Driveshaft Issue and Vibration?

Currently running a 3.91 gear in a 8.25 rear behind a A-833 OD trans (1.00 ration 3rd/.67 ratio 4th) and mild 318 in my Duster. Driveshaft is 2.75” diameter. After the gear change last month, above 2300 rpm/65mph in 4th/OD, there was a minor vibration/resonance. Had new u joints put in and re-balanced. According to ship, the driveshaft is not perfectly true, but almost with very little runout. That improved the issue down to what I can describe as more of a medium frequency resonance that actually intensifies slightly as I let off of the gas, and not really a vibration. It’s is still there to a lesser extent when I pop it out of gear. Feel it more up in the front under my seat. Rear tires are new and balanced, same for front.

Motor mounts are about 8 months old and ever since, the engine has always seemed rougher when free revving in neutral with those mounts, even before the 3.91 gear and prior to that, the 4 speed swap.

Does this sound like pinion angle slight Imperfection, or driveshaft half critical speed situation, or maybe slightly worn axle bearing? I am still driving the car, because I don’t run it on the dragstrip and it isn’t shaking me to death, but I don’t want to risk the driveshaft exploding (if that is even a legitimate risk or fear).

The car is talking to you asking you to fix its problem
I would concentrate on fixing now before it hurts you with a parts failure
Can you jack the car up and put it on jack stands high enough for you or a helper crawl under the car while running? If so do that and have someone either drive the car on jackstands or have someone look at the drive shaft under power in gear where the car vibrates to help identify where the problems are
Many things can cause problems like your having, a tire that is out of round and so on
Have fun and let us know what you find, that could help others on here later

I'm wondering if the mts had a slightly different thickness & changed the ujoint angles a bit or if they are just stiffer. the noise got worse with the gear change so that would point me there but what confuses things a bit is the shaft balancing helped. I would check the angles & confirm that the front yoke moves smoothly by hand back & forth in both planes & the rear ujoint is smooth in its one plane. too many changes at the same time does complicate diagnosing. Keep us updated on any info (we'll get it)

Yes, the motor mounts do make me wonder, as they were done before the trans and gear swap when the car was a 904 with 3.21s. The trans swap came about a month after and the gear swap about a month ago.

I think the ujoints and balancing helped, mainly because the front joint was so utterly shot after 6 years of service since the last balance job and new ujoints.

The car sits on Mancini XHD +1 springs, with lowering blocks to bring rear down closer to level with the front.

& another potential (I had that once) is not enough front yoke engagement into the trans (& or bad ext housing bushing). Mine was a PT vibration at random sort of (mostly deccel) & I swapped the correct shaft in & all good.

attached is picture of the current ride height...

The snout on the front of the driveshaft is better than 3/4 engaged into the tailshaft and the snout is roughly 6" long. Before the 3.91 swap, I didn't feel anything off at the same highway speed. Maybe bc the engine revs were lower.

I have run up to the same speed in 3rd and that is about 3300 rpm. Couldn't really tell if it was there, probably because of the added harshness from the engine. I don't think the motor mounts are the softest.

If the tailshaft bushing in the trans has wear, should I be concerned until I can get the trans out and freshen it?

I ain't for sure & it would depend on how sloppy it is. How much end play does the driveshaft have? (when you butt it all the way into the trans how much distance is there to move it back till the rear ujoint is seated). excessive endplay is more likely to be a cause than a worn bushing (unless it is real bad) & right now I'm thinking the bushing ain't the problem (it might not be perfect but I am thinking elsewhere) however too much end play can definitely be it. I like ~3/4-7/8, enough to get the ujoint over the yoke when installing plus a bit more measured with springs compressed at ride height/weight (stands under axle). plus we have the angles and the pig as potentials. As Cab says we gotta stay after it.

With the driveshaft bolted into the rear yoke, there is not front to back play obviously. With the new front u-joint, you can maybe move the front of the drive shaft side to side a bit (haven’t measured, but I would guess no more than .250”. This is the snout of the driveshaft only, and not the u-joint flexing with it. As extra information, there isn’t any play on the rear pinion/yoke back at the differential.

I am a skeptic that, although the gear change increased driveshaft speed and it’s probably at or approaching half critical speed of a 2.750” shaft, that it’s a major player in what I am feeling.

