Drive line Vibration

[Steve Kranz]

Finally got my 68 Cuda back from restoration. Of course, there are a few bugs. Mainly I have a consistent drive line vibration. I can hear the low rumble and feel it in the seat of the pants. It is consistent at nearly all speeds and under both acceleration and deceleration. The shaft and u-joints are new. The pinion angle appears to be in direct alignment with the shaft. One concern is the distance of the exposed slip yoke barrel from the tail of the trans. Is there a specified distance dimension? My concern is the drive line may be a little too short, causing the slip yoke to wobble? The trans was rebuilt and the diff appears to be in good shape. I will try to get a pic of the slip yoke location.
340, 727, 742 8 3/4 diff. Thanks in advance!

 

[H22A4]

Was the vibration there before the restoration?

While the driveshaft might be too short have you also checked to ensure the exhaust isn’t making contact with anything (ie: gas tank)? How about the tires; perhaps a belt has separated or they are out of balance?

What condition is your transmission mount in? Rubber or polyeurothane? Rear drums balanced/not missing weights? Front wheel bearings are in good condition? Rear end has been checked out too (bad rear bearing or bent axle shafts)?

 

[Steve Kranz]

Basically everything (with the exception of the long block and differential) is new or rebuilt. Exhaust, wheels and tires, suspension, steering, axles, etc. are all new. The diff shows no signs of excessive wear or damage.

 

[Bodyperson]

First thing I would check would be the pinion angle.

 

[340sFastback]

From what I read a driveshaft yoke should have minimum of 2 inches of spline engagement with trans output shaft. You can measure how much of yoke is sticking out, pull yoke out of trans, then based on length of yoke determine length of spline engagement.

Your pinion angle should be in the 2 to 4 degree range. Pinion and driveshaft should not be aligned. Have you measured angle of engine trans and then measured angle of pinion?

 

[Steve Kranz]

From what I read a driveshaft yoke should have minimum of 2 inches of spline engagement with trans output shaft. You can measure how much of yoke is sticking out, pull yoke out of trans, then based on length of yoke determine length of spline engagement.

Your pinion angle should be in the 2 to 4 degree range. Pinion and driveshaft should not be aligned. Have you measured angle of engine trans and then measured angle of pinion?

Good info. I will check the slip yoke engagement distance and drive line angles. Thanks!

 

[Steve Kranz]

From what I read a driveshaft yoke should have minimum of 2 inches of spline engagement with trans output shaft. You can measure how much of yoke is sticking out, pull yoke out of trans, then based on length of yoke determine length of spline engagement.

Your pinion angle should be in the 2 to 4 degree range. Pinion and driveshaft should not be aligned. Have you measured angle of engine trans and then measured angle of pinion?

Drive line and pinion shaft appear to be aligned. I will check for the correct 2-4 degree difference and adjust accordingly. And check slip shaft engagement distance too. Thanks!

 

[Oldmanmopar]

Don't ever assume the new drive shaft isn't the problem. Been there.

 

[skep419]

Does it change with speed?

 

[cpearce]

Plunge on the yoke should be about 5/8" and no more than 3/4" . Too much can lead to vibration.
As others have mentioned, aim for 2-3 degrees downward angle at pinion.

 

[Steve Kranz]

Don't ever assume the new drive shaft isn't the problem. Been there.

Good point. Had this one made about 18 years ago. Never used until now. Will check it out.

 

[Steve Kranz]

Plunge on the yoke should be about 5/8" and no more than 3/4" . Too much can lead to vibration.
As others have mentioned, aim for 2-3 degrees downward angle at pinion.

I'll check it out. Ordered the axle shims. Thanks!

 

[Steve Kranz]

Does it change with speed?

Pretty consistent at various speeds and under acceleration and deceleration.

 

[skep419]

Pretty consistent at various speeds and under acceleration and deceleration.

So it speeds up as the car speeds up?

 

[Steve Kranz]

First thing I would check would be the pinion angle.

Looks aligned. Ordered spring shims. Thanks!

 

[Steve Kranz]

So it speeds up as the car speeds up?

Yes

 

[Inertia]

If possible, take it back, get their opinion, and expect them to recommend something.

I always want to know about any aspect of my builds, cuz something musta moved, and I want the opportunity to repair any deficiency .

jmo

 

[Steve Kranz]

If possible, take it back, get their opinion, and expect them to recommend something.

I always want to know about any aspect of my builds, cuz something musta moved, and I want the opportunity to repair any deficiency .

jmo

Had the shaft made by Drivelines NW in Everett WA. That was over 18 years ago. Will still get it checked out tho. Thanks!

 

[Oldmanmopar]

I had one that was not clocked the same front to rear. They need to be perfect. A dent or one end welded on crooked.. It does not take much to get a vibration. Your description leads me to believe its the drive shaft.

 

[Steve Kranz]

I had one that was not clocked the same front to rear. They need to be perfect. A dent or one end welded on crooked.. It does not take much to get a vibration. Your description leads me to believe its the drive shaft.

Interesting. You have a good point. I will have the shaft checked for any flaws. Thanks!

 

[Steve Kranz]

Just checking online specs. One source says the shaft length for 68 Cuda with 340, 727 and 8 3/4 length is 48.88. Another indicates 44.88. Quite a difference.
I need get under and do some measuring. At 73 years old it could take some time!

 

[cpearce]

Just checking online specs. One source says the shaft length for 68 Cuda with 340, 727 and 8 3/4 length is 48.88. Another indicates 44.88. Quite a difference.
I need get under and do some measuring. At 73 years old it could take some time!

May I suggest, undo your straps at the pinion yoke, slide driveshaft all the way forward until it bottoms out on output shaft, scribe a mark on the yoke at the seal. Then slide back into pinion yoke and measure from scribe line to seal. The difference will be your plunge depth. This should be between 5/8 to 3/4"

 

[skep419]

I'd also check to make sure the u-joint caps are seated completely in the yoke, and that the caps have all the needle bearings.

 

[cpearce]

I'd also check to make sure the u-joint caps are seated completely in the yoke, and that the caps have all the needle bearings.

Great points!

I will add, make sure your u joint strap bolts are not too tight causing binding of the cap. Double check the torque, but it is low, like 15 foot lbs or 170 inch pounds.

 

[Steve Kranz]

May I suggest, undo your straps at the pinion yoke, slide driveshaft all the way forward until it bottoms out on output shaft, scribe a mark on the yoke at the seal. Then slide back into pinion yoke and measure from scribe line to seal. The difference will be your plunge depth. This should be between 5/8 to 3/4"

Excellent idea! Will do! Thanks!