Tires: harmonic vibration, ford street rod, stud holes

Question
My '35 Ford street rod has a definite harmonic vibration
beginning at about 45 mph. It is equipped with Cooper Cobra
tires. I felt as though the harmonic vibration was possibly
caused by the rear tires so I took those tires to the local tire
store and they used a machine that applies 1700 pounds of
force to the tire and said that one of the two tires was out of
tolerance. They broke the tire loose from the rim and rotated it
to the point where their chalk marks lined up. At first the
harmonic vibration seemed to have been elimintated but after
200 or so miles of driving the vibration was back AND the car
seemed to vibrate at a higher frequency too. Yesterday they
replaced those Cobra tires with new ones. The higher frequency
vibration is gone but the harmonic vibration (1 per second) is
still there. Am I on the right track in thinking the tires are the
problem? The u-joints were replaced and drive shaft re-
balanced. Where else should I be looking for a harmonic
vibration problem?

Answer
Howard,

1 cycle per second is just too low for a single tire.  It might be 2 tires that have relatively low vibrations that becauase of a slight difference in rolling diameter, come in and out of phase at the rate of once per second.

However, this could also be the wheels AND it could be the wheels not being centered on the hub.

Most OE wheels pilot on the center hole, while most aftermarket wheels pilot on the studs.  The studs (and the stud holes) are somewhat difficult to get centered - there are 5 of them after all.

I would try taking the wheels off and putting them back on very slowly and carefully and see if that changes anything.

Hope this helps.