Rear End Vibration

[Belltek821]

I'm looking for some input on a rearend vibration. I've got a 69 Valiant w/ 904 tranny, 8 3/4 rear, spool and 3.91 gears. Car drives and launches great but once I'm into 3rd grear above 3,000 rpm I get a winding and vibration in the rear that progressively increases with speed. Talked with a few guys, some say pinion angle, some say the rear isn't set right, and others say the drive shaft is out of balance. Any troubleshooting tips to get this started? Thanks

 

[RedFish]

could be a U-joint too. The drive shaft is where I would start.

 

[Supershafts]

Pinion angles are related to load and or drag.
95% of the time pinion angle is a issue under hard accel or decel.

Now you can have vibrations related to out of spec angles.. For example if you are beyond 7 degrees after 3500 rpm you will get a vibration and it will get worse as you increase speed.
Joints can only tolerate so much angle dependent on rpm.. As rpm increases angle needs to decrease..


I would point you at the shaft, the rear was just done, the shaft could have been thrown or dropped, making it have a need for balancing. Cap fell off, needle bearings are missing..


Another issue is you went to 3.91 and from what ? 2.73. Now the shaft at 3000 rpm is operating at a speed it wasn't originally balanced for..

A driveshaft is a precision piece, contrary to popular belief..

 

[Belltek821]

I bought the car with this rear end setup a few years ago. This is the last issue to work thru before it's done. The vibration is definately worse under hard accelaration, if I just ease it up to 3500 rpm in third I feel it but not nearly as bad. I'll pull the driveshaft and have it balanced to start. Thanks again

 

[cuda67]

I had this issue with my 67. Take your driveshaft to the shop, as I did. My driveshaft was balanced but it was sorta, kinda totally out of round, couldn't tell by looking. I had a new one made and no more problem. Just for info. I have the same 8 3/4 with 3.91/

 

[Supershafts]

The vibration is definately worse under hard accelaration, if I just ease it up to 3500 rpm in third I feel it but not nearly as bad. I'll pull the driveshaft and have it balanced to start. Thanks again

Under hard excel the pinion wants to rip itself from the rear and ride up over the ring, causing it to move the housing up....
When that happens, it can change the joints angle up to 4 degrees or more if the power is there and the springs are weak...

So if your angle is say 2 degrees different then shaft, you'll be susceptible to pinion angle vibes, meaning ..... if the shaft is at 0 degrees and the pin is neg 2 degrees, you'll have a problem.
or if the shaft is 2 degrees and the pin is 4 degree.

When you launch or excel hard, the pin will jump up 4 degree's putting it out beyond the positive side of acceptable...
(we are discussing a leaf spring car so do not even bring any other type suspension into this, each suspension is different)