At my wits end: Need help with driveline vibration

[Dennis H]

Might as well…A friend was driving the Coronet. He has a habit when cruising 70ish of pushing in the clutch, popping it into neutral and coasting to a known upcoming full stop. On doing this, thought I heard an rpm increase. Looked over at the tach, normal idle around 750. It was a somewhat high frequency but light feel and sound. Vibration. Think this started around the time the TKO was replaced by the SST but not sure. It’s complicated.
I don’t have this habit. Also, there is a constant high pitched slight vibration felt when holding the shifter at higher cruising speeds. Shouldn’t do that anyway. Not felt in seat or wheel. Discounted and ignored now.

 

[moparedtn]

Might as well…A friend was driving the Coronet. He has a habit when cruising 70ish of pushing in the clutch, popping it into neutral and coasting to a known upcoming full stop. On doing this, thought I heard an rpm increase. Looked over at the tach, normal idle around 750. It was a somewhat high frequency but light feel and sound. Vibration. Think this started around the time the TKO was replaced by the SST but not sure. It’s complicated.
I don’t have this habit. Also, there is a constant high pitched slight vibration felt when holding the shifter at higher cruising speeds. Shouldn’t do that anyway. Not felt in seat or wheel. Discounted and ignored now.

Fred has a bit of the same, but fortunately his is less prominent at 64-5 than it is at 59-60.
Definitely not wheels/tires, it's definitely in the driveline somewhere.

As the rear axle housing has never been moved, that leaves all the junk I've replaced over the years
(engine, transmission, clutch etc.). Further elimination is that none of the mounts were replaced during my care
for the last dozen years and none need to be now - and they look quite stock.

With the 18-spline being a fresh rebuild with less than 1k on it, I'm thinking driveshaft (came with the car, might
even be the original - it has the "doughnut" on it up front) or u-joint (they always check good but who knows).
Heck, might be the engine for all I know - it displays no other imbalance symptoms, though and the balancer is
new.

As the car has never been wrecked (and the alignment guy told me it's "damn near perfectly plumb" when he
pulled diagonals), I've ruled out bent body parts.
With all the mismatched components onboard though, who the hell knows?
Maybe some of 'em just do the harmonic thing and that's that?

 

[HawkRod]

Gentlemen: Thanks for all the recent replies. I'll respond to those shortly, but I have a set of driveline angles that I feel much more confident in. Please feel free to double check my math and logic!

Car setup
I set the car up in my garage with rear axle on jack stands so the leaf springs were holding the weight of the car. The front frame was on jack stands (just behind front wheels on each side), so front wheels were hanging in the air. Only non-ideal element here was that the back of the car was maybe an inch two higher than when it is actually sitting on the ground. All measurements were taken multiple times to ensure I got consistent results.
I measured off the frame just behind the front wheel and the car was sitting 0.1* high in the back.

Engine/ Transmission
I then measured the engine/ transmission. I found a really nice spot right on the yoke of the driveshaft that is machined and flat. I measured it to be 1.9* down (i.e. engine up higher than the transmission). So I'll call it 1.9* down 'absolute'. When I add in the error due to the frame being out of center, the engine/ transmission is 2.0* down relative to the frame. I verified this measurement using the front pulley and it was very close (2.0* down 'absolute', so within 0.1*).

Driveshaft
Measured the driveshaft directly by attaching the magnetic tool on the front part of the driveshaft. It measured 4.9* down 'absolute'. So driveshaft is 5.0* down relative to the frame.

Rear Differential
I took off the driveshaft off and measured the yoke itself by putting a flat piece of 1/4 " plate steel against the front of it and the tool attached to the plate steel. The yoke is pointing up (i.e. the part that attaches to the driveshaft is pointing up in the air) by 1.3* 'absolute'. So the yoke is 1.4* up relative to the frame

So the driveline 'rules' as I understand them:

• Engine/ transmission should point down 0.5* to 3.0*. My actual is 2.0* down. So this is OK.
• The rear differential should be very close to parallel with the engine and transmission. If it is not it will cause the harmonic as shown in the video posted by @1 Wild R/T. The difference between my two is 0.6*. There was another video I saw that said it really needs to be less than 1.0*. Mine is not ideal, but is probably OK?
• U joints angles must be at least at 0.5* but should be less than 3.0* to avoid excessive wear. My front U-joint is at 3.0* and my rear is at 3.6*. Reducing these angles would have to be done my having my rear wheels sit up in the wheel wells more (back of the car more squat). The Hemi leaf springs sit the up a little bit (give it some rake), but I am not ready to call these values bad. I will suggest these values are marginal but probably OK?.

