I'm so relieved.

[rhipsher]

I've had a vibration problem coming from the rear of my 90 Z for the past 1 1/2 or so and it's bugged the hell outta me as some of you already know. I've tried swapping the tires. No luck. I've replaced the rear wheel bearings. No luck. I've had a four point aligment done. No luck. Checked all of my bushings. Nothing loose. No luck. It's been bad enough that I couldn't even look out my rear view mirror because it shook so bad. I don't care how fast the car is it got to the point where it was no longer fun to drive anymore because above 60mph it felt unsafe. So I decided thats it I'm going to find and fix this problem if it kills me. I've taken this car to 179mph many times and now I can't even take it above 60mph? F that. And I found it and fixed it today damn it. It was my fault all along. A total rookie mistake that I can't believe I made. But it was something I had suspected all along. The last time I replaced my throwout bearing/fidanza and clutch disk I installed my drive shaft 180 degrees out. And let me tell you something! That makes all the difference in the world. When I spun the wheel by hand and watched the drive shaft I swear it looked like it had 1/8 in run out. When I rotated it 180 and bolted it back together and spun it by hand again it ran totally true. The shaft and the ujoint rear diff already had white paint on them to mark proper orientation and I swore I installed it right. I must have been in a rush. But it runs totally smooth now. No vibration or roaring noise. I felt like slapping myself over that one.

Thanks for sharing this valuable info and glad to hear it worked out for you.

[flyin ryan]

Simple mistake many have made over the years, don't beat yourself up over it .

[carter200]

Good to hear all is in proper orientation now

[Paul Workman]

Thanks for sharing!

However, balance is one thing...but runout is another. Runout... Hmmmm.... That surprises me. Unless something was a leeetle bit cockeyed when it was installed, I wouldn't expect you to have runout. (Just curious thinking out loud here.)

Glad you found the problem tho! I hope I marked mine before we (Lee and I) took it apart last August. I guess I've got a 50-50 chance of having the same problem (only w/ my luck, it is more like a 90% chance of getting it wrong!)

P.

[rhipsher]

Originally Posted by Paul Workman

Thanks for sharing!

However, balance is one thing...but runout is another. Runout... Hmmmm.... That surprises me. Unless something was a leeetle bit cockeyed when it was installed, I wouldn't expect you to have runout. (Just curious thinking out loud here.)

Glad you found the problem tho! I hope I marked mine before we (Lee and I) took it apart last August. I guess I've got a 50-50 chance of having the same problem (only w/ my luck, it is more like a 90% chance of getting it wrong!)

P.

Runout and balance are the same thing. I deal with it almost every day. I would say in this case it was conical runout . Coming from the transmission it was pretty true but the further down the line towards the differential it got the more it ran out. But if you get it wrong you'll damn sure know it. But only takes a few minutes to switch it around.

[Paul Workman]

Originally Posted by rhipsher

Runout and balance are the same thing. I deal with it almost every day. I would say in this case it was conical runout . Coming from the transmission it was pretty true but the further down the line towards the differential it got the more it ran out. But if you get it wrong you'll damn sure know it. But only takes a few minutes to switch it around.

While I don't agree that runout and balance are by strict definition necessarily the same thing, (allowing for variances in driveshaft wall thickness shifting the CG slightly), you're right: runout will result in CG shift due to the eccentricity - no argument thar!. but I always thought the driveline vibration was a function of the centerline of the driveshaft and that of the yoke(s) not necessarily being perfectly aligned. That is OK as long as the error is small and a counter weight can be added to the drive shaft to compensate for the collective (driveshaft & yoke) offset of CG.

Course, if the counterweight is on the opposite side; i.e., the drive shaft is 180º out of position...Um yeah the CG error is doubled. But, my surprise is that the runout would be so bad that you could see it w/ the naked eye...yikes!

P.

[rhipsher]

Well when I get a blueprint of a round part I have to turn and it has several o.d & I.d's and it says diameter A B C & D have to be concentric to eachother within .0015 TIR, that would be the same as the drive shaft not running concentric to the yoke and would cause it to be "Out of balance." Right? Now of course you hear the word balance used more when talking about tires because they can change their shape. Now when machining steel it is what it is. There is no in between. Its either in tolorence or its out.
So I hope that explains it. There's not counter weight that's gonna fix an 1/8" concentricity runout.

[A26B]

Not to add fuel to the debate about runout vs balance, but they are not necessarily the same thing. Run out most always results in imbalance in rotating mass when all diameters are concentric. However, eccentric imbalance could theroetically be offset by mass, such as increased wall thickness (mass) if it were opposite the runout.

Mass imbalance, such as that which could occur in the driveshaft, can be the result of any or all of the foloowing; (1)tube manufacturing tolerances, i.e. wall thickness variations, (2)tube/yoke welds or (3)yoke forging and machining variations.

Vibration aside, if you had 1/8" runout that was fixed by rotating the driveshaft 180 degrees, it was either an installation issue or some really bad parts.

Just my 2 cents worth...

[rhipsher]

Absolutely Jerry. I think the drive shaft could have a wall thickness variance and they may have purposely welded the end cap out of faze to help compensate for it. Which would explain why at 0 degrees it has runout and at 180 degrees it runs true. That makes sense to me. And I suspect my Z is not the only one that's like that. Some may be worse than others.