Originally Posted by Trav
It doesn't matter if its spinning 2.8 or 28 times more if it vibrates at a consistent speed and rpm. Where do you get the 2.8 number from? That depends on the gear ratio and tire size.
When the transmission shifts the drive shaft and cv axles rpm remains constant if the same speed is maintained only the engine and transmission internal rpm changes.
The 2.8 is from the JHZR2's final drive ratio, 2.82:1 to be exact. The frequency with the highest vibration amplitude at 60 mph is 13.6Hz captured by his phone accelerometer, which corresponds closely to the rotational speed of his particular wheel size (also given in his post) at 60 mph. This points to an imbalance that occurs at 13.6 times per second, assuming it is a single imbalance in the rotating system. If the prop shaft has an imbalance, it would be at a different frequency as the prop shaft rotates 2.82 times faster than the wheels.
The key is the 13.6Hz frequency, as it corresponds to an imbalance at wheel rpm at 60 mph.