At my wits end: Need help with driveline vibration

[HawkRod]

Some of you may have been following along with my HawkRod RoadKill restoration thread and read about this. I am also trying to leave in a couple of days on a cross country trip and really want to get this fixed. I am posting this here to try and get broader viewing in case there are some good ideas I have not yet tried.

Car specs:
1973 Plymouth Road Runner. Factory 340, 727, 3.55 sure grip car. Car is fully equipped, still has it's factory A/C, so it weighs in at around 3900 pounds.
Currently the car has a 340 based stroker displacing 416 cubic inches. I have also added a Gear Vendor's Overdrive.

Issue:
The car has a harmonic vibration that occurs around 60 MPH. An important note is that this vibration has been there since I "restored" the car in the mid 1990s (read - low budget fix it up).
I have tried almost everything to address it. I have

• Rebuilt and rebalanced the engine (now twice - even a completely different engine). The 415 inch stroker engine that is now in the car is a balanced and blueprinted engine and is 100% different (including the block) to the previous engine with no change in the vibration issue.
• Put in two completely different transmissions. I have put two completely different transmissions into the car. Vibration did not change at all, regardless of which transmission was in the car.
• added a Gear Vendor's overdrive. Adding this GV O/D did not change the vibration at all. No change when adding it.
• Balanced the drive shaft (now twice because it has been shortened for the Gear Vendor Overdrive). In other words, I had it balanced before the overdrive to try and solve the issue, with no change. After the overdrive install it had to be shortened and was balanced again. No change to the vibration.
• Tried a completely different rim and tire combo on the car. I moved all four rims and tires from my 70 (that is smooth as silk) to this car. No change in the vibration.
• I swapped brake drums from my 70 (that is smooth as silk) to this car. No change in the vibration.
• I had a slightly bent axle. I bought two new axles from Dr. Diff and installed them. No change to the vibration.
• I swapped the3.55 rear differential with the one from my 70 (that is smooth as silk). No change in the vibration.
• AND I have tried to shim the rear end up and down to no avail.

There are two elements to the vibration:
(a) At around 60 MPH, there is a harmonic vibration. This vibration oscillates in a rhythmic pattern. Tough to describe in words, but rrrRRRRrrr... rrrRRRRrrr... I would guess the harmonic is maybe 70 to 90 cycles per minute (each rrrRRRRrrr cycle). This vibration has been there since the mid-90s when I "restored" the car.

(b) When accelerating slowly, no issue. When accelerating hard, there is constant hard vibration. This vibration is fairly new since I have been playing with driveline angles to try and fix issue (a). This vibration occurred when I shimmed the rear of the transmission up too high. I since have removed those shims, but then I removed the ISO suspension and that has dropped down the axle and I assume has created the issue again. After measuring driveline angles, I added 2 degree shims to shim the pinion down 2 degrees. No appreciable change that I can feel.

Current driveline angles (measured without the car itself level, so only the difference between the readings really matter). I used a digital level and rear suspension at drive height - I held the car off the ground using jack stands on the axle.

• Engine/Trans (measured at the front pulley with a square). 0.6 degrees UP
• Rear Pinion (measured by putting straight edge along back of pinion). 2.7 degrees UP

So this meant there was 3.3 degree difference between the two. All I have read states these should be the same angle. So I added two degree shims to shim the rear DOWN. This should have given me around 1.3 degrees difference. Still no fix.

I truly don't know what else to try. If anyone has any plausible ideas I'd love to hear them.

Thanks in advance,

Hawk

 

[Big Bad Dad]

Does it occur only when under power?.. If you run up to speed, and then put the trans in neutral and just let the car coast, with the engine at idle, is the vibration still there?.. That will help determine if the engine is part of the problem or not.
And another question, do you have a clutch fan? And has the fan ever been changed to see if it could be causing the problem?

 

[1 Wild R/T]

but rrrRRRRrrr... rrrRRRRrrr...
Describes a pinion angle problem....

Watch this....

 

[6872n73]

Does the vibration happen in third and second and neutral? What is the balance of your stroker versus the balance of the torque converter? The balance of the tires?

 

[Charles Cook]

Have you removed the weights off the converter or it a manual?

 

[R413]

You measured the angles of the engine and rear, but not the height. You need to measure each joint angle separately from trans to driveshaft, and driveshaft to rear axle

View attachment 1277801 View attachment 1277802 View attachment 1277803

 

[AR67GTX]

Exorcism?

 

[Don Frelier]

I had the ujoint caps walking around in the pinion yoke straps.
Otherwise "what hasn't changed"?

 

[snakeoil24]

Get yourself a degreed magnetic based angle finder place on the top flat on your rear axel and hten the out put of transmission and read the measurement. Let us know. Also get a piece of clear tubing and place one end around area where you think vibe is coming from tie it down and put other end into driver compartment listen to the tube for sounds. Move tube around till you isolate it.

 

[HawkRod]

First, to ALL of you: A big thank you for your responses. See some answers below.

Does it occur only when under power?.. If you run up to speed, and then put the trans in neutral and just let the car coast, with the engine at idle, is the vibration still there?.. That will help determine if the engine is part of the problem or not.
And another question, do you have a clutch fan? And has the fan ever been changed to see if it could be causing the problem?

If I am at speed and I put the car in neutral the vibration will still be there.
I do have a clutch fan. I can't absolutely say if it has been changed since the issue started. But a key observation is that the vibration is based on vehicle speed, not engine speed. I can put my overdrive on and off and the vibration is still there at the same harmonic.

but rrrRRRRrrr... rrrRRRRrrr...
Describes a pinion angle problem....

