Vibration @ hwy speeds
my 2004 S/T (140,000km or 87thou) has annoying vibration from the rear at hwy speeds between 100km/hr (60mph) thru 115-118km/hr (71-73 mph).. tires have been balanced, rotated, and are relatively new. still from the rear...Smoothens out after 120km/hr (75mph)
Any suggestions to look at first? rotors? universals, driveshaft balance?...
someone posted info relative to '02's having a Ford recall for driveshaft replacement to aluminum. any benefit to this?

well it should not be your rotors, normally you feel that only when stopping. the first thing I would think of would be the balance because you do start to feel that around 60-64 then it breaks away smooth again but if you say it has been checked and passed the buck I guess its not that. There might be a chance that it is the wheel bearings but it shouldn't be. are you 4wd or 2wd, if you are 2wd then you shouldn't really feel anything from the rear wheels because its a straight axle. what type of tires are they? any bigger then the ones you had before. Oh and in regards to your drive shaft what does it look like? Is it all rusted up or looks new?

Still sounds like a tire issue. I've known many tires to have balance issues withing certain speeds. There's often not much you can do. You can try playing with the PSI in the rear tires.

thanks for your feedback and input.. Fyi, it's 4wd, steel driveshaft (rusty)...
haven't tried for vibration in 4wd mode... but was on the hoist again today and the tech showed me upper front left control arm ball joint is worn 25/30%, allowing the wheel assy to move a little, and upper front right starting to fade. Says that's the issue....

How is that going to translate into rear shimmy??? would it not be more pronounced at the steering wheel and coming from the front?

when you are saying "rear shimmy" I'm assuming you are feeling the shaking from your seat...right? try taking pictures of the ball joint, i'm interested to see what wear and tear looks like for that piece. You might not feel through the steering wheel until it gets worse.

I know that some ST's that had the steel shaft had problems at higher speeds and that is why the 05's came with a different metal...I feel lucky . :smile:

by the way, what tires do you have and what size?

Thanks again for the reply..Fyi, tires are "Rodian HT by NEXEN" P255/70R16
Yes, "shimmy" vibration has the passenger seat backrest bouncing when not occupied... and can feel it in the chassis at speeds noted above. Universals were also checked and good..(tight, no play)
Can't get a picture of the wear on the upper control arm ball joint until it's out and exposed... surprised OEM doesn't have a replaceable ball for the wishbone control arm. Seems logical...
Apparantly MOOG's replacement arm does but it's significantly more $$$ upfront. Still looking for alternatives... While I recognize a worn ball joint in front suspension needs replacing, I'm having trouble understanding the correlation to a vibration that appears to eminate from the rear. The shop won't guarantee this is the solution, hence looking for advice and opinion, or similar experience.

Did you find out what it was?
I'm having trouble myself:

http://www.sporttracclub.com/viewtopic.php?t=758

Hi Chatrhouse,

I have had the same problem with my 93 Taurus wagon, I put it up on a hoist and noticed that when rotating the rear tires they had about 1/8" of runout on them. You could try rotating your tires and see if the problem moves with the tires. Tire places usually put the "bad tires" on the rear so that the vibration is not as noticable.

The vibration you are feeling is only happening at a certian speed, this is called the "critical speed". It is caused by an imbalance in your drive system from somewhere between your output shaft all the way back to your tires, it could even be from several components not being balanced properly. The vibration is there at all speeds, but at the critical speed it sets up a harmonic frequency that is amplified by the system.

If your rear tires do not have any runout (side to side) and are not out of round you might try just rotating the rims one lug at a time and seeing if that helps. My buddy had the same trouble and it was caused by the tranny shop bending one of his u-joint ears.

I hope you get it figured out, and look forward to hearing what it was, something simple I hope.

Calvi