Thanks for the additional comments. In the dealership realm of things, with GM now having tires in the vehicle's basic warranty coverage, if the Hunter machine indicates that the tire's road force variation is not within the GM specs (from the appropriate TSB, for cars and light truck chassis vehicles), then the dealer orders a new tire(s) as necessary until the issue is "in spec". Each tire manufacturer also has some criteria for what they'll cover and not cover, though, regarding how long the tire has run before the RFV is checked.
According to a Goodyear rep, they don't have and will not honor a warranty claim for "road force variation", but call it "ride disturbance" instead. Another manufacturer states that the tire must have 500 miles on it before they will cover a RFV claim. But that is "them" talking and not "GM", per se.
I understand the relationship of the size of the drum that measures RFV on the balancer and how it might not completely relate to how the tire flexes on an actual roadway. I supposed that as long as the measuring technique is standardized (and the equipment is so calibrated), then the size of the rolling/bead seating drum would not be as critical. It's not that it is supposed to simulate the actual road surface, but that it's done in a standardized manner.
I concur that chassis stiffness is greater than it used to be, but I feel the issue is more in the weight of the tires versus the weight of the wheels they are mounted on being, generally, less than what they used to be in the 1970s (for example). It used to be that an H78-15 bias ply tire would have enough Fed Excise Tax on it to indicate a weight of 28-30 pounds. Now, if you can find the weight of the equivalent P225/75R-15 modern radial tire, it'll probably be more like 25 lbs. Not to mention the generally higher inflation pressures now used for fuel economy reasons. All of these things do change the resonant frequency of the tire itself, to a certain degree.
In the original radials, it was that resonant frequency (in the "boom" lower frequency spectrum) that resulted in more noise in the 45mph range on unit-body construted vehicles, but once the vehicle speed rose into the 50+mph range or slowed below the 40mph range, things got much smoother and quieter. Body/frame construction vehicles were less sensitive to these things, due to their greater isolation of the frame from the body structure. Of course, the resonant frequency was demonstrated to be different between different tire brands/models and how they interacted with the vehicle they were installed upon.
In modern times, the majority of cars are unit-body construction, reinforced to comply with Federal Safety crash criteria. Strut suspensions have replaced control arm suspension, typically, and have different ways of dealing with suspension compliance and noise transfer. Many suspension pieces are "light allow" rather than stamped steel, as are some of the front subframes on many fwd cars.
And, now, we have the trend toward tires of even larger diameters and shorter sidewalls. Less flex in the tire makes tire construction concerns much more critical. Less sidewall height means less room for road impact energy to be absorbed and/or dispersed. Add in the "tread caps" to make the tread itself stiffer, and that figures into the mix too.
I remember when the old Atlas Plycron tires (circa 1963+), say in 7.75x14 size, would balance with not more than .5 ounces of weight on each wheel. If they took more than that, something was wrong! Usually, they took less.
And the trusty bubble balancer, if the operator knew how to use it "right", worked fine for all but the most difficult balance issues. For something better, there was the Stewart-Warner Alemite spin balancer that spun the tire "on the car" to balance the complete rotating assembly at whatever speed the operator desired (depending on how long he kept the drive wheel held against the tire!). If you pulled the tire off for any reason, you marked the lug stud that matched the valve stem location so that the original balanced relationship could be maintained.
Things are different now, but I guess we're lucky that things work as good as they did (back then) and do (now).
Enjoy!
CBODY67