Another drivetrain vibration.

-
K

Kent mosby

FABO Gold Member
FABO Gold Member
Joined
Aug 24, 2018
Messages
1,225
Reaction score
1,282
Location
Idaho
I am trying to get my drivetrain smoothed out a bit. First, what I have. 1973 scamp ,512 ci, rebuilt 727 trans (by me), narrowed 8 3/4 rear end, 741 with 3.91 gears (bought here), front end completely rebuilt from drdiff parts, new rear axles from drdiff, new driveshaft from drdiff.

What has changed, I ordered the wrong u-joint for the driveshaft so I got the correct one from drdiff, BUT, I did not have the driveshaft rebalanced after installing it.

We welded on a back brace on the rear axle housing, Could it be warped. When assembling everything was perfectly straight.

I have a Prothane, urethane rear transmission mount. Could it be just transmitting more "normal"sensations?

I was down 9* on the pinion angle and I placed a 6* wedge shim in the rear perches.

What I am experiencing is that it seems less under mild to hard acceleration, but at 65-70 mph there is a harmonic surging vibration. Hit the throttle and it goes away. While cruising at 65 without the throttle and you shift into neutral it coasts smoothly as can be.

How can I track this down? What other things can I do or check? Rebalance the driveshaft? Try 4* shims instead of 6*? Thoughts??? Help, please.
 
On the lift the trans output is 2* down. The driveshaft is level and the pinion is pointing down towards the driveshaft 3-4*. If that helps at all.

Edited I just removed the driveshaft to measure angles better. trans down 1.5 and pinion down 2.
 
Last edited:
Almost sound like there is a little too much pinion angle because the vibration decreases with increased load. Perhaps coasting smooth is because of no load at all.

Double check the angles now after everything "settled in". Should be with driver weight, but I can't imaging it would make that much difference. I guess it's a drive on lift? (Had to ask.)

Check U-joint condition and make sure they are installed correctly and clips properly seated.

Too much heat and long welding runs on a brace can tweak the housing, but can't see how it would cause a vibration.

Make sure the joints don't have any lateral play, especially at the rear end yoke. They have been known to spread from abuse.

Check tailshaft bushing.

Solid or poly engine mounts? Poly trans mount will transfer a little vibration, but can't imagine it being worrysome.

(I had a bad vibration once from an improperly lightened ring gear which caused an imbalance there. Drove me crazy.)

Enough fore-aft slip spline yoke clearance in the tailshaft static & throughout suspension travel?
 
I just had my neighbor drive it. He is the president of a hot rod club. He got it up to 100 mph, laid down 100 ft of rubber, hard braking. Basically, he put it through its paces. He hears a little noise from the back of the car but no vibration. He said he would not be hesitant at all to take it to the track. I need to get back on the freeway to test for any more vibration.
@Locomotion i now remember that when assembling the u-joint it was a bit tight. Maybe I pressed it in too much. I figured that to would loosen up a bit. I need to remove the driveshaft and check its condition. Yes, drive on lift.
 
So r u saying that u checked the pinion angle against the driveshaft angle? If so forget about the driveshaft and check the pinion angle and the trans angle. 2 down on the tranny and just measure the pinion and make at least a 2 degree difference. So pinion down 4. Driveshaft is totally out of the equation. Kim
 
oldkimmer said:
So r u saying that u checked the pinion angle against the driveshaft angle? If so forget about the driveshaft and check the pinion angle and the trans angle. 2 down on the tranny and just measure the pinion and make at least a 2 degree difference. So pinion down 4. Driveshaft is totally out of the equation. Kim
Click to expand...

I understand that the driveshaft does not matter. I just included it for reference. I removed the driveshaft to be certain of my angles. The trans output is 1.5* down and the pinion is 2* down. So on acceleration it should come up to +2 or so, Correct? Do I need more down than what I have on the pinion angle?
 
also, Would changing the u-joint possibly throw the driveshaft out of balance? The first u-joint did not have a zirk and the new one does.
 
Yes, u should be atleast 2 * lower at the pinion than ur trans angle. U r only 1/2 * now. If u have lots of torque 3-4 down is better. Was it a lot better when u added the shims? Kim
 
Don't think the grease fitting would be enough to throw it off. But the angles are suppose to be in relationship to the driveshaft. (Always worked for me.) Driveshaft 1.5-2* down behind the trans and the pinion down compared to the driveshaft at that end. I believe leaf spring suspension recommendation is 3-5* pinion nose down.
 
oldkimmer said:
Possibly, but I don’t think enough to matter. But being tight will cause a vibration. Kim
Click to expand...

I checked the u-joint while the driveshaft was out and it moves by hand smoothly. Not loosely where you could flip it over and back. it just feels like grease resistance.
 
oldkimmer said:
Yes, u should be atleast 2 * lower at the pinion than ur trans angle. U r only 1/2 * now. If u have lots of torque 3-4 down is better. Was it a lot better when u added the shims? Kim
Click to expand...

Way better after adding them. Night and day difference.
 
You must log in or register to reply here.
-

Back
Top