Duster Driveshaft length causing Vibration?

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scott mattingly

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bardstown, ky
All,
Looking for help in troubleshooting a driveline vibration on my 74 Duster w a 360/727.(car had a few problems when I bought it including a slipping trans. & a bad driveline vibration at ~55mph). The past few years of ownership have included: confirming proper driveshaft balance & U-joints, rebuilt 727, new engine(a stroked internally balanced 360), new tires(balance checked twice), & some shimming work to correct poor alignment btwn trans output and diff.(they are within 1/2-deg of parallel). CURRENTLY: vibration has been reduced but still starts to be an issue ~55mph & drives me nuts.
Car was originally a 6-cyl./904 car & was converted it to 360/727 with an 8-3/4 rear by P.O. My latest research is leaning me to think that the driveshaft is simply too short and/or trans. pinion is not the correct one for my set-up. I AM thinking that these shortcomings are resulting in my driveline vibration. (See pics included).
Details:
a) Distance between C.L. of U joints from the pinion (fully seated into trans.) & the differential input is 45-3/8"
b) Length of driveshaft btwn the centerline (C.L.) of U joints at each end is 44" (This results in sliding the pinion out 1-3/8" from its fully seated position to reach the driveshaft.
c) As the pics show, when installed, my pinion extends just over 3" out from the end of the trans. (I do not have the full length measurement of the pinion but understand that there are at least 2 different lengths commonly used)
So,...after reading about a hundred different posts here concerning driveshaft length and proper measurement procedures, here is what I am thinking:
1) My 44" driveshaft is too short. (as per various posts, the standard length for my set-up should be 44.99" and exact target length should be determined after a full set of specific measurements)
2) The fact that my pinion shaft has 3" of length showing is Way too much (As per various posts, optimum should be btwn 3/4" to 1-1/4"...additional length "could" result in vibrations...)

QUESTION: Does my reasoning seem solid? Has anyone experienced similar driveline/driveshaft vibrations as a result of my car's current set-up? What other troubleshooting steps can I take?

THX-Scott

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According to the FAQ on Dennys Driveshaft website:

How much slip travel should I have?

In most applications the minimum is 3/4 inch and more than 1 inch is not necessary. This is measured with the rear suspension supported on safety stands or with the rear end on the ground.
Click to expand...

Dennys Driveshaft FAQ Page Frequently Asked Questions and Answers

Denny's Driveshaft Specialist has answers to frequently asked high performance driveline questions. Very informative for the racer or street rodder who needs answers
www.dennysdriveshaft.com

You may have other issues as well, but you clearly have way too much travel on the slip yoke. You push it all the way into the transmission and then pull it out 1 inch. That is where the yoke should be when the driveshaft is bolted to the differential and the car sitting on the ground on the tires.. Fix that first and then try it, before making any other changes.
 
Yes, you should address the excess stick out by either replacing or lengthening the drive shaft first and then see how it drives. I would also inspect and replace the tail shaft bushing as needed. Since there's so much stick out, that whole front u-joint is wanting to follow an elliptical path, instead of just spin like it's supposed to. That's what causes the vibration. If you fix that, the vibration will likely disappear.
 
Gents, Thx for the feedback so far...I have also fully reviewed the Denny's driveshaft web site material from my previous research and found it very helpful.
I do intend to pull the driveshaft in the near future and re-asses the full set-up using your advice and what is posted at Denny's site. Others in my local club have worked with a shop called "Republic Diesel" in Louisville and I intend to engage their services in producing a new(longer) driveshaft and re-assessing if my Yoke is "O.K.?" (Anybody have experience with them?)
If anyone else has other ideas/recommendations, bring them on....In any event, I will re-post updates as I make changes..

THX-Scott (Bardstown Ky.)
 
I just measured the drive shaft from my 71 demon 340 car with 727 and 8 3/4 diff. I used this shaft in my 74 duster with a big block & 727 & 8 3/4. It was a perfect fit. 45 inches ctc of the u joints. Never any vibration. I agree your drive shaft is 1 inch too short. Lower the pinion at least 2 degrees more than the trans angle. Kim
 
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You are not referring to the "pinion", you are referring to the "slip yoke" and I agree, it's likely out too far. a good tight one with no spline wear may be OK that way, but it seems way excessive

Also, you say the transmission centerline and rear axle pinion "are within 1/2 degree" This too may be wrong. Under power, acelleration the rear axle will "wrap up" the springs and come up. You may want it down as much as 2 more degrees
 
THX all for the feedback (Yes my terminology was poor in that I was referring to the "Slip Yoke" in my original thread notes. Was typing it late at night & my brain was fuzzy.....)
Regarding the alignment btwn. the rear axle pinion and trans. centerline: when I measured my original set-up, my trans. output centerline was down ~2.5 to 3-deg compared to the driveshaft and the rear axle pinion was nearly dead-level with the driveshaft. As part of troubleshooting my vibration issue, some friends and i installed a set of 2-deg wedges on the leaf spring perches so that the rear axle pinion is now at ~2-deg upward compared to the driveshaft. This work made basically ZERO difference in the vibration at 55mph. (Not sure if it really matters, but my car also has what appears to be non-stock leaf springs...they only have 4 leafs from the axle forward with the 3 bottom leafs supporting the rear of the car. The car does sit a little low in the rear and feels "stiff" riding in the rear to me.)
 
All, Just wanted to close this string out in case it helps someone else in the future. Here is a summary of details/specs/results:

1) Distance from C-C of my fully inserted yoke to the differential U-joint mount is 46-1/4"
2) Length of my original driveshaft was 44". (This WAY short driveshaft was not allowing my yoke to reach the bushing inside the trans casing, which resulted in LOTS of vibrations)
3) My shiny new steel, 3-1/2' dia., driveshaft fabricated by Republic Diesel in Louisville, Ky is 45-1/2" long. (This gives me 3/4" of travel between the c-c measurement above for installation and allows for full engagement of the Yoke into the bushing inside the trans. casing.)
4) I installed the new shaft yesterday and tested it on the road at ~65mph. My vibrations are GONE! car run smooth at highway speeds for the first time since I have owned it(~2-1/2 yrs). Problem solved...Hoo-Yah!

THX for the feedback and support. Also, wanted to plug Republic Diesel for their work and affordability(Total cost was $297.00). For this price, they re-used the 2 end pieces(U-joint mounts) from my old driveshaft, installed 2 new U-joints, checked the whole shaft-yoke assembly for balance, applied a nice coat of gloss black & had it done in a week.

-Scott
 
That's a very decent cost for a new drive shaft.
 
scott mattingly said:
THX for the feedback and support. Also, wanted to plug Republic Diesel for their work and affordability(Total cost was $297.00). For this price, they re-used the 2 end pieces(U-joint mounts) from my old driveshaft, installed 2 new U-joints, checked the whole shaft-yoke assembly for balance, applied a nice coat of gloss black & had it done in a week.

-Scott
Click to expand...
In a rare case of weird economics, a shop in West Sacramento does the same for about half that price. and has it done in a day.
Glad that you got yours figured out.
 
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