Exhausting Ductility - Bending the lid, and your bike.

The wonderful property that distinguishes metals from other elements is ductility, the ability to deform without breaking, much like an imaginary superstrong taffy. On the one hand, ductility makes it possible for us to form sheet metal by rolling or pressing it into almost any desired shape. And on the other, a material that bends or stretches takes a great deal more energy to break than a brittle material such as a ceramic. Many metals yield 10 to 30 percent before breaking, a process which requires exerting high force over significant distance and amounts to considerable energy. That is toughness.

Try to drive a ceramic railroad spike and it would instantly shatter, but steel spikes have been happily doing their job for nearly 200 years. The silicon-nitride big-end needle rollers used in the last generations of Honda two-stroke roadrace engines retained hardness well at elevated temperature, but the slightest clearance in the bearing would break them.

Old Metal

Yamaha’s 250 TD and TZ production racers were a wonderful gift to generations of riders, being fast, relatively inexpensive, and readily available. Many a pro racer’s career began with those 250s. They had limitations, of course, and one of those was that their engine mounts would eventually break. We welded them, but then they just broke again, and sooner. And so we came to regard a year-old chassis as “old metal”: something had happened to it that made it more and more susceptible to cracking. This added purpose to many riders’ decisions to upgrade annually. In general, a TZ250F was a step forward in performance over a TZ250E—and it had “new metal” in its engine mounts!

Let’s look into Professor Bela Sandor’s little book Fundamentals of Cyclic Stress and Strain. On page 48 he introduces a lovely concept: “exhaustion of ductility.” This describes a material that has been subject to repeated plastic strains, in effect “using up” its original ductility. That was the “old metal” in our bikes’ engine mounts, exhausted from so many trips to the track.

We experience this same process in our own kitchens when we open a tin can. Often there’s one narrow bit of metal holding the lid to the can’s body, and we bend the lid back and forth until after several cycles the lid breaks free. Here we need to understand the difference between elastic and plastic deformation: Subjecting ductile metal to repeated plastic strain (that is, deformation beyond the material’s ability to elastically return to its original shape and dimensions) in effect “locks” the dislocation mechanism that permits said plastic strain. It “exhausts the ductility” in the material affected by creating tangles of dislocations that prevent further yielding. The resulting very stiff material breaks instead of bending.

Fatigue and Motorcycle Frames

This resembles the state of metal in a motorcycle frame that has been subjected to roughly 10 million cycles of back-and-forth yanking as the 10,000-rpm, 180-degree crankshaft performs its natural rocking motion. The chassis was designed for a 7,500-rpm street engine, ridden most of the time at below 4,000 rpm. Inertia forces increase as the square of the rpm, so instead of lasting almost forever, the metal in the race chassis fails after 20 to 50 hours of operation.

And that is why, when Yamaha began to replace steel chassis with lighter but much more fatigue-prone aluminum in the late 1980s, the engineers had to stop vibration at its source: in the engine. A self-balancing 90-degree V-twin replaced the earlier parallel-twin design, and a small balance shaft addressed the slight remaining rocking motion caused by crankpin offset.

When the late Gavin Trippe and others had the intuitively attractive idea of creating a new, low-cost roadracing class, they took old aluminum-framed TZ racebikes and replaced their two-stroke engines with modern four-stroke 450 MX singles. But the 450s’ violent shaking quickly exhausted the ductility in those chassis; they cracked and broke in an hour or two of running.

Man up—vibration means power!

How did today’s population of old-timers get the weird idea that tolerating vibration is manly? Public-relation flacks are experts at magically transforming a vice into a virtue, hardly a new concept. When Pierce-Arrow’s gear department couldn’t eliminate the annoying whine of its new worm-drive rear axle (a design requirement since it permitted an appealingly lower floor), PR solved the problem with the slogan “Pierce-Arrow—the car with the aristocratic hum.”

With the 1937 Triumph Speed Twin, Edward Turner created the classic British twin. The vibration of its two smallish pistons and light, forged aluminum con-rods moving up and down together was less than that of the thudding 500 singles it replaced. But when those 500s grew to 650 and then 750, they vibrated a lot more thanks to the increased weight of their bigger pistons. Anyone who complained about it to the dealer was told to man up—vibration means power!