73 Dart Swinger Vibes and Solution?

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mario03srt

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All,

I own a 73 Dart Schwinger 360 cu in with a 727 and a 8 3/4 rear with a 489 hog head, also SS springs. It's is all new to include the drive shaft and the diff internals. The car has low low miles since completion. It's only been out on the freeway twice this year up to 70 or so mph.

I have noticed that I have the dreaded rythmic vibes/harmonics at freeway speed. I have spoken to some and have used this site for info and details, thanks! Before I press forward with any changes here is what I learned and pics of the associated drivetrain.

A local expert told me after seeing a pic of the yoke engagement into the trans, he indicated that this was the issue at least initially as it appears to be too far out.
Dart 727 Yoke.JPG


So I measured the shaft and it came out to be 49" and 5/8" approx. Subtract 2" from pic see below.

Old Shaft length before yoke referenced.jpg
Old Shaft length before yoke referenced rear.jpg


I inserted the Yoke and pulled it out 1" and remeasured. The results are 50" as best as I can tell from the tape.

Shaft length with correct Yoke insert.JPG

Shaft length with correct Yoke insert from rear.jpg


So the previous measurements I did not have the Yoke inserted properly before measuring when I had it made. It's appox 5/16 to 3/8 too short.

Sooooo, before I trot off and have my shaft lengthened I have some questions? Is this out of spec condition really enough to cause the vibes? I can see it being quite possible but I need some conformation/peace of mind.

Another issue that I looked into is the angles of the rear pinion and the trans.

Front Trans Output Shaft angle.PNG
Rear Pinion Angle.jpg


From what I have read with the SS springs that the rear angle is supposed to be 7 deg? As you can see, the front output shaft is basically at Zero angle. It's not shimmed and the car did not have the vibe issues AFAIK before the rebuild of the car. Is this angle correct and an acceptable point to measure the shaft from?

Pinion.JPG


Thanks for any input you have on this worn out subject, sorry but it's my turn in the barrel so to speak and don't want to waste and $$ and time.

Again, Thanks Marion

Old Shaft length before yoke referenced.jpg
 
Driveshaft looks maybe a hair short, but, I don't think it's enough to cause a vibration. Your angle off the trans doesn't look good to me though. You might try adding some spacers under the transmission mount to raise the tailshaft giving it more of an angle and give it another test drive to see if that changes anything. If it cures the problem, you'll need to make a more permanent modification, but, the shims will work for test purposes. Adding the shims will make the driveshaft even shorter, so, you may end up having to do both, including changing the rear pinion angle. Depending on how thick your test spacers are, it may change your shift linkage adjustment, so, you may have to readjust that too.
 
If you are going to spend the $ to get it lengthened I wouldn't for 3/8 ". I would shove it in the trans and see how much room between the rear u joint and the yoke top of flange, most all my cars if I push it all the way in the trans it is a tight fit to get in the rear yoke.
 
Just Incase you don’t know this:
To measure for drive shaft length-
make sure you jack up the car by the rear axle and put on jack stands the axle as well. If you lift the car and the axle hangs your adding to the below 0 axle angle and the length of the drive shaft. I personally like about 1” exposed of yoke out of the back of the transmission but with the large below seal you need less exposed like 1/4” because the actual seal is inside the bellow about 1”.

again- just Incase you don’t know.
Also, I had that rymic vibration at a certain rpm. I changed my center chuck to a different ratio (3:23 to 3:91) and it is gone. Mine was more of a drone at like 2200rpm from my mufflers. Load and rpm change made all the difference.
 
Thanks, Yes the car is on stands. I have returned the DS to the Ohio Driveline Shop and splained the issues. It has very little use, so they will re-spin and check the balance and go from there. They did confirm it's about 1/2 inch under optimum measuring center to center. I see that on the old worksheet they had me measure from the output shaft to rear center. That's why I did not remember indexing the yoke on the tranny when ordering initially. Maybe they will cut me a deal to make it exactly right!

Marion
 
Driveshaft looks maybe a hair short, but, I don't think it's enough to cause a vibration. Your angle off the trans doesn't look good to me though. You might try adding some spacers under the transmission mount to raise the tailshaft giving it more of an angle and give it another test drive to see if that changes anything. If it cures the problem, you'll need to make a more permanent modification, but, the shims will work for test purposes. Adding the shims will make the driveshaft even shorter, so, you may end up having to do both, including changing the rear pinion angle. Depending on how thick your test spacers are, it may change your shift linkage adjustment, so, you may have to readjust that too.
cudamark,

I was looking this over tonight. I had my driveshaft rebalanced and if that does not cure things I'm looking at the shims you mentioned, once I re-measure the angle.

So I'll need to drop the cross member and remove the 2 mount nuts and put in the shim between the trans and the mount raising it up? Any idea what thickness? I saw some factory pieces on ebay, are these what the factory had in there?

Thanks,
Marion

Mopar Rare Trans Mount Rectangle Spacer Shim NOS Quality Pair Hemi 440 383 340 | eBay
 
No way to know how thick the shims need to be without knowing the driveshaft angle. I start with something about 1/2" thick and see if it changes anything. For test purposes, I'd just get some washers and stack them until you find the thickness that works. then make a permanent one piece plate if this cures the problem.
 
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