I,ve ridden enough Harleys to know that the vibration increases in intensity with an increase in speed, you just can't feel it much on a RubberGlide, the vibrations at idle are more from the uneven power pulses than from engine imbalance, a Sportster with a solid mounted engine turns into a butt numbing vibrating machine at around 5,000 rpm and has been known to cause liscense plate brackets to break from metal fatigue. So try some other argument. An out of balance prop is barely noticeable at idle but at speed can vibrate the tail off of the plane.