Tracking Down Vibrations

[davidbrady]

A smart phone can sometimes be used to track down vibrations. Download one of the vibration analyzer apps, I use "FFT Vibration Analyzer" on my Android. This tool will give you a graph of vibration frequencies versus amplitude. Our tires rotate at approximately 500 rotations per minute at 60 mph, or 8 Hz. Assuming a sixth gear ratio of 0.639 and a final drive of 4.56, we can calculate the following approximate fundamental frequecies for tire, driveshaft, and engine vibrations at 60 mph:

Tire: 8 Hz

Drive Shaft: 8 (tire Hz) * 4.56 (final drive ratio) = 36 Hz

S60 Engine: 1450 (engine rpm) / 60 / 2 * 6 (number of cylinders) = 72 Hz

8V Engine: 1450 (engine rpm) / 60 * 8 (number of cylinders) = 190 Hz

Of course it's not as simple as it sounds because each of the vibration sources will generate harmonics at multiples of their fundamental frequencies; however, the fundamental is the highest amplitude.