And yes, just go to stay after it. I am more OCD than anything about so called issues like I am describing.

the front to back distance would be measured with the rear ujoint loose just like you were first installing the driveshaft (& that is a good time to check that) when you butt it all the way forward to clear the rear yoke then move it back to seat into the yoke, that distance. Right now the side to side slop the front yoke has in the extension housing bushing you mentioned, 1/4 seems like a ton or did I misread something.

That 1/4" guess is probably way more than it is, so that was misleading. I can push the snout side to side when installed, but only a very small amount.

There should not be any movement with the driveshaft installed in the car , no up or down and no side to side movement, EVER It should rotate smoothly in a straight plane, no wobbling
Wobbling(up or down or side to side) makes vibrations, straight and true do not
It sounds to me that maybe the front U joint tranny yoke is junk It should not move up or down or side to side when installed on the tranny output shaft
Get that good looking rascal up in the air on jack stands so you can run it in gear to find the cause

Yoke not far enough into the trans will do this. Sounds like he's currently about 2" from bottoming out, should be less than 1".

Driveshaft is too short.

Did you put poly mounts in the trans yoke as well?

I am not sure what you are referring to. You mean the trans mount? if so, it is just a regular mount, not a poly.

Well fellas the only different thing is the TRANS, so I'm thinking the tail shaft bushing or the bearing. I can't remember if the 904 and four speed use the same length shaft.

Mostly true. Used 3.21 gears for first few months with the A833 (which was not rebuilt) and didn't feel anything between 65-70 mph now, but maybe that has some to do with the driveshaft now spinning at a higher rate of speed that with the 3.21s.

Almost immediately after I left the diff shop for the gear change, I felt the resonance on the highway above 60mph. Two days later, the driveshaft was balanced and new Spicer u-joints were put on the front and rear since they were shot. That re-balance and new joints helped immensely, but as said, still not completely gone.

I should mention that this is the same driveshaft that I shortened to use behind the previous 904 when I swapped in the 8.25 rear. That was the first time the driveshaft was balanced and new joints were installed. The very same driveshaft was used when swapped over to the A833 trans, with ZERO modifications to the driveshaft done.

I will see if I can take some pictures of the driveshaft if I get home the office early enough tonight.

that's what Sherlock Holmes/Columbo and lieutenant Kenda would (all) call a major clue. Keep It simple Sam. Unless they messed up the shaft somehow during the swap (ujoint clips/caps can be delicate/problematic) I'd be thinking about swapping the pig at least for a test even with an open gearset. I know it is a pain.

Yes, I had thought of that, but, being that it is not an 8 3/4 with a removable third member and is a 8 1/4, that isn't exactly easy. The 3.21 setup previously had a Spicer limited slip added and functioned great. The 3.91 gears in it now were slightly used (about 2000 miles, but all teeth were in great shape). The same limited slip unit with 3000 miles on it was used. Very slight quiet whine from gears at any speed, but not louder on deceleration like what would occur with bad pinion crush sleeve.

What about an axle bearing being knackered a bit during the gear swap?

As mentioned, the problem really isn't a vibration more than it is a deep, almost cyclical hum that you can feel resonating mostly below the driver side seat and mid to front of drivetrain. Not to say its not felt further back as well.

I missed that it ain't an 8&3/4. do keep us updated.

I will. Going to take some pictures of the driveshaft and post in a few hours.

If it ends up being in the rear gears from the install, I might just have to deal with it for a while. The axle bearings and all components were reused from a rebuild and add of a limited slip about 6000 miles/2 years ago..

Maybe the gears/carrier & bearings just didn't take a good set this time,could be somethings off there.

I agree in theory but every driveshaft that I can recall had some slight play at the trans. The splines are not an interference fit, they do slide as they rotate.
My Charger has the best collection of parts of the cars that I own and even it has a small degree of slop. Zero drone, zero vibrations other than the engine.

Here are a few photos. After checking front snout slop again, there really isn't any movement. I'll try and post a video.

