Glen,
At 60mph, a mile a minute, the rear wheel rotates at approximately 780 revs per minute. This is a very low frequency compared to the 4.5 times that, very high frequency associated with engine vibration.
I have a Rapide that vibrates quite badly to the extent it has split the rear of the tank even with the tie bolt in. The bike had been involved in an accident and a thought was that the flywheels had shifted. Dissassembly of the engine showed .004 round out, hardly huge but room for improvements. The engine has 8 : 1 specialloid pistons in it and reverse calculations show the balance factor to be with in 6 grams of 35%. I was loath to deviate from this figure as the engine is in very original condition and quite good inside. Dynamic balancing only needed two small holes drilled partially into the flywheels on the crankpin side, one 3/16" on one side1/4" on the other flywheel. Truing the crank to .002 and reassembly, I had I hopes that it would be very smooth. Sadly I was dissappointed as it still vibrates quite badly. Up to 60mph it is smooth, over that and the vibrations creep in. By 70 mph the bars are "expanding in diameter" and the tank is "doing an orgital sander" I have tried different tensions on the cylinder head bracket bolts but with no improvements. My Norvin, norvins known to be a bit more vibratory than a standard bVincent, is markedly smoother than my HRD. I'm at a loss to explain it so just keep riding it.