Drivetrain unbalances

[EdFury]

Dear readers, your expertise is requested:

My 76 Fury Sport has been suffering from unbalances in the drivetrain since day 1.

This unbalance (which usually but not always) would occur over let’s say 110 km/u (approx 70 MPH) and differ in intensity (as if the position of the rear wheels towards each other would matter).

What I did over the years:
- new rear leaf springs (needed anyway),
- new shock absorbers (needed anyway),
- tried different wheels (3 sets) ,
- new tires (needed anyway),
- had x-joints replaced,
- rear brake drums changed in position,
- then new rear brake drums,
- rear axle angle checked.

At a certain point, a new unbalance started to occur at about 80 km/h (50 MPH) so I thought of parts wearing out.

This prompted me to have the sliding piece of the propeller shaft resurfaced and also to have the propeller shaft rebalanced (and x-joint replaced).

The 110+ km/u (approx 70 MPH) unbalance has finally disappeared but to my horror, the 80 km/h (50 MPH) unbalance is still there and not even less.


Any suggestions about the cause(s) and how to repair it?
Thanks in advance for your replies.


Remark: it may not be connected, but sometimes when coasting in long right hand turns, slight grinding sounds can be heard from the rear axle.

 

[moparnation74]

Have you ruled out your axle bearings?

 

[EdFury]

Have you ruled out your axle bearings?

Thanks. Yes, I had them checked and they felt OK but I'll better check them again to be sure.

 

[dryheat]

Where do you feel the vibration?

Is it in the steering wheel?

Does the vibration seem to come and go or is it constant at the speeds you mention?

 

[PurpleBeeper]

Yes, what dryheat said (see above)…."where" do you feel the imbalance?

On another note, have you checked to see that you don't have a bent wheel OR a tire that isn't balanced (weight fell off) OR is starting to go bad (separated belts inside tire)?

 

[EdFury]

It is not always there that strongly. It can oscilating; getting stronger and weaker again.

I don't think it is in the steering; the steering wheel doesn't move although I feel that the brake discs are slightly wobbly

 

[Darter6]

These are hard to figure out. As dryheat stated if you feel it in the steering wheel it usually is front end/suspension. If you feel it in the seat of your pants it usually is driveline/rear suspension.
The other is harmonics.A vibration of sound waves at different spinning speeds. At one time Chrysler used rubber in the driveshaft to absorb harmonic vibration. My 67 Dart had a shaft that the rear yoke was vulcanised rubber.
Some of the late 70's Mopars had the transmission crossmember/torsion bar mounted in rubber along with front suspension K frame that turn to mush after awhile. I changed all these bushings in my 79 Chrysler 300 B body.
Last but not least is the transmission output shaft bushing worn. That's about all I can think of.

 

[Darter6]

Was also thinking of engine mounts and transmission mount.

 

[Dennis H]

Can you engine off briefly at speed and left/right for changes and to isolate. Neutral.

 

[slepr1]

Reminds me of chasing the vibrations. I went as far as balancing the driveshaft and replacing the u-joints.

 

[EdFury]

Dear Friends,

First of all thanks for your contributions and my apologies for the belated reply:
I don''t have the car at home and little time anyway to spend on it.

Upon your replies I made along trip to have a good feeling on the problem.
Then I had my wheels balanced carefully and checked on bends or unroundness - nothing worriesome came out.

Finally I want to an experienced race car tuner.
He had the car checked on all possible cleranaces, worn out suspension, rubbers, bearings etc and came to the conclusion that it must be either an out of balance torque converter and/or flexplate.

This is e.g. because with the car parked and in "N", the vibrations occur too when the the engine is revved to about the same rev as when the vibrations happend when driving.

Now comes the point of having the lot balanced which is a very expensive operation.

However, a Mopar expert tells me that out of balance torque converters are very rare with 381 engines as they are balanced on 0 (zero) unlike 360's that are balanced externally (basically meaning the 360 have a torque converter that is adjusted to the a.o. turning parts of the engine plus flex plate)

Any thoughts from you before I may decide to spend much money on balancing with still doubts that that may not be the solution to my Fury-sorrows?

Thanks in advance

 

[Darter6]

The flex plate that attaches the converter to the crankshaft may be cracked.

 

[David Stillie]

Perhaps someone at sometime put in a converter for an externally balanced engine. Also check inside the front crank pulley for mud, dirt build up. Alot of sludge can accumulate in there

 

[kiwigtx]

The 360 must have the correct torque converter weights and the flexplate is specific to the 360.

You need to check what is in your car now...otherwise the converter can do damage.

Small Block Mopar 360 is a difficult one to get right if the car has been apart, and put together again using pieces from other cars.

Sorry, but your transmission needs to be removed to ascertain exactly what your car is equipped with.

I have been through this recently, and is a PITA until you have the correct parts installed.

 

[Paul_G]

I had a bad torque converter on my 72 Charger 360. It had a vibration above 50 MPH. Finally figured out the vibration was above 3000 RPM. It would shake like crazy over 3000 RPM in neutral or driving down the road. Does your car do that?

Also, it had no stahl. The engine would want to pull as soon as I took my foot off the brake.

 

[66Satellite47]

EdFury, not sure what motor you have. You Mopar expert is talking about a 381? Maybe he means 383, but that would not be stock in your '76 Sport Fury. Could be a 400 & those are external balanced. I'd be looking at the balance weights on the converter or be sure it has the correct damper(front crankshaft balancer). As others said, if a 360 small block(distributor at the back), it also is externally balanced & will have weights on the converter.