Do you have a angle finder, compass?
If so measure the pinion angle and driveshaft angle and let us know what you find yours are, if you don't have one or can't borrow one go get one (beg, borrow or steal one ) and check your pinion and driveshaft angles with the car on the suspension like it driven
In the sideways pictures of your driveshaft and pinion U joint is the rear end on jack stands or is the rear end hanging down on the cars rear suspensions?
Me thinks your driveshaft is to short The yoke needs to be in more when the car is on the suspension, so if it is not on the suspension in your pictures ignore this comment

No sir, not on the rear tires. On jack stands with the rear suspension relaxed and jack lowered.

Here are pics of front and rear yokes, perfectly level and camera level, with rear on ground. Rear yoke might be at ever so slight steeper angle than front, but is very miniscule. I'll try to properly get the reading though...

Edit: pic of rear yoke is off a bit. Really at same angle as front.

the first thing I would do right now is pull the rear ujoint caps & move the shaft all the way forward & measure the distance back to where the caps seat in the yoke.

I wonder of I am seeing it right. It appears that the yokes are not in alignment. They have to be within a few degrees of each other to avoid vibration. The u-joint connection speeds up and slows down at every rotation. I'd expect trouble if the u-joints didn't both do this at the same time every rotation.

That picture of the rear didn't come out right; the camera is tilted a bit. If angled the right way, the tires are level with the ground.

I will check that out. About 1.75" exposed of snout and no less than 5/8 of snout is engaged in trans.

If the car is on the ground in the last set of pictures the driveshaft is about an 1" short.

The car is on the ground. Yes, the driveshaft could be a tad short, but I am not grasping why it's now an issue when it wasn't prior to the gear swap.

nothing was changed that would move the housing a tad rearward?

Not since the gear change. Motor mounts done a month before the trans swap, which was a year ago.

Can you verify both U joints are in line on the driveshaft? The driveshaft looks to me to be too short...If it's not into the tail shaft bushing far enough it does wobble and will cause a vibration. The splines and the tail shaft do not control the yoke as much as the bushing in the tail shaft housing does.

Yes, both u joints are in line with each other.

I was told you want the driveshaft to be able to slide 1" forward before it bottoms out in the trans.

There is a tool available to R&R the rear trans bushing without pulling the trans. It cost about $150. I bought one and did mine chasing a similar high speed vibration. I have yet to find it, and it only occurs above 95mph.

Where the rear pinion bearings changed? Maybe that front pinion bearing is bad, nut tight? That might be the place to check since the "noise" came after the gear change. I thought I read you replaced it all at the same time.
Also since you had to change front slip yoke, is it bent? Have to watch it turn running in gear. Only way to check the driveshaft for harmonics is to try another one.

No, nothing was changed when the gears were changed to 3.91 recently. Everything was replaced in the rear a year prior when the sure grip unit was installed with the 3.21 gears previously, so nothing was touched this past time around.

The front slip yoke was sourced and put on the front of the driveshaft back when the 4 speed was put in early last year. Nothing appears bent and it all functioned without issue with the 4 speed/3.21 combom before going to the 3.91 gears.

My dad is checking for engagement of the driveshaft into the trans today while I am at work, making sure that the driveshaft isn’t backed out more than .750-1.000” after pushing it in as far as it will go. Will report those results later.

Came in here for this. Have you checked your pinion angle? You can exacerbate other driveline vibrations if your pinion angle is bad.

Verifying that both yokes are parallel to each other today and if adjustments are needed.

Here you go boys. 1.5 inches exposed and the rest is engaged, which is 4.5 inches.

The drive shaft is now spinning about 1000 rpm faster with the 3.91 gear. The faster it spins, the harder it shakes.

The shaft looks too short coupled with the fact that the shaft has some runout according to your machinist equals vibration.

Kevin

Grabbing at straws here but I had one in a pickup years ago that did what your describing after having new U-joints installed...and old mopar guy told me to "Spin the driveshaft half a turn" take it loose and spin it half a turn and reinstall. It worked....scratch my head to this day over that one...
I'd say it's worth a try...

Measure the inside spline length
I like to see from .750 to 1.0 yoke extension out of the tranny tailshaft housing from bottomed out with the car on the ground loaded ready to drive You have way more than that on yours
The one picture of the front tranny yoke on the driveshaft, it looks like to me that the drive shaft is not centered in relation to the yoke center I'm sure it is not out of center due to if it was as bad as I see it you wouldn't be able to drive it over 20 MPH without the drive shaft breaking the output shaft and tail shaft housing on the tranny

Sorry about that, yes the mount. I had the same exact deal going on in my car and never got it figured out. Someone said it was the pinion angle. The addition of the poly mount didn't help either in my case, but it was not a DD so I lived with it.

