I am no expert on this, but it is interesting and I would hope we could have
some
intelligent discussion, whether or not you like my physics.\

Mechanical systems may have several modes and frequencies of resonance.
Just because it is resonant at several frequencies, however, does not mean
that
it will go into oscillation unless it is driven with energy...something that
out of
balance suspension components are all too adept in supplying.

If a wheel or tire is out of balance, then it will generate a vibrational
frequency
which increases with the increasing speed of rotation.  The intensity or
energy
of the vibration due to unbalance conditions will also increase with
frequency
as a function of centrifugal force.

BUT, the situation will be most noted when the vibrational frequency is the
same
as one of the undamped mechanical resonant frequencies.  When the induced
vibration is far different in frequency from one of the natural resonant
frequencies,
the vibration should be less noticeable...  Hence you will seem to go into,
and
perhaps out of, speeds at which more, or less, severe vibration is
detectable.

I dont know whether any of the wheel balancing machines are set up to detect
chassis mechanical vibration modes.  It wouldnt be a hard thing to do, at
all, from
a physics standpoint. You could excite the chassis with sound waves, for
example,
and measure the intensity of the response at various locations.

I have wondered what some automakers have done to improve the ride so
drastically on
some models of cars.  Some of the brands and models in recent years are as
smooth
as an eel in an oil barrel.  On some other models, I can feel a slight
annoying jitter.
In some cases, this may be due to faults in tires, balance, etc.

For example, some models of Renault, Peugeot, Toyota, and Volkswagen (and
others,
I am sure) can be so smooth, it makes me wonder how the manufacturers made
such
a jump step improvement.

Recently I drove a Hyunda Azera, and although it had rather good quality, it
had the
little bit of annoying jitter.  Our 98 Buick drives well, but it is not as
smooth (and age
must count) as some of the cars mentioned above.

Do the manufacturers tune out vibration by better damping, do they match
components
better, are tolerances so much improved?  Dunno....  maybe someone here
does.

Someone complained on one of these groups a long time ago about buying a
custom
machined set of wheels from a well known manufacturer, only to find them
away out of
round (but still within the 'specs' of the manufacturer.) You can't remove
out of round
vibration by balancing, although they CAN be mechanically balanced.

If you were to buy one of Boyd Coddington's special one-off rods, would it
drive as smoothly
as one of the best above, or would it drive like a home build rod??

Ideas?