Steel companies likewise invested in large research departments to map out phase diagrams showing the way to new alloys and heat-treatment processes. Lubricating-oil manufacturers spent millions on developing processes by which they learned to reform the great variety of natural hydrocarbon molecular structures into particular desired species of superior performance.
I just finished reading German Fighter Aircraft in World War I by Mark C. Wilkins. Rather than being the usual “buff book” listing horsepower, top speed, and daring deeds, it relates in detail how the actual manufacture of fuselages, wings, and tail surfaces evolved in Germany through the war years between 1914 and 1918. It’s easy to dismiss aircraft of this time as mere wire-braced motor-kites made of sticks and cloth, but in fact their evolution from that beginning was rapid and continuous. By the end of the war the internally braced wing (free of those drag-inducing external wires) had become a reality, as had semi-monocoque fuselages so light they could easily be carried by one man (“monocoque” meaning that its strength is in its skin).
Such developments are of special interest to me because motorcycle frames have also come far since the 1950s, when Norton’s Featherbed twin-loop steel tube chassis weighed 42 pounds. Modern bike chassis are often half this weight thanks to two innovations:
- The “two Phils”—Phil Vincent and Phil Irving—all but did away with the traditional motorcycle chassis by using the engine instead. The chassis of their postwar Rapide V-twin was just a box beam of welded sheet steel, one end carrying the steering head and the other bolted to the engine’s two cylinder heads. Why? Because in post-WWII Britain steel was still rationed.
- The weight of conventional steel-tube chassis stiff enough to handle the increased grip from slick tires was rising unacceptably. In 1980 Yamaha gave Kenny Roberts’ 500 GP two-stroke a light aluminum Featherbed-style chassis made from square tubing, but it was too flexy to be stable at high speed. Two alternatives appeared: Eric Offenstadt’s welded-aluminum monocoque of 1972, which elaborated into 1980′s aluminum box-beam Kawasaki KR500 chassis, and Antonio Cobas’ large, thin-walled twin-aluminum-beam frames.
In WWI, the fuselages of early kitelike 60-mph airplanes quickly evolved into structures of welded steel tubing covered with fabric, paired with thin wire-braced wings (too thin to contain spars strong enough to support the fuselage). Such wings remind the viewer of the slightly curved slats of Venetian blinds.
Everyone knew that bracing wires and struts added drag, reducing top speed. From what I’ve read it’s unclear whether it was aerodynamics pioneer Ludwig Prandtl, discovering that much thicker airfoil shapes did not increase drag; or if was the likes of Hugo Junkers, who decided to do away with bracing wires and make his experimental wings thick enough to be self-bracing. Either way, someone discovered how to eliminate bracing wires; if not during the war years, then shortly after.
Another big discovery was that using thin plywood to sheath a rectangular ash or spruce framework of longerons and formers created a box structure with far greater stiffness-to-weight ratio than steel tubes, internal bracing wires, and cloth covering. Here’s an example of monocoque construction closer to home: Notice how flexible an open cardboard box is, and how stiff it becomes once the open top is closed and taped. Its material, distant from its center, gives it maximum leverage over loads.
A similar comparison can be made between steel rod and steel tubing. Under compression, a rod buckles long before a tube of the same weight per foot because its material is too close to its centerline to have useful leverage against the buckling forces. And remember, the Featherbed frame did not load its parts in either pure compression or pure tension—there were also bending loads that a tube (again, of equal weight per foot) resists far better than a rod.
At present, motorcycle chassis design is split between the semi-monocoque aluminum twin-beam concept and Vincent-like semi-frameless construction employing welded steel tubes to join engine and steering head.
After discovering the remarkable stiffness of a box fuselage, the aircraft designers saw something we can still see when a heavily loaded box trailer passes us on the highway: diagonal ripples in the trailer’s stressed skin; the thin plywood skins of box aircraft fuselages did the same. How to further stiffen the fuselage? The next step was to eliminate flat surfaces, replacing them with curves that are, like tubing, better braced against buckling. The aircraft builders laminated thin plywood onto a carefully shaped form that could then be withdrawn. Then internal longerons (necessary to widely distribute engine, wing, and tail loads) and formers (to prevent skin buckling) were installed inside the plywood skin with screws and glue.
In the 1990s both Honda and Yamaha would construct their racing chassis using extruded aluminum side beams with lengthwise internal stiffening walls. Around 2000, the coming of greatly improved casting methods made it possible to cast parts for production bike chassis with accurate control over varying wall thicknesses, making the “skin” thicker or thinner as needed.
The German fighter fuselage builders had borrowed their plywood lamination techniques from the way wooden rowing shells were constructed. Later, internal longerons and formers were erected on alignment jigs, and the very thin plywood covering—now molded separately in longitudinal halves by a variety of wrapping or scarfing techniques—was added to it. This work was labor intensive and required much training.
In 1953, new jet aircraft required that sheet-metal aerostructures be replaced by integrally stiffened, machined-from-solid elements. Such large, complex, and three-dimensionally-curved pieces could only be machine-made; there simply were not enough senior machinists in the world to “turn the cranks” to make such parts. That need forced the development of computer numerically controlled (CNC) machine tools. Cycle World has elsewhere shown how that CNC technology, radiating from its aviation origins in the 1950s, has been privately used by Richard Stanboli to build his own CRT (Claiming Rule Teams) MotoGP bike chassis from a pallet load of aluminum billets (see “Stanboli Attacks MotoGP,” April 9, 2012).
When human technology encounters problems, people’s minds invent solutions.