Once pushed all of the way in and backed out 1 inch, there is about .750" of slip yoke exposed, versus about 1.5" installed. There is currently 4.5 inches of slip yoke in the trans. The splines start about 1" in.

Nope, just a bad picture on my behalf. The angles really arent off that much at all and damn near parralel with each other.

Just by chance when they were installing the u-joint maybe a couple needle bearings got buggered up? Just thinking out loud.

Well, I would ponder that as well, except I had the driveshaft balanced and u joints put on a month after the vibration began immediately after the gear change. The ujoints and rebalance actually took 80% of the vibration away to its current level.

If the DS is indeed a bit short and any minor vibes now exaggerated by the recent gear swap, is there any major risk of dropping the DS in the meantime while sourcing a new slightly longer 3 inch piece?

The splines in the tail of the trans are all in good shape still and 3.5 inches of the 4.5 inches of total slip yoke in the trans is in the splines. Slip yoke in total is 6 inches long.

Assuming the circlips are all installed on the cups, my money is still on the shaft has some known but unquantified runout that has reared its ugly head now that driveshaft speed has gone north of 3000 rpm at cruise.

Also the yoke looks a little worn where it rides in the bushing which likely means the bushing has some excessive wear also. If you grab the yoke and shake it while it's in the trans, you should have to pay attention to feel if there is any slop as in less than .010".

If it has much more than that you should look at a new yoke and tailshaft bushing. If it seems excessive and you have another yoke that doesn't look as used as that one does, slip it in and see if it's tighter.

I went thru this with a Ford top loader after I went from a 2.75 to a 3.50. I changed the tail bushing which helped but my yoke looked like yours and when I swapped it out it drove like a new one after.

Kevin

One other thing to consider is what RPM the drive shaft was balanced at.

You need a high speed balance.

and most shops that do general driveshaft work for everything, [cars,trucks,big rigs] do NOT do high speed balancing. it seems that even though it costs a few dollars more, one might find a necessity to use a race type driveshaft company to insure the shaft is not only high speed balanced, but also to insure it has almost zero runout.

eyeballing your driveline angles is not valid

Measure them

come back when you have numbers

First, get a driveshaft that's 3/4-1" longer.

Second, you said less of a vibration and more of a hum... any chance that you're getting exhaust resonance with the higher engine RPM while cruising?

Yes, high speed balance. Totally forgot that aspect. Might want to consider that. Isn't high speed balancing based on high rpm harmonics rather than sustained speed?

Yes, I plan to check the actual angle…once I source an angle finding tool this weekend.

No, definitely not an exhaust drone.

Just confirmed that the driveshaft was not high speed tested. It was only taken to about 500-700 RPM and brought in to no worse that .005-.007” runout.

I am beginning to look at having a 3”, .0083” wall thickness, driveshaft built, that is .750” longer on the body to get more engagement in the trans.

I would butt the yoke you plan to use all the way into the trans then measure the C to C you have at that point then subtract 3/4 from that dimention & this is with the suspension loaded (axle on stands). give that number to your shop. EDIT I would locate the front/rear axle position first & I like to have the bottom of the rear shackles point slightly rearward so that the springs "work" when they are compressed when you get on it as opposed to raising the car when the springs arc over center which they do if the bottom is pointing forward from "vertical". keep an eye on shock fitment when you change that position.

That is pretty much how the DC books say to do it.

As fro finding the angles, I have two of these I use.

I have bought from that eBay shipper "Zoro", no issues. Definitely get a digital one as opposed to a cheapie with a magnet & gravity needle.

Uppn more measuring, the body of the driveshaft definitely needs to grow 3/4" to be right. Should have a new DS tomorrow.

Using a bigger 3” driveshaft, am I still okay to use the same 4 bolts and caps for the rear yoke? They are all in good shape.

As log as your keeping the same size U joints your good on the straps and bolts
Do NOT over tighten them

No sir on over tightening; it is bashed into my brain from testing and design with fasteners as part of my every day gig.

