Like all materials, metals consist of atoms, which in turn are made up of a positive nucleus of protons and neutrons, surrounded by a vague cloud of negative electrons orbiting at various energies. It is electrical forces—attraction between positive and negative charges—that hold materials together.

Metals are special because their outer electrons are so weakly bonded to their nuclei that they wander through metallic solids like a gas. The high mobility of these outer electrons is what makes metals generally good conductors of both electricity and heat. That electron “gas” also contributes another feature of metals: their malleability, or property of bending or stretching under stress, rather than snapping. This allowed early human metal workers to hammer or bend metal into useful shapes—blades, points, axes—that made life easier and more productive.

Pol Espargaró
Has aluminum reached its limit? Like Ducati and Honda, KTM is using a carbon-fiber swingarm in MotoGP competition. Pol Espargaró has said the non-metal material doesn't change the character of the bike, but, “It gives you something else when you need it, like when the grip is not so nice.”Courtesy of KTM

When enough stress is applied to mineral or ceramic materials to open a crack, there is no mechanism in them to limit or arrest that crack. The crack therefore relieves that stress by propagating through the material at the speed of sound. This is brittle fracture. While useful if we are flaking a flint nodule into a fist axe, this property is less so if we are trying to chop down trees with such brittle axes to build a dwelling or protective palisade.

In metals, the presence of those roving electrons allows material under stress to rearrange itself, as atoms broken loose from their neighbors by stress can rebond just as strongly to new neighbors. This is malleability. This makes metals inherently tougher than hard, brittle materials because in all that bond-breaking and bond-making, energy is consumed; it takes a great deal of energy to make metals yield. This is toughness, often measured as percent elongation before failure. For brittle materials, this can be less than one percent but for metals of high toughness it can be 30–50 percent. To break such materials, you have to keep pulling and pulling with great force, expending a lot of energy. This is how crumple zones in cars protect us jelly-like humans during collisions.

The diamond cutter of old Amsterdam examined a rough stone to identify its planes of fracture, and then, precisely positioning a punch, struck it with a hammer to split this hardest of all substances in an instant. Had that diamond been a lump of steel, the cutter could spend the rest of the week with his hammer and achieve nothing: Steel is tough but much less hard than diamond.

A basic way to compare the abilities of metals to resist deformation is their Young’s Modulus of Elasticity. Think of Young’s Modulus as the material’s “spring rate,” just as in motorcycle suspension.

Material Modulus (million psi) Density (times that of water) Density (percent of steel)
Steel 30 7.8 100
Aluminum 8–11 2.7 35
Magnesium 6 1.7 22
Molybdenum 42.7 9–10 122
Titanium 15–18 4.5 58
Beryllium 43.9 1.8 23
CFRP 26 1.6 21 (non-metal)

The clear winner among metals is beryllium, weighing less than a quarter what steel weighs, but with a higher Young’s Modulus. Trouble is, beryllium is really expensive and its dust is poisonous. It has its uses; as electronics racks in satellites, for example, it saved money in a time when launch cost to low earth orbit was $10,000 a pound.

Titanium looks pretty good too. Alloys of titanium can be heat-treated to strengths equaling many high-strength steels, yet it’s light—only 58 percent of steel’s density. Then our hearts sink when we see that titanium has only half the Young’s Modulus of steel. This is why Formula 1 engines have swung back to steel connecting rods from titanium. When the piston decelerates at thousands of gs approaching top dead center, a titanium rod stretches twice as much as one made from maraging steel, and that requires the all-important squish clearance between parts of the piston and the head to be made larger to keep pistons from hitting the head. And that, in turn, makes squish less effective at bottom- and mid-range rpm, resulting in less engine torque. When a rider friend was given a titanium front axle for his Yamaha TZ750 roadrace bike, he tried it in a practice, then said, “Take it out.” It was certainly strong enough for the job but was only half as stiff, making the bike’s steering feel vague and imprecise.

Aluminum, the workhorse metal, is one-third the weight of steel but also has only one-third of steel’s stiffness.

Magnesium has less than a quarter the weight of steel but then it has only one-fifth as much stiffness. In addition, magnesium is subject to low-temperature creep, defeating most efforts to make engine blocks or cylinder heads of it. And so, we see magnesium on bikes mainly as engine covers and as ultra-light wheels, usually for racing.

Aluminum, the workhorse metal, is one-third the weight of steel but also has only one-third of steel’s stiffness. Lacking magnesium’s low-temperature creep, aluminum lends itself to use in major castings. Modern motorcycle chassis are frequently aluminum, and engine cases and heads always are at present.

I've included carbon-fiber-reinforced plastic (CFRP) just for comparison; the bulk material is close to the modulus of steel but its density is only one-fifth as much. Fabrication is expensive, requiring special techniques to produce the highest properties. A few years ago, folks were expecting miracle carbon-fiber engines but that didn't happen. Right now, the carbon excitement in motorcycles is the higher lateral flexibility that can be built into swingarms for MotoGP bikes (metal would be too vulnerable to cracking if made thin enough to act in the same way).

Why does materials progress take so much time? I fall back on the vaguely mystical statement that, “Time has to exist because it’s what keeps everything from happening at once.” That gives us the pleasure of anticipation.