Choosing Materials For Motorcycle Manufacturing Is More Complicated Than You Think

Steel and aluminum are the norm, but why not more carbon and titanium?

Kevin Cameron's Top Dead Center banner
Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

Despite all the trick titanium bolts or the occasional connecting rod, carbon-fiber-reinforced plastic (CFRP), and the odd bearing with silicon nitride balls, motorcycles are still mostly made of steel and aluminum. Do you recall the promise of plastic car engines that one person could easily pick up? Or NASA’s fun and games with no-weight carbon-carbon pistons? Or the wonderfully weird-looking carbon filament-wound con-rods that were so light that 25,000-rpm engines were just around the corner?

All these wonderful possibilities do actually exist, but making them practical takes serious money of the kind that Boeing invested in its 787 “Dreamliner,” much of whose structure is CFRP.

One measure of material performance is specific modulus, which is the stiffness of the material divided by its density (weight per volume). In arbitrary numbers, aluminum, magnesium, titanium, and steel all have closely equal specific moduli. Then we look at beryllium—off the chart with a number several times that of the other metals! NASA has used the stuff for electronics racks in satellite applications, but the $10,000-per-pound cost of rocketing loads into low earth orbit makes it worth considering—even though dust from machin­ing it is poisonous, and the metal is troublesome and expensive to produce.

Okay, but how about unidirectional CFRP, tested along the grain? Its specific modulus is 4.5 times greater than that of the common metals. Boeing is making a go of it; why can’t we? A motorcycle is pretty different from an airplane. Airplanes are basically a large beam (the wing) supporting a big tube (the fuselage), and the important stresses are mostly distributed as lift over large wing and tail surfaces.

On a motorcycle frame, stresses are concentrated—at the steering head, swingarm pivot, and rear suspension anchorage, plus the several points at which the engine is attached. Some way has to be found of diffusing such concentrated stresses into the CFRP structure, and while this has been done successfully it has never been done easily. It also adds weight to durably bond metal fittings (good at handling concentrated loads) into CFRP, eating away at its weight and stiffness advantages. In the present era, metal motorcycle chassis are mostly made of cast elements, welded together by robots, but high-class CFRP requires skilled handwork of the kind that has only lately been banished from chassis production lines.

If you investigate high-end bicycle technology, you find that riders think of aluminum bicycle frames as being disagreeably stiff. How can this be, when the Young's modulus of aluminum is only one-third that of steel? Here's how it comes down. The stiffness of tubes increases as the fourth power of diameter, so to make a light bike that's strong enough for the forces applied to it, you increase tube diameter and decrease tube wall thickness. With steel—almost three times denser than aluminum—you quickly reach a wall "thinness" that is vulnerable to denting or outright buckling. But when you switch to aluminum tubing of the same weight per foot, you have three times the wall thickness, allowing you to go farther with the bigger tube/thinner wall concept before denting and buckling rear their ugly heads. As a result, aluminum bike and motorcycle frames have bigger tubes than do steel frames. That bigger tube size gives a very stiff structure—even though aluminum is one-third the stiffness of steel. And that is why many bicyclists find aluminum frames uncomfortably stiff. The stiffness comes from the way the material is used (in large thin-walled tubes) and not from the material's own properties.

Yes, Ti has only 57 percent of the density of steel but properly alloyed can be heat-treated to equal the strength (which is different from stiffness) of fancy steels.

We all admire the beauty of forged magnesium wheels, but we know that magnesium yearns inexpressibly to become ore; it is very susceptible to corrosion. This is why street wheels are aluminum. Also, aluminum wheels are cast close to net shape, though they need hub and rim machining they are cheap to make. But every surface of forged mag wheels has to be machined, turning more than half of the weight of a blank into chips (which because of their extreme fire hazard need special handling).

Ever do any fiberglass layup? It’s not too bad—the heavy glass cloth drapes well and mostly stays where you put it in the mold as you daub on polyester resin and inhale the hearty fumes of MEK peroxide. But carbon? If you handle the raw fibers, be ready for them to float into any unsealed electric motors or switchboxes in your shop, where they cause shorts. The stuff is stiff, so it rises up out of the resin you’ve blorped onto it. That’s why the dominant technique is to use unidirectional layers pre-impregnated with uncured resin (keep this pre-preg in the fridge until the moment of use, to prevent curing), held against the mold by vacuum bagging, with the whole assembly going into an autoclave for controlled-temperature curing. Oh, and those unidirectional fiber plies? They have to be laid down in sequence, oriented to give the desired properties.

In other words, more demanding than strolling into your 24-foot boat mold with a chopper gun and shooting it with chopped fiber and resin.

Wow, titanium. Yes, Ti has only 57 percent of the density of steel but properly alloyed can be heat-treated to equal the strength (which is different from stiffness) of fancy steels. What are we waiting for? Let’s make everything out of it! Not so fast. First, because Ti is less stiff than steel, replacing steel axles with titanium makes your bike feel, well, floppy. And when con-rods are made of the stuff, you need two or three times the piston-to-head clearance required with steel rods. By forcing you to make your squish areas thicker, it makes them less effective in fending off detonation. So it’s back to steel rods for some applications.

Titanium valves? You bet—but only after hard-facing the seating surface and coating the stem to prevent galling or seizing. And putting a hard lash cap atop the stem.

Weigh all the fasteners on your bike. Is saving 43 percent of that worth the price of titanium?

Bottom line? You can’t just scratch out where it says “heavy, boring, obsolete material” on your parts drawings; write in trick lighter material, and expect to feel significant improvement on your next ride.