The ujoints will be the same. Specified to the builder that the rear yoke WILL NOT be changing and he is going to work around that.

Are you going by rear ujoint endplay (I hope so) or by front yoke appearance/it's end play?

In the existing form, there was 1.5-1.75” of slip yoke exposed with the driveshaft installed. Pushing the shaft all of the way into the trans then backing out .750”-1” left about .500-.750” of slip yoke exposed.

.750” longer on the body should push the slip yoke into the trans .750” further.

I would suggest .750" max (springs loaded)

Yep, .750" additional length is the max and what I specified, to be safe and conservative.

The additional diameter of the driveshaft and the extra engagement should help eliminate the driveshaft from the potential problem list, even if this doesnt solve the issue. I do believe it will though.

Source another front yoke and pony up the coin for new u-joints...

Checked the front yoke and all seems well for now. I'll pull it apart later if I need to.

New ujoints in the same number and size as the 2.750 inch are being added onto the 3 inch unit.

might be too late, but Inland Empire is not too far from you.... I've had pretty good results using their stuff on my race car. A little more reading from their web site

https://www.iedls.com/?a=Critical_Link#Power_Train

UPDATE: new 3" shaft is in. Engagement into transmission is improved. Pinion angle is good. Same issue is still there.

Drove it more tonight and even felt that minor soft vibration between 35-40. Drove it in 3rd at highway speed and couldn't tell if its still there, due to increased engine vibes. Starting to feel this more from the fron, maybe something in the OD gear wearing, or elsewhere in the trans. Maybe those new motor mounts are not the best at dampening harmonics? I am going to check the wheel bearings in the front.

At least the driveshaft is now eliminated from potential issue list.

It sucks when you think you have a problem pinpointed and spend money only to see that the symptoms continue. ALL car guys deal with this!

I have a friend whose Mopar mechanical and automotive skills I have great respect for, insists that a driveshaft yoke shoved into a transmission bushing with too much clearance is the root cause of many unexplained intransigenet driveline vibration issues.

He recommends a new bushing which is too tight and honing it out to fit the specific yoke you are using. He suggests with the transmission out of gear to relieve stress on driveline, to attempt to move the yoke up and down. Any permitted movement is indication of potential problem.

I will forward this thread to him for comment.

And have we checked pinion angles yet?

It verified right in line upon measuring everything up. Angle is good.

Oh I am quite alright with it. It needed a bigger unit for down the road and this one has more engagement into the transmission. It was a good investment.

Sounds good. Yes, although there really wasn't any play at all on the slip yoke, I am starting to think the tailshaft bushing is the culprit. Realistically, the transmission as a whole is kind of tired. It leaks a bit from somewhere on the left side after sitting overnight and has for a while. I add fluid when needed every 2 or 3 months. First gear (not in it long because it is so short) whines some.

All and all, I think it needs a freshen. Until I can, I'll just ride around and not be too harsh on it. Since the mentioned minor pulsating vibration is more evident in 4th/OD, I will just keep from using it as much as possible except on the highway (just in case it is something in that gear as well) I usually do that anyways.

Just me I would borrow another 8&1/4 rear to confirm or elim that. in my head I keep coming back to that. Do post the outcome (no matter how long it takes).

As my friend is not a member of this forum but he did send this response to me after I had forwarded a copy of this thread to him. The following is a "copy and paste" of his response.

Roger:

It looks like you explained it well but I'm happy to provide you something you can cut and paste as I am not a member there. However some members on this site may carry over to the FBBO forum and would recognize me by my handle Meep-Meep.



The sign of slip yoke vibration is to drive the car close to or at freeway speed and find that spot where you "float it" by not being under power or deceleration. Just modulate the throttle until you hear that grumble sound and you will find you are not loading the drive line, thereby allowing the yoke to rattle around in the tail housing. This is independent from a balance issue since an out of balance shaft will try to orbit around it's center of mass even with a properly fitted bushing.

It's been my experience lately with slip yokes that the clearance is way too loose even when replacing the parts with new. And by way too loose I mean over about .007" is too loose. For a quick test on an unloaded drive line (tranny in neutral and rear wheels off the ground) grab the front yoke and attempt to move it up and down and you should have little to no perceptible motion. I don't have the actual dimensions off the top of my head but for sure the new yokes that I measured are small and the bushings are perhaps bigger than they should be. My yoke reference is taking the measurement on the non worn part on a factory yoke. One car I had measured over .060" and others are in the .020" range if memory serves and all have the grumble to some varying degree. Granted taking a dial indicator measurement way out at the U joint will show a pretty big number but it's still a reference.

