Originally Posted by TasmanianDevil
Unless you have a balance problem or a bad wheel which you damaged why would any vehicle vibrate after a certain speed...
Well, it you had an perfect flat road, perfectly balanced tires and wheel,
perfectly balanced driveshaft, all moving parts balanced, incl. engine, then yes, no vibrations.
But, asphalt are wavy, engines not balanced (I6 and V12 are better), wheels with
pebbles, etc. All influencing the car with forces of various frequency and amplitude.
You may think of the springs and shocks as the suspension of the car, but
all those forces with higher frequency than bumps in the road, are supposed to be
taken up by all the different bushings around the car.
Engine and transmission support bushings, suspension arm bushings,
tie-rod bushings, steering damper, idler arm busing,
front and rear axle carriers bushings, differential bushings, etc.
All of them are there to prevent that other equipment come into vibration (eigenfrequencies).
The sizes and physical properties of the dampers/bushings, are carefully
calculated, so any deviation from that (by replacing with wrong part, poly-bushing
old bushings), will give changed characteristics.
Myself, I have now replaced upper arms and lower arm bushings (and they
were clearly broken), but still have some vibrations in the steering wheel,
so the idler arm is next...
I guess what I mean is; its a combination of unbalance (that you'll never
eliminate completely) and ability to mask those by suspension,
that in the end gives the impression of vibration.