So the bottom line of my driveline measurements indicates that while they may not be "prefect", I'm not sure I would call any of these a smoking gun and say they are the problem.

What do you guys think about the measurements - any comments?

Edit: My second bullet is the setup for a passenger car without leaf springs. As a leaf spring goes under load it arches the front of the pinion up. So at rest, it should be further down than being parallel to the line of the engine/ transmission. But by how much???
The image shared by @1 Wild R/T shows the engine/trans should be 3 degrees down, and the rear should also be down by 3 degrees. My engine is down by 2 degrees. So should I set up my rear to be down by 4 degrees? Wow. I already have 2 degree shims shimming it down....

 

[HawkRod]

I promised to respond to many of your suggestions, but I am quickly running out of time.

I truly appreciate all the brain trust and suggestions here. My path forward is as follows: Tomorrow I will get some additional shims and shim the pinion down further. That's it - work or not, then we head for our trip.

@Nxcoupe has very kindly offered to help me diagnose the issues with his tester. Short of a miracle that miraculously fixes this thing, we will try to drive instead to Ohio and see if we can get a definitive problem identified.

In between getting, installing and testing the shims, I have to think about packing, tools, camping gear, etc. It's going to be a couple of late nights, but that's the RoadKill Runner for you. It is what it is!

I will keep everyone posted on progress (or lack thereof).

Thanks again,

Hawk

 

[zyzzyx]

We will be waiting! God Speed!

 

[bearman]

Look at misaligned rear diff. Had a truck do the same thing. Found that the diff was not straight on the springs. Replaced bushings had all axle alignment problem gone. The funny thing about the deal was the tires didn't really so any funny wear issues.

 

[matthon]

Were the new mounts holding the engine higher or lower than the old ones?
Side-to-side a little different? To what do you attribute the new ones causing the problem?

No, neither higher or lower, side to side same.

The new ones were just poorly made, internally, and they showed zero signs of being compromised at all, plus I pulled them a few times to check.

The old ones appeared to be rock hard, but not blown up or torn apart.

I had other issues at the same time, carb and charging, which increased my frustration.

My 2 biggest mistakes, trusting new parts, and failure to invoke the k.i.s.s. principle.

Moment I discovered my stupidity:
318 Poly Vibration

 

[WileE]

I mentioned the rear axle housing in post #20. Have you checked it ?

 

[Outlawd]

When or if you ever solve this please share. I've got a couple threads here on same issue, never did find problem. I ended up going from 373 gears to 323's just to push the vibration to a higher speed that I very seldom hit so now I can at least drive and enjoy the car. I'm totally convinced MY problem is pinion angle because the vibration is 100% drive shaft speed related. I have replaced every drivetrain component possible and got my pinion angle to a nats *** and never ditched the vibration, hell it didn't even change at all while experimenting with different shims, same freak'in vibration....got pissed and changed gears so it comes in around 80 mph, I don't drive that car that fast anymore anyway. Good luck

 

[HawkRod]

Look at misaligned rear diff. Had a truck do the same thing. Found that the diff was not straight on the springs. Replaced bushings had all axle alignment problem gone. The funny thing about the deal was the tires didn't really so any funny wear issues.

I will check this when I swap shims today.

I mentioned the rear axle housing in post #20. Have you checked it ?

I know this is a potential issue. What I don't know is how to check it. I do not have a special tool needed to verify. Only way I know to do it is with a special tool that puts a big bar through an empty differential. It uses a special center section and hold the bar straight through the axle. I do not have access to one.
If there is some reasonable way I can check this in the garage I am all ears.

 

[Big Bad Dad]

> "I know this is a potential issue. What I don't know is how to check it. I do not have a special tool needed to verify. Only way I know to do it is with a special tool that puts a big bar through an empty differential. It uses a special center section and hold the bar straight through the axle. I do not have access to one.
If there is some reasonable way I can check this in the garage I am all ears.

