vibration after 3.23 to 3.91 swap

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skep419

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1969 Dart 360, 727, 8 3/4. Noticed bad vibration after gear swap. Doesn't do it in neutral or any other gear besides the top end of 3rd. Suppose it could have done it with the 3.23 but its not often that I drive 130+mph. hard to tell in the video but its a viscous vibration/shake thats felt in the floor/steering wheel. It changes on decel which makes me think it has something to do with the driveshaft.

 
1969 Dart 360, 727, 8 3/4. Noticed bad vibration after gear swap. Doesn't do it in neutral or any other gear besides the top end of 3rd. Suppose it could have done it with the 3.23 but its not often that I drive 130+mph. hard to tell in the video but its a viscous vibration/shake thats felt in the floor/steering wheel. It changes on decel which makes me think it has something to do with the driveshaft.


I had a similar problem
My rear u-joint wasn't perfectly centered in the third member yoke.
I carefully centered it...even measured it with calipers and the vibration went away.

Jeff
 
Are the balance weights still on the drive shaft or did any get knocked off during the swap? Look for roughly 1-1/2 inch long by 1/2 inch wide y 3/16 inch thick tab weights plug welded to drive line. Is the drive shaft straight and not too long? Were bores clean in the pinion yoke to keep from crushing bearing cups to an oval shape, flattening needle bearings ? Is it a crush sleeve rear end? Is end play on axles correct? Its hard to tell by a video,but it sounds suspiciously like a rear transmission driveshaft bushing that has an egg shaped bore or a pinion bearing. Was it a gear swap only or an entire rear-end swap? Hope this helps, but with out actually being able to crawl under it to shake things around it's kind of a crap shoot...
 
gear swap only. The driveshaft was purchased from Dr. diff a while back. 1350 u joints with nut style straps.
Come to think of it pretty sure it did the same thing last time I had these gears in. (so that means its done it with a different block, 4 speed trans, different drive shaft, 904 trans, 3 different converters.........................)
 
Try turning the drive shaft 180 * at the rear yoke, that might solve the problem.
 
Try turning the drive shaft 180 * at the rear yoke, that might solve the problem.
pretty sure that should have happened by chance by now but I will give it a try.
wondering if rear thrust angle would have anything to do with it? (alignment still needs some tweaking only had time to set toe last time on the rack)
ca8c939c-2665-40b6-bde0-8cbb6182f151-jpeg.jpg
 
As posted earlier, check your u-joint, c- clips should center it in yoke but check it. I had a vibration a month or so ago turned out to be one saddle of yoke had alittle wear and the joint bearing cup had about 010" play no matter how tight the strap was. That's all it took to vibrate at speed
 
Remove the driveshaft and place a straight edge near the pinion yoke. Rotate the pinion and see if it spins true. I've seen the forward threads on the pinion bent from people beating them with a hammer. You should have to lightly tap the 1350 into the yoke, if it slides in to easily something is wrong. Just because your driveshaft is new doesn't mean it's straight so check it out. Also check the u joint phasing.
 
If it changes on decel, it's probably a changing pinion angle.
It has nothing , or very little to do with thrust angle.

With 3.91s; 130mph is 6100 rpm @ zero slip, so close to 6500 with that 727. Under acceleration and at WOT you would likely be well past max power, but with the pinion still jacked to the moon. Then when you take your foot off the gas, Compression braking will slam it in reverse direction, now pointing it down.
Under power, the pinion is winding up the diff and trying to raise the front of the car.
Under closed throttle decel, and high rpm, the pinion is raising the rear and maybe yanking the front end down.
With soft springs the range of motion could be 7 to 10 degrees. maybe more.
Your job is to either limit the range or find the sweetspot.... or both.

I went thru the same crap years ago. I added a second eye-to-eye mainleaf, and clamped the front spring sections, and briefly tried an adjustable pinion snubber; but my car was too low for it. Those changes, as well as changing the pinion angle several times, minimized the vibration, but it never completely eliminated it. Along the way I did some other things but they were inneffective. I replaced the tail bushing, had the driveshaft balanced TWICE, changed the rear yoke from 7260 to 7290. The front was already a 1350 , shifted the New U-joints, took them apart and inspected them. I ran it on the drive-on hoist with the wheels removed, the drums removed and the axles removed. I swapped to about 4 different chunks, and, 3 different sets of tires, but could never find the source of the vibration on the hoist. That's what got me started on the Pinion angle trail. I eventually took the rearend out and apart and sent the housing out to Moser's for narrowing, and straightening, along with a set of C-body axles for shortening. And you guessed it no difference.
I finally solved my problem by installing 3.55s and a 3.09 low gear, and a GVOD; and used the GV as a gear splitter. The driveshaft-rpm is waaaay slower than 4.30s, but the ratios are waaaaaay better.
Between the pinion-angle tuning and the reduced driveshaft rpm, it is now acceptable.
At 130mph my driveshaft rpm with 4.30s was ~7030, but with 3.55s is ~5800; that's a reduction to 82.5%
For you with 3.91s your driveshaft rpm at 130 is ~6390 with an effective tire rollout of 84 inches.
 
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Same problem from same gear set... pull chunk and check runout on ring and pinion gears with a dial indicator, and possibly pull pinion and check pinion bearing register on a lathe and length of pinion for runout, it acts like pinion bearing register setting on an angle or pinion possibly bent during heat treatment or previous usage
 
How did the bearings look after you pulled the 3.91’s out?
 
When I said 130 mph I was talking about the 3.23 gears. I’m still using the same speedo gear from the 3.23’s so the mph is off in the video. The vibrations start probably around 75mph and I let off probably around 90.

