First, we are accustomed to a degree of motorcycle engine vibration and may even like it because it tells us the engine’s running and everything is normal. People on the ocean liners of 1900 knew something was amiss when the steady (even reassuring?) thump of their large triple-expansion piston steam engines ceased. What’s happening? I feel the same when I come to a stoplight in a rental car that has the start-stop feature. Mm, something’s wrong. Then I remember.
The battery-electric LiveWire motorcycle has close to no vibration at all, so a little bit was engineered into it. When “switched on” but still at rest, its electric traction motor is made to oscillate, reminding the rider it’s ready to go.
On the other hand, there’s a reason why so few riders traveled long distances on the many air-cooled Harley Sportsters produced: vibration described by some as near the double-vision level. Even rubber engine mounts failed to tame their mighty shaking.
Beyond ordinary human discomfort lies a world of broken parts. Kawasaki’s original 1975 KR250 tandem twin (two crankshafts, geared together, one ahead of the other) had such a tremendous fore-and-aft rocking couple that it chronically broke its front cylinder’s exhaust pipe and California racer Harry Klinzmann’s ride broke four of the six frame tubes supporting the steering head.
When the engine was switched to both pistons moving together with a 100 percent balance factor, later bikes (250s and 350s) were smooth and won eight world championships in the hands of Kork Ballington and Anton Mang.
The lightweight KR3 500 two-stroke triple built by King Kenny’s team near the end of the ‘90s had nothing to deal with such engines’ strong side-to-side rocking couple. Parts breakage required parts to be reinforced, increasing weight, but that didn’t stop erratic carburetion resulting from the shaking (what do you call a milkshake made from hydrocarbons?). Retired Honda engineer Yoichi Oguma offered to redesign the engine with a balance shaft. This was successful and the machine had just begun to show its promise when the two-stroke era ended in 2001.
Outstanding in the annals of vibration-driven problems was the development of Wright’s wartime R-3350 18-cylinder radial, which powered the B-29 bomber. Troubles such as badly galled propeller centering cones, repeated cracking of nose case, carburetor mounting adapter, and supercharger housing, plus failures of such details as magneto drives, starter mounting flanges, and carburetor control linkage were attributed to “normal developmental problems” and no doubt some of them were. But when improved engines were finally supplied to Boeing to enable flight test to begin, that company’s independent vibration measurements showed the presence of substantial secondary torsional vibes.
That couldn’t be ignored because it presented the possibility of vibration-fatigued steel propeller blades throwing high-velocity pieces in flight. B-29 flight testing was therefore delayed by six months while eight gear-driven balancers were designed to squeeze into each engine, type-tested, and put into production.
There are many more examples of projects in which both time and money could have been saved by considering vibration early in the design process, rather than having to take last-minute emergency measures against it.