Drive Shaft: Avanti rear end vibration

[Springstreet]

I need advice. My Avanti vibrates in the rear ... enough to cause the shift knob to shake. I have rotated wheels/tires, had the custom driveshaft balanced, and replaced the rear drums. The car has composite springs mounted with small lowering blocks, new SI trailing arms and new Koni shocks.
My LS engine is mounted to the stock Avanti engine and transmission supports ... the 3.07 rear end is stock.
Is it possible that the driveshaft mounting flanges of the transmission and differential are not parallel ... because of the blocks or non-stock transmission? Could my rear end not be perpendicular to the driveshaft ... because the frame is bent or leaf spring shackles not aligning the solid axle properly? Could I have used the wrong hole for my front leaf spring ... there are three and I used the lower rear hole on each side?

Any thoughts?

[sweetolbob]

IIRC the drivers spring goes in the lower hole and the passenger in a higher one. I think mine only have two holes but it's been a while.

If you are convinced it's the rear end and it's speed related, the first thing I would do is check the run out on the rear tires and axles as you seem to have done most of the other obvious fixes. Square should be easy to check but I'd think that a bent frame or front mount position would cause other problems than a speed related vibration. You might also put it on jack stands and have someone accelerate while you watch the rear tires et. al to see if anything is visually an issue.

Just a couple of thoughts. Good luck,

Bob

[StudeRich]

What are "New SI trailing Arms"?
Since Studebaker never used "Trailing Arms", Studebaker International would not have them.

[bezhawk]

I bet it's a terminology point. I'm sure he means radius rods, as Studebaker called them.
I think he has a drive line angle problem. The factory used a spacer under the drivers side if it was an automatic, and Two spacers under the rear mount (between the bell housing and mount) if it was a manual.
There are wedge shims available at speed shops to get the pinion nose of the rear to the same angle as the front U-joint. If the front U-joint is 2 degrees down, then you want the rear U-joint 2 degrees up. In other words, the angles can't diverge, or converge they should be parallel.

[dynolou2]

So Brad, how do you get a good measurement on the angle of the front u joint? The X frame makes it difficult, of course having one of the few 1985's with the T - 5 manual trans makes it a less than fully developed fitment. Lou Cote

[Ron Dame]

I suspect driveline alignment, either phasing of the U Joints, or as Bezhawk shows. It is also possible it is a bent axle, and if it has tapered axle shafts, one could be twisted.

[spokejr]

Let's back up a moment.

I'm guessing you just made an engine swap? Did the car run and drive before, in otherwords, do you know how the car drove before the LS swap?

[grobb284]

I bet it's a terminology point. I'm sure he means radius rods, as Studebaker called them.
I think he has a drive line angle problem. The factory used a spacer under the drivers side if it was an automatic, and Two spacers under the rear mount (between the bell housing and mount) if it was a manual.
There are wedge shims available at speed shops to get the pinion nose of the rear to the same angle as the front U-joint. If the front U-joint is 2 degrees down, then you want the rear U-joint 2 degrees up. [ATTACH=CONFIG]67836[/ATTACH]In other words, the angles can't diverge, or converge they should be parallel.

Brad,
You are correct when dealing with most cars. However Studebaker, Land Rover, and others beg to be different. They do in fact converge, but are equal angles. A Studebaker may point 5 or 6 degrees down at the transmission, while the nose of the differential may point down the same amount. This is discussed in the Avanti shop manual. Having said all this, when we built our Avanti frame we set up the driveline conventionally as you discussed.

[Mike Van Veghten]

StudeRich wrote -
What are "New SI trailing Arms"?
Since Studebaker never used "Trailing Arms", Studebaker International would not have them.

I believe just Springstreet used the wrong term, the Avanti has traction bars attached to the top of the housing tube , then to the frame.
Could be considered "Trailing Arms"...sort of...I guess..!?

Mike

[bezhawk]

So Brad, how do you get a good measurement on the angle of the front u joint? The X frame makes it difficult, of course having one of the few 1985's with the T - 5 manual trans makes it a less than fully developed fitment. Lou Cote

You can measure the angle, anywhere along the driveshaft. It remains constant, unless it's a flex shaft like a Tempest. . Use one of these magnetic protractor angle finders.

[dynolou2]

So I do have one of those protractors, so if I measure the angle of the drive shaft say 2 or 3 degrees . Then the rearend should be down 2 or 3 degrees. This is being measured with the car supported on heavy duty jackstands under front A frames, the rear supported on the rear sway bar mounts on the rear spring plates. Lou Cote

[Springstreet]

Thanks for the responses. My mechanic was (is) convinced that driveshafts should not be in line with differentials ... so (in theory) he made mine 2 +/- degrees off. (Note: the car didn't run before the LS was installed ... without wedges). However, we changed lowering blocks (composite spring arch was wrong) and the front (and rear?) springs settled over time. Also, I have read that as a drive line is torqued, the differential will lift ... which changes alignment. And, it would seem that a live axle will constantly be changing driveshaft alignment during each bump cycle ... and this leaf spring movement will not be completely vertical as compared to the transmission. So, (after I raise the passenger side leaf spring to the upper hole, keep the traction bar/trailing arm unchanged and check the axle and run out) it sounds like if I abandon the idea of installing a Jag IRS, then I can cut down one side of my lowering block to get the pinion nose and u-joint in line ... and hope for the best.