Smooth Operators

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

A Spintron is a system for monitoring an engine’s valve motions by laser interferometry: While an electric motor spins the engine, an interferometer uses coherent monochromatic light from a laser, reflected from a mirror on the moving part, to count the number of light wavelengths it has moved. Laser light is used because it is all “in step” and has a single frequency. Two beams of light—one from the source and the other reflected from the mirror—are superimposed upon each other. As the mirror moves, the intensity of the superimposed beams varies from close to zero (when the two beams are 180 degrees out of step and consequently cancel each other) to a maximum (when the two beams are in phase). All the system has to do is digitally count up the cycles of light and dark as the valve moves, then multiply times the wavelength of the light being used, to know how far the tiny mirror on a part has moved in a given time.

Degreeing a Cam

Many of us have done this the old way while degreeing in a camshaft. Instead of beams of laser light to measure valve lift, we have placed a dial gauge so that its foot touches the valve-spring retainer. Instead of using the Spintron’s electric motor, we have slowly turned our engine with a wrench. With a degree wheel on the crankshaft plus a pointer to indicate crank position, this is all we need to graph out valve motion—and the all-important measuring points that will tell us whether or not the cam is opening and closing the valves at the timings scribbled on the card that came with our new cam(s).

But here’s the problem: In real engines, operating at very high rpm, the actual motions of their valves may not be as shown by the smooth sweep of the dial gauge’s hand. The crankshaft in a running engine does not rotate smoothly and at a constant rate, but instead accelerates somewhat each time a cylinder fires and slows down again as each firing pulse dies away. This constant speed variation, or flutter, is transmitted through the cam drive (by chain, gear, shaft and bevels, or toothed belt), which itself is not perfectly stiff, but in fact has some springiness. The camshaft has cam lobes along its length, each of which requires sudden torque to open its valve. As the cylinder firings spin the crankshaft, it too twists and untwists slightly; the same is true of the camshafts as they lift the valves.

Like any oscillating system (a pendulum, a bouncing ball, a ringing bell), the crankshaft has natural frequencies. At an rpm when the cylinder firings are in step with one of the crank’s natural torsional (twisting) vibration frequencies, the amplitude of its torsional vibration will become larger. A device called a torsiograph can measure the amplitude of this vibration over the range of an engine’s operating rpm.

Such torsional vibrations will in part be transmitted by the cam drive to the camshaft, so its rotation will not be smoothly continuous either, but will also oscillate slightly.

Valve Float

Here is an example. If we connect a single-lobe camshaft to a heavy flywheel, we can be pretty sure that its rotation will be very close to being smooth and steady. Driving this rig on the Spintron we can now find the camshaft rpm at which the valve mechanism ceases to accurately follow the cam profile. This occurs at some high rpm when the cam is moving faster than the pressure of the valve spring can move the tappet and valve. We call this “the rpm of valve float” because when the cam gets ahead of valve motion the spring’s pressure is no longer enough to hold tappet and valve against the cam profile, so they “float”—rise off the profile slightly, then crash down against it a few degrees later. This can be clearly seen on the valve-motion trace generated on the Spintron. The valve-motion measurement is highly accurate because the wavelengths of visible light are centered on 500 nanometers, so the valve motion required to produce one light/dark cycle of laser interference is 0.00000003937 inch.

Now that we know this rpm of valve float with this combo of cam-lobe shape, valve spring, and valve and tappet weight, we can compare it with what happens in a real engine, which does not have a heavy flywheel on its camshaft to smooth out its rotation.

When Honda’s HRC made such a comparison, it found that valves in a running Formula 1 engine it was developing began to float at roughly 1,500 rpm earlier than in the above-described test with a heavy flywheel closely coupled to a single cam lobe. Why? The rapid variation of cam-lobe-instantaneous rpm, caused by torsional vibrations in the crankshaft and camshaft, was causing cam lobe rpm to “flutter.”

I felt a glimmer of understanding. Both Dick O’Brien, Harley-Davidson’s long-serving racing manager, and Rob Muzzy, the successful Superbike engineer and team manager, had independently told me the same thing: “Every time I’ve tried to run a lightened crankshaft at Daytona I’ve lost top speed.” The lighter the crankshaft, the larger its speed variation at each cylinder firing. And that speed variation is transmitted through the cam drive, leading to degraded accuracy of valve movement—and loss of power.

Removing rpm flutter from crank and cams isn’t easy, but it is essential if valve motion is to accurately follow the cam profile. In F1, engine builders apply a variety of damping technologies to cranks, cams, and cam drives in an effort to make each cam lobe turn smoothly and steadily, with minimal rpm flutter.

When I visited HRC in February of 2020 I had the privilege of conversation with the Large Project Leader on the latest CRB1000RR Fireblade, Mr. Yuzuru Ishikawa. I asked him why there is all this talk in MotoGP that V-4 engines have a power advantage over inline-fours, “I like inline-fours,” he said. “I’m an inline guy. And in the rpm range that is usual in World Superbike (up to 16,000) they work very well. But when you add the extra rpm of MotoGP (currently peaking around 18,500) the inline crankshaft becomes torsionally active, but the shorter crank of a V-4 remains stable.”

What this means is that a manufacturer running an inline-four in MotoGP (Yamaha, Suzuki) may have to budget for considerable extra R&D to stabilize the rotation of its crank and camshafts—developing suitable dampers to smooth out their motions.

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