With the 3.23 gears it would happen at a speed that would never be reached.

I forgot about the vibration and never checked the bearings. just threw it in because my coworker has a built 86 camaro with a 406. Figured if he got cockey I’d be ready.
 
we had a broken belt in a tire and it would manifest itself at 75mph on the nose. Warranty replaced, smooth as silk.
 
The only thing that's changed besides the gear-swap, considering equal mph, is the driveshaft runs at a higher rpm now. This alone could make any off-balance issues show up.
So it would make sense if the issue lies in the driveshaft or the way it's installed.
Is the yoke play-free in the trans?
 
If it changes on decel, it's probably a changing pinion angle.
It has nothing , or very little to do with thrust angle.

With 3.91s; 130mph is 6100 rpm @ zero slip, so close to 6500 with that 727. Under acceleration and at WOT you would likely be well past max power, but with the pinion still jacked to the moon. Then when you take your foot off the gas, Compression braking will slam it in reverse direction, now pointing it down.
Under power, the pinion is winding up the diff and trying to raise the front of the car.
Under closed throttle decel, and high rpm, the pinion is raising the rear and maybe yanking the front end down.
With soft springs the range of motion could be 7 to 10 degrees. maybe more.
Your job is to either limit the range or find the sweetspot.... or both.

I went thru the same crap years ago. I added a second eye-to-eye mainleaf, and clamped the front spring sections, and briefly tried an adjustable pinion snubber; but my car was too low for it. Those changes, as well as changing the pinion angle several times, minimized the vibration, but it never completely eliminated it. Along the way I did some other things but they were inneffective. I replaced the tail bushing, had the driveshaft balanced TWICE, changed the rear yoke from 7260 to 7290. The front was already a 1350 , shifted the New U-joints, took them apart and inspected them. I ran it on the drive-on hoist with the wheels removed, the drums removed and the axles removed. I swapped to about 4 different chunks, and, 3 different sets of tires, but could never find the source of the vibration on the hoist. That's what got me started on the Pinion angle trail. I eventually took the rearend out and apart and sent the housing out to Moser's for narrowing, and straightening, along with a set of C-body axles for shortening. And you guessed it no difference.
I finally solved my problem by installing 3.55s and a 3.09 low gear, and a GVOD; and used the GV as a gear splitter. The driveshaft-rpm is waaaay slower than 4.30s, but the ratios are waaaaaay better.
Between the pinion-angle tuning and the reduced driveshaft rpm, it is now acceptable.
At 130mph my driveshaft rpm with 4.30s was ~7030, but with 3.55s is ~5800; that's a reduction to 82.5%
For you with 3.91s your driveshaft rpm at 130 is ~6390 with an effective tire rollout of 84 inches.
.
Isn't that closer to 17%.
 
Thank you for being gentle...I hope I can return the favor someday
At 130mph my driveshaft rpm with 4.30s was ~7030, but with 3.55s is ~5800; that's a reduction to 82.5%
You are almost correct; I said TO 82.5%.
What is 17% of 7030? it is 1195rpm, and
What is 7030 less 1195? it is 5835.
Which is not 5800, so you are almost correct.
5800/7030=82.5%
Its like the difference between your gallons and our gallons; yours are 128 oz and ours are 160oz. So yours are 80% of ours, and ours are 125% of yours; which being 25% greater; and not 100% less 80% =just 20%.

Or did I quote the wrong thing
 
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Great thread, this will no doubt definitely be helpful with the upcoming 4.10 Dana and what I thought was going to be a super stock spring installation, thanks much AJ/Form for the info... I believe I’ll probably go with Cal-Tracs now, believe I’d rather pay now than fight aggravation and spend twice as much later...
 
I’d like to add that we put up on the alignment rack jacked up the rear tires and it did it on the hoist.
Driveline looked smooth
 
The tires and axles are the same as before the swap, so if they were not the cause before, then they won't be now.
Like you said, it doesn't do it in Neutral, so it ain't the engine,nor the TC nor the front half of the trans guts. . And it's only apparent in third gear at higher speeds....... so it has to be somewhere between the trans geartrain after the forward drum and before the ring gear. And the go-to guy is the driveshaft and/or the pinion angle.
Check the driveshaft run out., front,rear and a couple of points in between, as well as the the trans yoke bushing, for play.
In case you don't know, pinion-angle is the angle that the pinion centerline makes with respect to the driveshaft centerline. The smoothest running will occur when it is equal and opposite to the front angle, with all 4 wheels on the ground. But if you have soft springs, this will allow the pinion to wrap up undr power, to as much as 10 degrees depending on how soft and how much torque is input . This can snap u-joints. So to compensate, the pinion-angle has to be set to no more than about 2 to 4degrees higher than the front when under power. This might require up to a 7* nose down static setting to achieve....... and then under light throttle, there is nothing you can do about the vibration
Slapper bars or a pinion snubber can limit the wind up, allowing less compensation., as can stiffer springs.





I battled the same vibration with any rear gear from 4.30s to 5.38s . I cured mine by going to a 3.09 low gear in my A833 and putting the 3.55s back in. This gave me the starter gear equivalent of 4.12s but reduced my driveshaft rpm drastically.
Good luck
 
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I had a very soft & oil soaked transmission-mount in a '64 Newport that would eminate vibrations through the car at certain speed-increases.
Then again, the Newport was also lengthened by 2 feet in the middle and a 2-piece driveshaft was made for it at the time.
But the soft-rubber mounted center supportbearing in combination with an oil soaked transmissionmount made for some serious vibrations in the car.

Took some time to find out by accident. This was at a time I just started wrenching on cars.
 
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