Hey y’all,

I’ll quickly touch on current suspension mods. 275s on all four corners, wheel spacers, KYB gas-a-just shocks about 15,000 miles ago maybe, poly trans mount and torque arm bushing. Front control arm bushings are in the process of being replaced with poly, with the exception of the rearward LCA bushing, which is OEM rubber. Only driver side is done so far. Let me explain my issue.

For legal reasons this happened in Mexico. When I allegedly go around 130 for funsies and I either clutch in, go neutral, and sometimes decelerating, I cannot exaggerate it when the body violently feels like it swings side to side like a bull ride at a festival. I also get weird vibrations around 74 cruising that smooth out and sometimes come back as I speed up. I suspect maybe panhard but aggravating the condition when on the clutch at high speed is strange, could be the driveshaft. No idea. Would love input and smoothing out unexpected vibration.

Thanks in advance.

EDIT: The side-to-side sway can happen going over credts and dips and the likes. Makes me lean towards suspension or diff. Last time it was on stands the wheels spun in opposite directions in neutral which concerned me.

 

I don't have any ideas on the violent shaking @130mph, but I can help with this:

That is completely normal and exactly how the Torsen LSD in your 2000 Z28 should operate when unloaded. 1998 was the last year that an Auburn LSD was issued in a Z28 (it continued to be optional via SLP in the SS cars); starting in 1999 the Torsen unit became the standard LSD and it behaves as an open rear (wheels spinning in opposite directions) in an unloaded state (such as in neutral on jack stands).

 

I'd upgrade the rear panhard bar to an aftermarket unit.

 

It sounds to me like you are getting a lot of aerodynamic lift at speed, and with variations in pavement, this phenomenon is exacerbated. In addition to the panhard bar, I would install lowering springs - especially in the rear if your car has a high rear rake - and look at an enhanced chin spoiler or splitter up front.

 

My 02 at that speed does not do that. I am still on the factory springs and orange Delphi shocks on the rear and front Delphi coil overs on my front. My car has just over 70k. I have a brand new set that I just put on Eagle F1 GS-D3 275/40/17s. You are sure you don’t have something off with your suspension right? Wheels aren’t bent? Tires all worn evenly? Road in Mexico was smooth as well?

 

Your sway bar links and bushings good? It is more for cornering stability but just thinking out loud here.
The other thing is if your suspension is not the same side to side (you've done poly bushings on one side but not the other) that could in theory cause some weird behavior.

 

So a few things. 1) a couple days ago I had a wipeout taking a left intersection turn and the passenger rear tripped the median in a 180. This shifted the rear axle towards the passenger side by eye’s view which sealed the deal on a new panhard. UMI double roto-joint adjustable. Should help when I get a proper alignment. 2) I chocked the front wheels off and put the rear on jack stands and did some testing. My 74 MPH vibration still happens on jack stands and I also discovered idling in 6th on jack stands (essentially 40 MPH, 500 RPM for the wheels) the right wheel had some vibration to it and the left rode straight. Swapped wheels and same thing (now-left vibrating, now-right straight), so the wheel is giving me trouble. I’ll have that either fixed or replaced. No vibrations with no wheels and just the axles spinning. Must’ve been some trouble with it before my wipeout anyways from maybe a curb or aggressive bumps. 3) I’ve no words for the rear sway bar bushings, but the fronts are pretty hardened and need replacing. 4) The LSD clarification was helpful. Does that count even if you spin one wheel and the other wheel stays still? Still checking for a bad diff because that happened when I put the rear in the air.

All in all I agree with the consensus that the panhard is to blame and will be rectified, as well as my slightly warped wheel. As far as front uneven bushings, I had this issue before bushing replacement so for now that’s out of the question.

 

Quick bump. Since my last post I’ve installed a UMI double-rotojoint adjustable panhard bar and had my wheel straightened. The bucking is less, but not gone. I also found what I believe to be the source of my vibration: the driveshaft U-joint caps move in and out ever so slightly while bolted down on the axle end. The straps aren’t doing their job and will be replaced, and eventually I’ll finish my bushing job and get an alignment and come back with more updates.