Aluminum is everywhere that either lightweight structure or high thermal and electrical conductivity are required. The typical sportbike has an aluminum cylinder block, head, and crankcases, plus a welded aluminum chassis and swingarm. Within the engine, the crucial aluminum application is its pistons, which by conducting heat so well are able to survive exposure to combustion temperatures far above their melting point. The wheels, coolant and oil radiators, hand levers and their brackets, top and (often) bottom fork crowns, upper fork tubes (in USD forks), brake calipers, and master cylinders are likewise aluminum.

We have all stared in admiration at an aluminum chassis whose welds resemble the fabled fallen stack of poker chips. Some of these chassis and swingarms, such as those of Aprilia’s two-stroke 250 racers, are graceful works of art.

Aluminum can be alloyed and heat-treated to strengths greater than that of mild steel (60,000 psi tensile), yet most alloys machine rapidly and easily. Aluminum can also be cast, forged, or extruded (which is how some chassis side beams are made). Aluminum’s high heat conductivity makes its welding require a lot of amperage, and the hot metal must be protected from atmospheric oxygen by inert-gas shielding (TIG or heli-arc).

Although aluminum requires large amounts of electricity to win from its bauxite ore, once it exists in metallic form, it costs little to recycle and is not lost to rusting, as steel can be.

Early makers of motorcycle engines quickly adopted the then-new metal for crankcases, which would otherwise have had to be of cast iron weighing nearly three times more. Pure ­aluminum is very soft—I remember my ­mother’s ­anger at my dad’s use of her 1,100-alloy double-boiler as an improvised BB trap: Its bottom became a mass of dimples.

The increased strength of a simple alloy with copper was soon discovered, and it was such an alloy that auto pioneer W.O. Bentley used in his pre-World War I experimental aluminum pistons. In back-to-back testing against the cast-iron pistons then dominant, Bentley’s first-try aluminum pistons immediately boosted power. They ran cooler, heated the incoming fuel-air mixture less, and preserved more of its density. Today, aluminum pistons are universally used in auto and motorcycle engines.

Until the coming of Boeing’s carbon-fiber reinforced-plastic 787 airliner, it was a basic fact of aviation that almost every airplane’s empty weight was 60 percent aluminum. Looking at the relative weights and strengths of aluminum and steel, this at first seems odd. Yes, aluminum weighs only 35 percent as much as steel, volume for volume, but high-strength steels are at least three times stronger than high-strength aluminums. Why not build airplanes out of thin steel?

It came down to the resistance to buckling of equivalent structures of aluminum and steel. If we start with aluminum and steel tubes of the same weight per foot, and we reduce the wall thickness, the steel tube buckles first because its ­material, being only one-third as thick as the aluminum, has much less self-bracing ability.

During the 1970s, I worked with frame-builder Frank Camillieri. When I asked him why we didn’t use larger-diameter steel tubing of thinner wall to make lighter, stiffer frames, he said, “When you do that, you find you have to add a bunch of material to stuff like engine mounts to keep them from cracking, so that weight saving disappears.”

Kawasaki first adopted ­aluminum swingarms on its factory MX bikes in the early 1970s; the others ­followed suit. Then in 1980, ­Yamaha put Kenny ­Roberts on a 500 two-stroke GP bike whose frame was fabricated from square-section extruded aluminum tube. A lot of design experimentation was necessary, but eventually, using the ideas of Spanish ­engineer Antonio Cobas, Yamaha’s GP road-race frames evolved into the familiar large twin aluminum beams of today.

Certainly there are successful chassis of other types—Ducati’s steel-tube “trellis” for one, and John Britten’s “skin and bones” ­carbon-­fiber chassis of the early 1990s. But twin aluminum beam chassis have become dominant today. I’m confident that a workable chassis could be made of molded plywood, provided it had durable bolting points and the usual proven geometry.

Another significant difference between steel and aluminum is that steel has what is called a fatigue limit: a working stress level below which the lifetime of the part is essentially infinite. Most aluminum alloys lack a fatigue limit, which is why aluminum airframes are “lifed” for a planned number of hours’ use. Below this limit, steel forgives us our trespasses, but aluminum remembers all insults in the form of invisible internal fatigue damage.

The beautiful GP chassis of the 1990s could never have been a basis for mass production. Those chassis consisted of pieces welded ­together from machined, pressed, and cast-aluminum elements. Not only is that complex, but it requires that all three alloys be mutually weldable. Welding costs money and time, even if performed by production robots.

The technology that has made today’s lightweight four-stroke engines and cast chassis possible is low-turbulence mold-filling ­methods that do not entrain the films of ­aluminum oxide that instantly form on molten aluminum. Such films form zones of weakness in the metal that, in the past, required castings to be much thicker to achieve adequate strength. Cast parts from these new processes can be quite complex, yet today’s aluminum chassis can be assembled with welds countable on one hand. It is estimated that the new casting methods save 30 or more pounds of weight in production motorcycles.

Together with the wide variety of steels, aluminum is a basic workhorse of human civilization, but it’s more than that for modern motorcycles. It’s the meat of a bike, so ubiquitous that we barely see it or acknowledge how much of the machine’s performance we owe to it.