Vibration Investigation

The graph above is a Frequency Spectrum from my 1973 240Z with a Chevy V8 under load at 50MPH.  The accelerometer was positioned on the drive shaft tunnel.

My complaint was vibration under load.  I noticed the vibration at many speeds.  I chose 50MPH to investigate the vibration.  The tires are 24.7" tall. the rear gear ratio is 3.54.  The engine has 8 cylinders.  I plugged these parameters into the calculator  and it produced the excitation frequencies listed above.  To run the actual test I "rode the brake" while opening the throttle to load the engine.  I held the speed right at 50 MPH for 16 seconds while collecting data.

The frequency spectrum shows peaks at 40, 80, & 160Hz.  Looking at the excitation frequencies in order of decreasing amplitude, note the forcing frequencies are U-Joint, drive shaft balance, and engine power stroke. (185Hz may be a floor board resonance)  Because the Y axis Amplitude is measured in dB, the U-Joint vibration level is actually about 3 times greater than the others.  The U-Joint is the only significant vibration under these conditions.  (Considering the weeks I spent modifying parts to ensure a perfect drive shaft alignment, I guess any further improvement will have to come from using a drive shaft with large dynamic damper similar to the one used in the mid 90's Camaro's.)  Actually it's not that bad, I just wanted to know where the vibration was coming from.

Note: Most vibrations experienced by the average car owner are caused by tire and wheel imbalance and tire non-uniformity (eccentric assemblies).  These problems become even more prevalent when using aftermarket rims which often are poorly machined. 

Back to My Car Section Starter Page