1996 Z51 suspension question

['96-Z51]

I'm looking here for helpful comments from someone who has experience driving a well-maintained (ideally '96) Z51-equipped car AND at least one other late C4 with standard suspension. I bought my '96 Z51 LT4 CE last summer with 44,000 miles on it. Showroom-looking condition, but with a vibration kicking in at around 70 mph. Thought it was the crappy generic cracked tires and aftermarket wheels, so these were replaced (now on Continental DW Extreme Contacts). The vibration didn't go away, turned out the main driveshaft had a strange scoring on it, and was possibly twisted but otherwise unbalanced, and the U-joints were pretty stiff, so I had it replaced with a custom balanced driveshaft. There's still some vibration at highway speeds, and it's making me want to postpone doing track days until a solution is found. Both my regular mechanic for over 25 years (who also works on several other C4's), and our local Corvette specialist garage, have test-driven and gone over the car looking at wheel/tire roundness and balance (balanced twice), wheel bearings, suspension joints, all of the usual indicators for vibration. I've also had a four-wheel alignment done at a local race-prep shop. Am I chasing nothing at this point? Is the Z51 suspension simply more sensitive to road surface irregularities at higher speeds? I have owned other sports cars, from Lotus Seven to Jaguar XKE, and would expect this Corvette to ride a bit more smoothly, more toward the Jag end, but is this what I get with Z51? I love this car, my first Corvette, but I'd enjoy it more if I could relax about the vibration!

 

[mr.beachcomber]

I have a daily driver 89 Z-51 Coupe (w/o FX3 option). Although this suspension is very stiff and doesn't like rough, uneven surfaces, I've never had a vibration like you describe. Can you tell if the vibration is in the front of the Vette or in the rear? Can you isolate the vibration down to either the passenger or driver side?

If the vibration seems to be coming from underneath the Vette, I would suspect a worn/damaged pinion that is out of alignment. Similarly, worn support beams connecting the ZF-6 to the Dana 44 might be at fault also. I've installed C4-beam plates around 80K miles to correct this problem. One area that you didn't mention is shocks. I just recently had a rear shock lose it's oil seal and start to squeak a low speeds. It's possible that a worn shock could cause a noticeable vibration at only certain speeds.

Since the vibration only occurs around 70 mph, try taking a drive with a passenger along smooth pavement at that speed to see if the two of you can isolate the source of the vibration. I'd try no radio, windows down, and rear cargo carpeting out to help trace the vibration.

Hope this helps.

 

['96-Z51]

Your thoughtful response is much appreciated! I'll discuss each of your ideas with my mechanic when I see him next week. During the online part of my research into the problem I did come across the ZR51 Performance people, but didn't see mention of the beam plates; will follow up on that too.
I'll post any new developments here.
Thanks again.