What Causes Vibration In A Motorcycle Crankshaft?

Cycle World Technical Editor Kevin Cameron answers your motorcycle engineering and mechanical questions

I want to talk about "elephant-ears-flapping" vibration in crankshafts. That wonderful description comes from Cycle World's Italian correspondent Bruno dePrato, who was speaking of vibration that can occur in crankshafts. Here is an example of a two-stroke crankshaft, but this movement can also happen in four-strokes.

In this case, a pair of flywheels is joined by a crankpin that is pressed into the two flywheels. These two flywheels are constantly being accelerated by combustion pushing down on the piston. The result is the two flywheels vibrate toward and apart from one another, as Bruno puts it, like elephant ears flapping.

This system of two masses and a crankpin is reduced in frequency by adding the mass of the ignition rotor. When the Yamaha RD400 came out, it had larger and heavier flywheels. Therefore, they vibrated at a lower frequency. When the mass of the ignition rotor was added, the vibration was considerable.

A discerning racer coming home from a weekend at the club races took off the cylinder and put a feeler gauge between the flywheel and the connecting rod. Instead of the usual 0.017 to 0.018 of an inch, he found the clearance had increased to 0.024. He went to another club race and when he came home 0.024 had become 0.028.

Here’s what was happening: The ignition flywheel was slowly walking off the crankpin. With the extra mass of the ignition rotor, the elephant-ears vibratory mode was slow enough to come into step with the engine’s firing frequency. The bearings were not enough to completely prevent this vibration.

Eventually his crankshaft would have gotten so much wider that it would crowd the bearing between the flywheel and ignition rotor and then destroy it. Many people have encountered this charming elephant-ears-flapping crankshaft vibration. It’s happening in every engine all around us. Most of them manage to survive it, but occasionally there is a failure.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”