I guess I am old. Back when I was hot rodding my old cars, I would have just yanked a rear end out of a junker and swapped it into my car. Of course, it is not as easy and cheap to do that any more. Sometimes I really miss the early 1980s'.... LOL

 

[64fury74charger]

I had a situation where the axle housing was twisted for lack of a better term. My car was a 65 sport satellite and was regularly abused. Seemed like there were several different vibration issues and pretty much all were coming from the back of the car.
Compared the bare housing with another and it had a twisted tube. Replacing the housing solved the problems and smooth as silk. Took a little while to get to that point and the rear end had been out numerous times.

 

[AR67GTX]

Hopefully someone has a good suggestion on the best way to check your rear axle to determine it is square with the car. Maybe some gage holes in the frame but after 50 years you have to consider is the body still in tolerance.

 

[barbee6043]

Way back decades (90s) ago I bought a nice 68 GTX 440 4 back home in Ga. and headed back to Mo. I stopped off at a friend to see some parts he wanted it I sold it to him. He was into drag racing. He gets it out on the interstate late night and opens it up. It ran great till he got it up to???? 70? 80? don't remember. He opens it up and it has 5 440 rods and 1 heavy six pac rod!!!!

 

[KENTON]

In the video, even when the angles are the parallel he twist the driveshaft out of phase. This brings back the pulsating vibration. So is proper phasing when the ends of the driveshaft the same? i.e. the holes for the U joints line up with each other. Have you checked this?

 

[bearman]

Having yokes not matching is a problem and can be out one tooth on tail shaft of Trans or diff.

 

[HawkRod]

I guess I am old. Back when I was hot rodding my old cars, I would have just yanked a rear end out of a junker and swapped it into my car. Of course, it is not as easy and cheap to do that any more. Sometimes I really miss the early 1980s'.... LOL

I would gladly throw in another axle if I had access to one. I have access to several pre-73 axles but they wont help!

In the video, even when the angles are the parallel he twist the driveshaft out of phase. This brings back the pulsating vibration. So is proper phasing when the ends of the driveshaft the same? i.e. the holes for the U joints line up with each other. Have you checked this?

I just got my 4 degree shims and will head out to the garage now. I previously checked the driveshaft and visually it looks fine. I will try to measure it more carefully, but it has been cut and shortened and balanced again, so I would think the likelihood of me having the same problem before and after would be fairly small.

 

[HawkRod]

RESULTS:

So I got 4 degree shims this morning from my father-in-law. Cost me two hours of driving, but it was worth it.

So I added the 4 degree shims in addition to the 2 degree shims. So basically the pinion is now shimmed 6 degrees down from where it was with no shims.

Now I won't say the vibration is gone, but it is darn near gone. Not perfect but good enough for RoadKill! Assuming this holds and is a true result, here is my take home lesson from this in relation to driveline angles:

1. You will read that the engine/trans needs to be parallel to the pinion. There is also a great video I mentioned before that shows why. If you want to check it out, see post #3 posted by @1 Wild R/T or click this link https://www.forbbodiesonly.com/mopa...th-driveline-vibration.244451/#post-912223119
   The general rule is engine/ trans down 3* and the pinion up 3* so they make parallel lines.
2. #1 is true only if you DO NOT have leaf springs (in other words, coil springs). With leaf springs, the general rule is engine/ trans down 3* and pinion DOWN 3* so they miss being being parallel by 6*. This is shown by a diagram that (again - you'd almost think they guy knows what he is talking about! ) @1 Wild R/T posted in post #23 or click here https://www.forbbodiesonly.com/mopa...veline-vibration.244451/page-2#post-912223290

Doing # 2 for my car seemed to work. Now I haven't remeasured my driveline angles, but based on the angles I measured before, I should now be at about 2.0* down for engine/ trans, and 2.6* down on my pinion.

A special thanks and good guy alert to @Nxcoupe who was willing to try and squeeze us in last minute and help us diagnose the location of the vibration.

A huge THANK YOU to all for your help, ideas, brainstorming, etc!
THIS is why FBBO rocks!
​

Now to try and do some last minute tuning and car prepping and packing. Our goal is now to leave Tuesday (only 1 day delayed from our original scheduled start)

 

[barbee6043]

The correlation ( angle) between eng. and pinion going down the road is what makes it work! But you know that! Looking forward to your RoadKill drives!!!!

 

[1 Wild R/T]

Good to hear you got some positive results..... And a suggestion... I'm sure you noticed as you added shim the locator pin loses projection & with 6 degrees it's probably pretty short... After your road trip I'd suggest cutting the perches off the housing & re-weld them in the correct orientation....