Drive Shaft: Rear end shimmy

[Springstreet]

My Avanti has a slight shimmy from the rear end (coasting and under under power) that I can feel in the steering wheel. I remounted and balanced the tires and the replaced the (chipped) brake drums ... but still the vibration. Because of a transmission swap, I do have a new drive shaft that was installed with a +/- 4% vertical angle to the original Dana rear differential and new leaf springs. After that, the front wheels were aligned to spec. My question is: could the driveshaft be out of alignment horizontally thus causing the u joints to bind ... or could the leaf springs have been mounted on one side to the wrong front hole?

[doofus]

My bet would be a warped or bent driveshaft. maybe you have lost a weight from drive shaft. an old trick is a few large hose clamps together on shaft. mark position, try it out, move clamps 1/4 way around shaft and retry. this works best with car on 4 stands, 2 under front suspension, 2 under axle. good luck Doofus

[Warren Webb]

I would set the car up on jack stands as doofus said, then with the rear wheels off, run the car through the gears. If you feel the vibration then you know it's not the tire/wheel combo. I would check the pinion angle too while your there to make sure that isn't a factor. If the driveshaft is bent it will show now too. I take it your car has flanged axles, correct? When was the last time the rear wheel bearings were packed?

I'd have to go out to look at my Avanti to see which holes the leaf springs are mounted in. That could possibly give you a very slight pull but I doubt seriously it would give a vibration.

[PackardV8]

My Avanti has a slight shimmy from the rear end (coasting and under under power) that I can feel in the steering wheel.

The suggestions received are good. Should it turn out not to be the tires/wheels/brakes/bearings/driveshaft, then start checking the engine and transmission mounts. What year is yours? If a '63-64, the Stude V8 is so tightly packaged in the Avanti, it's possible to have exhaust manifold to steering box interference, bad engine or transmission mounts; all of which can come through the steering wheel.

jack vines

[karterfred88]

All sorts of potential areas to look at. Trans swap-is the new trans longer than original? Changes the angle at the front U joint with a shorter drive shaft, may have to shim the trans mount higher to restore the front u joint angle. Driveshaft as per above suggestions as well as "clocking" of the U-joints to each other on the new shaft ( I think I read somewhere, that because the drive train is a "broken back style" there was a different offset-not 90 degrees as logic would say they should be. Also, is it possible one of the u-joints got damaged when installing, or not centered exactly. That may show up as you watch the drive shaft when jacked up. I know this sounds wrong, but did you check for "out of round" tires when balancing-I've had some really "oval" tires that even moved the steering wheel back and forth while rolling at slow speed, that were "balanced" but shook the steering wheel like mad. The spring mounts are probably not the cause, only would let the car "list" to drivers side when only the driver is in the car. Another strange thing I notice on my older S10, that isn't driven much, is that the rear end vibrates for the first few miles then goes away, I think because the tires "flat spot" from sitting in one place too long, I don't feel it from the front tires because I think the greater front end weight flexes the tires more, and gets rid of it sooner. May be due to the tire belt material. Good luck, hope it's an easy fix.

[Springstreet]

Thanks for the input ... I'll put it up on stands and check for vibration without the wheels on. As to the rim/tire/brakes/etc. ... before they would chrome my rims, they spun balanced them (and I had to get one new one!) and my Kumho tires are also new (and hopefully not out of round) and after mounting were checked for balance twice. The vibration continues after driving so I don't think the problem is flat spotting. My swapped LS-2/six speed used new Summit engine brackets so that the new stock engine/transmission rubber mounts fit in the original Avanti mounting holes. But, my new composite springs (unexpectedly) needed lowering blocks to get close to the original stance (still a little high) ... maybe this (and the front mount bracket) made a problem with the front u-joint angle that we did not properly allow for (is 4 degrees enough?). The new (bigger and shorter) driveshaft was (in theory) properly constructed and I don't see where a balancing weight has fallen off. My 77 has the later style axles and we believed they didn't show signs of wear (but I guess I should recheck). Oh, and we didn't rebuild the 3.07 Dana differential.

[jclary]

Have you driven it on any road besides...SPRINGSTREET?

Sorry...but I couldn't resist the irony of your forum name along with the subject of this thread.

[doofus]

Manual calls for 4 1/2 to 5 1/2 degrees angle, believe that's at both ends. luck Doofus

[karterfred88]

I thought I had read about joint phasing on here at one time-I did:http://forum.studebakerdriversclub.c...-configuration
Remember that on original Stude cars the engine is also angled to clear the steering gear creating not only the down at the front, up at the rear, but also a kink from the trans output to the rear end input. Gotta love this stuff!!!