The fix is to fit the bushing to the yoke with the proper clearance, and since the yoke is the part more likely to be replaced or changed, it would make sense to make an undersized custom bushing and hone it to fit the new yoke. For honing I will take my next tail shaft and yoke to a machine shop that has the Sunnen hone like the kind used to resize rods and tell them it's like fitting a piston pin (but not piston pin clearance). This will ensure a non tapered round bore. The only catch is the bushing bore needs to be in proper alignment with the output shaft bearing all the way up to the pilot bearing or front pump bushing.



Alex Lobodovsky

This months issue of Hot Rod (April 2017) has an article helping a guy with the same problem your having,only his was worse. Hot Rod Rescue is the article. He went through everything, even taking it to a 4x4 shop because he was being told his angles were off. It was a combination of a lot of things, including transmission dowel pins out of alignment. Right now your just speculating and throwing money at a problem hoping to find a cure. Pick it up and read, very enlightening.

Very good information and thank you. Grumble is a great description and yes it is in the highway float zone, not under acceleration. Considering I have eliminated a few things and will look into the rear today, this is starting to look like the likely culprit. There is still a hair bit of play, probably .030" or less, in the pinion slip yoke, but it is play nontheless.

I'll probably deal with it for a while if so, until I can rebuild the transmission as a whole in the summer.

My dad actually showed me this article yesterday and we were looking it over. Very enlightening. Pinion angle is taken care of and confirmed a non issue as of yesterday. Alignment all looked good.

I am ok with the money I had spent so far, since new back tires were needed and a more robust driveshaft was preferred.

Progress report!!!

Drove it a few days to work and back last week, about 130 miles total, mixed driving. Probably will again this week if weather warms up more.

Grumbling, minor pulsating vibration is still there. Can feel in throttle and steering wheel, in the area of the transmission or very front of driveshaft. Still mostly perceptible in the throttle float zone when cruising above 55mph. Not really discernable during acceleration. Can feel some of it when I push in the clutch at highway speed.

Driveline angles are essentially in line, with the slip yoke off maybe .15-.25° compared to the rear yoke at most. Certainly feeling that it is more likely that the source of what I am feeling from the rear of the transmission, more specifically the tailshaft bushing. There is a bit of side to side play.

The transmission is in need of a refreshing soon to begin with, since it is a leaker from the speedometer gear area.

FWIW my buddy had a vibration like you describe in his 66 fairlane with 4 speed, pulled apart the trans and had 2 bad bearings along with a bad trans mount that made it worse.

Yeah, wouldn't surprise me if I have similar with my tired transmission. The trans mount is fairly fresh, but I have an extra poly bushing that I might try.

Update: drove 40 miles into work today. Grumbling resonance/light vibration is most present between 55-65 mph in 4th and I feel it more through the accelerator pedal, clutch pedal when I press it in and in the shifter handle. Not really under seat or rear ward. Pushing clutch in, it’s still there but much reduced. Same when popping into neutral (greatly reduced). Centering more and more on somewhat worn tailshaft bushing.

As mentioned, roughness while holding revs north of 1800 rpm in neutral is present, but has been for quite a while since the new motor mounts were put in.

Other than that, car ran good on the way into work; all vitals are good, temp and pressure wise.

Just wondering if you got to the bottom of this. I just had my Goodyear polyglas tires road force balanced and got some improvement. The increased rpm may bring out a tire balance issue that you didn't have previously. Let us know what you have found.

The sensation/issue is still there. The front slip yoke does have movement, but not a ton and it has not worsened.

I have put about 500 additional miles on the car since my last post. No change or increase in severity. What I have noticed is what seems to be a decrease in the grumble, or at least less noticeable, above 65 mph or 2400 rpm in 4th/OD. After that point, the light vibration/grumble seems much less pronounced. Starting to wonder more about the trans mount, too hard of motor mounts, or a bearing in the trans making some commotion at specific RPM in 4th/OD.