I dont understand how a 1" longer driveshaft would fix the problem only because I shifted the rear axle forward to make up for the short driveshaft. Is there some geometry being thrown off because I dont see how that makes sense...

 

I think your correct and will only know once the tailshaft bushing is replaced.

 

One more thing. When setting the rear pinion angle, make sure you have at least two folks sitting in the car for weight.

 

+1 on the 1/2" of stacked grade 8 1" washers on top of the trans mount. My vibrations got a LOT better and it gave me better clearance for my DS loop.

Hope this helps...

Rob (Bad30th)

 

AH HA! So there is a bushing in the tailshaft I believe thats where some of my vibration is comming from. Anyone got a part # so I can look it up and have it replaced? I'm also on a used tailshaft housing because I busted my 1st one.

 

So I've put some miles on the car now. The vibration is much better, but still there. At 130mph the car is smooth as glass and no vibrations at all. It vibrates from 60 to about 105mph then the vibration goes away completely. Sound like a pinion angle problem?

 

The 60 MPH sounds like DS imbalance to me. I had it once too.

 

I will have it checked next. Though I would think an out of balance driveshaft would get worse the faster you went, especially since its a brand new PST driveshaft :\

 

Not necessarily. It can phase in and phase out at various speeds or shaft RPM's.

 

Lets look at this from another angle.

After recently tweaking on my pinion angle a little bit, I noticed that I still have a tiny vibe around 70MPH. I determined that I need a new set of eyes to look at the setup since I have gone brain-dead trying to set it myself. I brought a mechanical engineer from work to the house and we noticed some things that have been complicating the setup.

My current setup is:
Rear of the tranny output shaft pointind down -2.3°
Rear of the driveshaft pointing down -1.7°
Rear of the pinion poiting up +1.1°
Front u-joint operating angle is .6°
Rear operating u-joint angle is 2.8°
The drive line looks like this from the right side \_/
Two details wrong here. First thing wrong is the front u-joint angle is backwards ( _/ from the right side of the car) and the second thing wrong is the two u-joint operating angle differences are >1.5°. The ideal front u-joint angle should look like this ( /¯ from the right side of the car) and the front operating angle should be closer to the rear operating angle than what it currently is.

We are thinking that the rear coil springs have sagged and this is causing the problem. We then raised the rear of the chassis (not the axle) up one inch to simulate what new rear springs might look like and this is what we came up with.
Rear of the tranny output shaft pointind down -1.4°
Rear of the driveshaft pointing down -2.3°
Rear of the pinion is level 0°
Front u-joint operating angle is .9°
___________Rear operating u-joint angle is 2.3° __
The drive line looks like this from the right side _/

BOTH OF THE ABOVE ANGLE FINDINGS USED THE EXACT SAME TORQUE ARM ADJUSTABLE SETTING.
The above setup is what you want to have for perfect driveline geometry. However in order for me to achieve this I need to install new rear coil springs that will raise the rear of my car by at least an inch. Any more than that should be a bonus to even further dial it in just right.

Clearly, rear spring sag will cause headaches when it comes to setting the driveline angle so be sure to facter that in. We were blown away at how simply raising the chassis 1" (not the axle) made that front u-joint flip the opposite way into the position it is supposed to be. I do believe the rear is lower than it used to be from measuring the distance from the tire to the fender plus it just appears lower to the eye.

I am going to buy new springs tonight and check things again.
I will post my results ASAP.

 

dude you better post up your answer to this haha. I am currently in the same exact boat as you. I am trying to hunt down some stock springs to swap into my car for the same exact reason. Let me know what you find out

 

My car used to be much lower when I had my stock 10 bolt before I changed out the springs and it was vibration free. Now knowing that the pinion is lower on a 9" than on a 10 bolt, wouldn't that kind of negate the fact that your car may be too lower? And can't this problem be fixed by just adjusting the pinion angle at the rear axle to compensate for the car being lowered? Just set your pinion angle equal to your engine angle.

 

i tried that and got the same vibrations. I think the concept is the u joints need to be at a certain angle minimum. I adjusted my read to be what it needed to be and it started making an odd noise if I were to just let off or get on the gas a very small amount rolling... if that makes any sense. My driveshaft was damn near flat with a very small angle pointing downward.

Also looking at just about any rear end, they all point upward... i dont know if this is for a reason or not but mine is point damn near horizontal. So raising the back end up will change the angle of the trans output thus changing the location in height from where the trans out goes and diff in is. Allowing us to point the rear end up and get the desired working angle for the u joints.... heck that makes more sense in my head than re-reading it haha

 

I bought some new OE spec springs and installed them. They raised the rear of the car up almost an inch like I hoped they would. I also made a minor tweak to the torque arm adjustment.

Here are my current angles:
Rear of the tranny outputshaft pointind down -1°
Rear of the driveshaft pointing down -1.7°
Rear of the pinion is level 0°
Front u-joint operating angle is .7°
___________Rear operating u-joint angle is 1.7° __
The drive line looks like this from the right side _/
On paper I would expect this to be nearly perfect and a good deal different then they way I previously had it. I wont be able to drive it till tomorrow. I will post my results.

 

When the torque arm is adjusted, it is rotating and not moving linearly up and down. The newer taller springs are making a linear adjustment independently of the angle adjustment. The lower pinion compared to the 10 bolt is what is frigging things up here. This is also the reason I had to raise the back of the tranny up, to counter the lower pinion position.

 

Wouldn't a lower pinion mean that you would have to lower to car, thus bringing the pinion back UP to where the 10 bolt pinion was?

 

Oh no. I am starting to go brain-dead again from thinking about it too much. This **** will make you nugging futz in a hurry.

All I know is I have spent countless hours jacking with it with the old springs and NEVER found the "ideal" geometry. I kept going in endless circles.

With the new springs, I now I have ideal geometry again at least on paper and have had it confirmed by an actual mechanical engineer. The front joint is now the correct angle which would not happen period with the old springs.

The only other thing that would help would be to get the rear of the tranny up even higher. Right now I have it 1/2" higher than stock and if I raise it any more, there wont be any threaded stud left on the bottom of the rubber mount to put the nut on.

 

Haha you're telling me, I've had this vibration for 2 years now, and what caused it? Spending 4,000 bucks on it. YAY!

 

Update.
Success. Absolutely no vibes up to 100MPH period. Clearly the new rear springs helped a good deal to get the ideal setup. Might look into that guys if its still giving you fits.

 

You said you are running an OE spec spring. Does that mean stock ride height? That would suck :\