irocman7

I couldn't figure out whether to put this into drive train or suspension, I went here because the problem revolves around my panhard (I believe).

So I have been tracking down a vibration in my Iroc for many years. It is only at high speeds (worst starting at 60 best starting at 70, more on that in a sec). It is a drivetrain vibration high speed almost a roar. I have been through various wheel/tire combos to no avail.

I have been all over trying to fix it. I have swapped out all components in my rear at different times, the rear bushing out of the T56, new drive-shafts including a 4th gen aluminum, now a custom 3 inch steel. It has a 9 inch with a aftermarket billet yoke which required a drive-shaft 1 inch longer (was hoping running a shorter drive-shaft was the problem but it wasn't).

The pinion angle was a concern, I followed Sphons directions originally measuring driveshaft/pinion angle (using their torque arm) which I now know was wrong and adjusted it based on the transmission output angle relative to pinion. No change in vibrations.

While having the rear end in the air I noticed my rear alignment was a little off (so I thought) so I adjusted the panhard towards the drivers side. Took it for a drive and the vibrations were about half as bad and moved up to starting at 70 mph instead of 60. This was the first time I have ever noticed a difference in the vibration from anything I have done.

I checked where the tires just sitting on the ground and realized I was about a 1/2 inch too far to the drivers side, so I adjusted again so it was centered while sitting on the ground. Took if for a drive and the vibrations are twice as bad and back down to starting at 60 again.

Everything I have read on the rear end alignment says panhard adjustment won't affect vibrations but it seems it is not true at least in my case. I am wondering if I adjust it past 1/2 inch biased towards the drivers side would the sound decrease, going to try this next.

Any theories on what is going on here? Should the rear be biased towards the drivers side considering the design of the rear? I know pinion angle being off can produce vibrations, why couldn't horizontal angle between tranny and rear pinion cause vibrations as well?

Here is the setup

LT1 T56, Spohn cross member/Torque arm (set to -2 pinion angle), Spohn poly transmission mount, custom drive-shaft balanced, Currie 9 inch, Lakewood adjustable panhard, J&M tubular LCAs, Comp Engineering 3 ways

Any help would be greatly appreciated!

roysatikas

iTrader: (1)

I had a similar vibration drive me nuts after an engine build and keep thinking I screwed something up. Long story short I had a bad brand new TQ converter from B&M. Swapped the TQ out and problem solved. I know you don't have a modified 700-R4 like I do but perhaps something in your engine/trans matchup is not balanced or bad. Or something internal to the trans? I have many of the same rear suspension parts as you without any issue. Good luck as I certainly feel your pain!
Roy

JamesC

iTrader: (5)

I couldn't live with the vibration caused by the Spohn poly mount. I finally swapped back to OE rubber. The mount may have nothing to do with your issue, but the following may be, at least, an interesting read:

https://www.thirdgen.org/forums/tran...ison-pics.html

JamesC

irocman7

I have a T56, I took the drive-shaft out and ran it up to 100 mph with no vibrations at all. Thanks for the idea though.

irocman7

I remember reading that post, and it was the reason I bought the Spohn mount. I can't remember if it made that much a of a difference (6 years ago). Have you had problems with the OE rubber?

JamesC

iTrader: (5)

Zero problems. Smooth as silk.

JamesC

irocman7

Just to update, in case someone else runs into this similar problem.

Aparently I had a non visible crack around the pilot bearing in my 9 inch thirdmember, I couldn't see it. It finally cracked off and exploded the bearing. The vibrations got worse, but I made it home and took it apart.

I bought a new case, and the vibrations are gone, after so many years.

Bow_Tied

iTrader: (1)

Wow, who'd have guessed... thanks for the follow up.

articwhiteZ

iTrader: (56)

how was the yoke...did you use a new one..or the old one

irocman7

It was a 1350 billet yoke, it was fine I just reused it. Why do you ask?