Watch this....

Thanks. Yes, I have seen that video (it's a great one by the way!). I agree, which is why I looked at the driveline angles. However, maybe I didn't do that right. See below response to R413.

Does the vibration happen in third and second and neutral? What is the balance of your stroker versus the balance of the torque converter? The balance of the tires?

It happens at any engine speed and in neutral. It is based on vehicle speed, not engine speed.
My previous 340 was externally balanced. After a performance rebuild, it was balanced externally through a weighted flex plate. The weight on the stock torque converter was removed. That engine (and block) died a horrible death when it threw a rod out the side of the block some 20 years after it was built. My new stroker is also externally balanced through a different weighted flex plate. I also now have a new, neutrally balanced torque converter. Through all these changes, no change in the vibration.

Have you removed the weights off the converter or it a manual?

It is an automatic 727. See response to 6872n73 above.

You measured the angles of the engine and rear, but not the height. You need to measure each joint angle separately from trans to driveshaft, and driveshaft to rear axle

View attachment 1277801 View attachment 1277802 View attachment 1277803 View attachment 1277804 View attachment 1277805 View attachment 1277806

I am convinced there is some problem with my angles, so perhaps I measured wrong. I will try to set the car up again and measure it again. I won't be able to do that until tomorrow night, but I will respond here with the results.

Exorcism?

It certainly feels like the damn thing needs it - at this point, I'm willing to try it!!!

I had the ujoint caps walking around in the pinion yoke straps.
Otherwise "what hasn't changed"?

I felt the U-joints and didn't feel any roughness or issues, but I will acknowledge that I don't remember when they were last changed. Maybe I should do this just to remove them as a potential problem...

Get yourself a degreed magnetic based angle finder place on the top flat on your rear axel and hten the out put of transmission and read the measurement. Let us know. Also get a piece of clear tubing and place one end around area where you think vibe is coming from tie it down and put other end into driver compartment listen to the tube for sounds. Move tube around till you isolate it.

I do have a degreed magnetic based angle finder. I will get some measurements tomorrow night and post them on this thread. I will also try to take pictures of where I took those measurements, in case I did something stupid.

 

[Curiousyellow71]

Bad spool motor mount?

 

[HawkRod]

Bad spool motor mount?

I inspected my motor mounts before I installed the new stroker engine. I saw no issues with them, although I certainly could have missed something. However, I'm not sure how a bad motor mount could cause the vibration even when the car is in neutral and coasting. Putting the car in neutral when driving at 60 MPH does not change the vibration.

 

[66 Sat]

Something dumb like the exhaust hitting the body somewhere, at that speed (60 mph) the air flow under the car causes it to rub? Hard acceleration causes the same rubbing?
Trying to think left field here.

 

[1 Wild R/T]

Like I posted above (and I'm sure Hawk got the message) the rrrRRRRrrr... rrrRRRRrrr is very specific & points in only one direction.... If you didn't view the video I linked you should... Once you hear the sound you'll remember it....
Every symptom described fits to a tee....

 

[HawkRod]

Something dumb like the exhaust hitting the body somewhere, at that speed (60 mph) the air flow under the car causes it to rub? Hard acceleration causes the same rubbing?
Trying to think left field here.

Thanks. I have tried everything I know of so some left field thinking might be what we need here!
We did recently check and adjust the exhaust. One of the hangers was touching the frame, and it would make a ratatat sound in certain cases. We then spent some time and adjusted the exhaust to ensure it was sitting right where it belongs.

 

[Curiousyellow71]

It's likely a long shot. Some motor mounts will transfer vibrations.
With the engine running it still could be transferring a vibration in neutral. Possibly get the car to speed and put it in neutral and shut the engine off. See if it changes.
If the engine has a bad damper, they can cause unusual vibrations as well.

 

[HawkRod]

Like I posted above (and I'm sure Hawk got the message) the rrrRRRRrrr... rrrRRRRrrr is very specific & points in only one direction.... If you didn't view the video I linked you should... Once you hear the sound you'll remember it....
Every symptom described fits to a tee....

I did get the message for sure! I want to really measure all the angles carefully. I have two post lift, so it is no good for measuring the angles since the car needs to be at ride height on the suspension. This means it is a bit of a pain to set it up on its wheels high enough off the ground so I can measure it. But I will get it back on the jack stands tomorrow night. (Jack stands will support the rear axle, not the frame. The front of the car will be supported by the frame rails, however.)

It's likely a long shot. Some motor mounts will transfer vibrations.
With the engine running it still could be transferring a vibration in neutral. Possibly get the car to speed and put it in neutral and shut the engine off. See if it changes.
If the engine has a bad damper, they can cause unusual vibrations as well.

New engine got a new damper, so that is not likely the issue.
Around my area, it is hard to shut down the engine on the highway, but if I get the opportunity and other things have not panned out, I will try this too just to verify. I strongly believe the vibration will still be there.

 

[Curiousyellow71]

I dont know what damper you have? But most replacements dampers are build overseas. Most aren't of the same quality as '73. If you shut the engine off at speed and it still shakes you should be able to narrow it down dramatically.

 

[moes]

• Engine/Trans (measured at the front pulley with a square). 0.6 degrees UP
• Rear Pinion (measured by putting straight edge along back of pinion). 2.7 degrees UP

If this was your starting measurement. I would make the engine 3 degrees down, with your 2.7 degrees up. The reason I say this is I find anything over 3 degrees on the motor is too much and makes it harder to balance. Universal joints flex around 3 degrees at the max, or so. Just my 2 cents worth.

 

[WileERobby]

Rear axle housing bent.