The vibration is INTENSE. It nearly shakes the passenger seat off of its frame. It is definitely coming from the rear because the steering wheel doesn't shake...and I can just feel it.

Ok...when I bought my car, I had BMR tubular subframe connectors installed. The shop lifted the car by the frame and welded them on, but the door seams lined up, t-tops fit fine, and everything seemed alright so I didn't worry about it.

I replaced the stock 16" rims with 17" Boyd Coddington rims and Kumho tires...that is when I noticed the vibration at 65-75 mph.
I returned the rims to the shop that mounted them and had them checked and rebalanced if necessary...again everything was fine.

So, when it came time to rebalance the tires (5000 miles) the tech said that one of the wheels had a wobble...woohoo! Problem solved right? No...returned wheels to Boyd Coddington only to find out that the wheels are all true and need to be installed "specially" on the balancer. So had the tires remounted at another place and WATCHED them do it...everything was fine.

So on to the driveshaft...mine has almost 100,000 miles on it, so I replaced it with a Denny's HD driveshaft. Vibration got a little better...but still 95% there.

I do have SLP Eibachs w/ SLP Bilsteins, a poly trans mount, BMR subframes, BMR LCAs, PHR, relocation brackets, poly sway bar mounts.

Does anyone have any suggestions? Could the driveshaft angle be off with the car lowered and the poly trans mount? Is it possible to do a 4 wheel alignment on this car? I am at the point where I don't like driving it because of the vibration.

 

Sounds to me like you broke a belt in one of the tires. What happens is at lower speeds it's fine, but as the rotational speed on the tire increases, it pulls the belt apart, and allows the tire to bulge where those belts are seperating.

 

I do have cheap azz Kumho tires, but when I rotate the tires from the back to the front, the vibration is still present in the rear. The tires do seem to be wearing poorly and are slightly cupped...but as I said...things vibrated when the rims and tires were brand new...check revealed nothing wrong with rims, tires, or balance.

 

Cupped? Has anyone followed you down the road to watch the car? See if there is a tire bouncing?

 

Maybe something with the rear end, bad axle bearing, etc?

 

Bent axle? Mine had a bent axle, replaced it and it got rid of some of the vibration, I think the rest is in the tires.

 

Thanks for all the ideas, guys.

Rear axle: The stock rear-end was in the car when the vibration started. I replaced the whole thing because the bearings were going bad (howling). It was cheaper to replace it ($350 used w/ 6,000 miles versus $550 for new GM bearings) than replace the bearings. The howling was gone, but the vibration was exactly the same.
There is some play in the axles, but it is lateral because of the c-clips...I assume that is normal on a c-clip axle. They dont feel loose in any other direction.

I sold my stock rims and tires, so I can't reinstall those. I do have a friend with a 2000 WS6 with stock rims and BFG tires and his car is very smooth. He will be here on the 27th, so I will see if I can borrow his wheels/tires for a test run. That will at least let me know if the tires are sh*tty. There is some indication of poor tire wear and cupping on my tires.

The bearings in the rear-end are quiet with the exception of the pinion bearing (howls on deceleration at 55 mph), so I would assume they are mostly ok. A 12-bolt is in the future for this car, so I am not willing to put much $ into the 10-bolt.

The clutch is an 02 LS6 clutch with upgraded hydraulics. The vibration was present before and after the clutch install. The transmission could very well be out of balance with all kinds of bent stuff...but as stated earlier, the vibration "seems" to be coming from the rear axle area.

The brakes are new EBC Green Stuff pads and slotted/drilled rotors...they have about 5-7000 miles on them and do not show any signs of warping.

After hearing your ideas, I think I should explore:
1. Borrow a set of rims/tires that I know run smooth, and test them on my car.
2. Check the driveshaft angle. If necessary, install a 1LE trans mount and aftermarket torque arm to correct a misalignment.
3. Check for play in the rear-axle bearings.
4. Strap the car to a dyno and visually check the rear end and tires for any problems.

 

Dude I HAVE HAD this issue until today hehehe
I first thought was rotors, replace no go
Replace drive shaft no go
replace rear end (with a 9 inch of course), no go
rotated tires no go
Tire balancing? tried it no go

I REPLACED all4 tires, and WHAMO!!!!

 

Well, new Goodyear F1 GS-D3 are in the very near future for this car....maybe they will solve the problem. We also have a road force tire balancer at a local Nissan dealership...maybe that will help some also.