Can anyone help me understand rear drive shaft angles?

I have stock 96 xj, leaf spring helpers in rear and 1inch spring spacers in front. Seems like minimal change in angles but but have a terrible vibration and noise when under heavy load such as up hill or sometimes when speeding up to get on highway.

Here is a picture of my rear drive shaft with the angles marked.




Do I need shims? Everything I've read just causes more confusion.

U joints are new, no missing weights on drive shafts, vibrations continue with front shaft removed. In good alignment on front, may need wheel balance but this has happened even when tires were new.

Thank you for any help!

 

I'm working thru the same issue right now with my 2001 XJ. Put a 2" lift (new springs) in the back and picked up some rear driveline vibration. The secret is that the transfer case and rear diff angles have to be the same. Looks like yours are 4 degrees apart right now.


Lots of folks put a transfer case drop kit in, which should help. One mine, I put a 1/2" drop kit in last weekend, and it only changed my overall transfer case angle 1/2 degree. Now my angle difference is down to 1.5 degrees, and I still feel a little vibration at certain speeds.


My next step will be to add a 2 degree shim to the rear axle and bring it down by that much. Should get me to within 1/2 degree total difference, which I probably won't be able to feel.

 

Typically such a small lift like that wouldn't cause any altercations with your pinion angle. I'm stumped on the vibration, but an idea for the noise could possibly be differential related? When was the last time you changed the fluid? And if you're doing any towing or up hill driving as you said, what weight are you using?

 

To simplify, for the stock 2-joint driveshaft the angle of transfer case output shaft to drive shaft (6*) should be equal to the differential output to driveshaft angle (10*).
Vibrations under acceleration is a sign of pinion being too low. Also, lift shackles and lift blocks can make this worse due to axle wrap, the changing of pinion angle due to warping of leaf springs during acceleration.

Also, anytime you change the ride height the working position of the joints in driveshaft also change. If the joints are worn, than the new position can cause a vibration as joints cycle through those worn spots.
http://www.hotrodhotline.com/md/html..._harmonics.php

 

I have had this Jeep for about a year. I haven't changed the fluid in differentials but I have pumped some out just to see if it looked good and it wasn't dirty.

I am really wondering if my springs just desperately need changed. Maybe it is the axle wrap you mention.

I do have the upcountry package. So with my current one inch lift I am sitting right at where I should be with the upcountry. Not at 2 inches like you'd expect.
Which means those springs are pretty worn out.

 

I just went thru exactly the same exercise, after a 2" lift (new rear springs in back). Had some vibration after the lift, and tried a 1/2" T-case drop (helped some), then tried a 2.5 degree rear wedge to try and get the t-case and rear diff angles the same.


This made it worse because after dropping the diff angle down to nearly match the t-case, the driveshaft-to-diff angle was large enough the a still had the vibes.


Bottom line - what worked was getting rid of the wedge, and dropping the t-case a full inch.

 

The wedge or degree shim is generally used in conjunction with a SYE, slip yoke eliminator, and a 3-joint double cardon drive shaft like the front one. It is set up differently.

 

drop the t-case or go sye, playing with the shims probably wont give the results you are looking for.

 

Sorry for delayed update. Just had a baby, been a busy few weeks.

Attempted to install 1 inch T case drop but am having trouble extracting the studs so I can install the longer bolts.

I have tried to tighten two nuts on bolt but all it does is strip the threads on lower bolt.

I have tried really tight vice grips with same result.

I even purchased the Irwin grip bolt extractor but the smallest size in that set cannot grip the end of the bolt.

Thinking about trying the smaller Irwin grip set but it looks more geared for screws and don't know if it will be strong enough.

Any suggestions?

 

You're having the same problem I had - those old frame bolts get rusted and stuck in the frame and are very tough to get out.


What finally worked for me was lots of PB Blaster, heating the bolt several times with a torch, and then using two locked nuts on the exposed part of the bolt. Had to work it, heat it, and PB it repeatedly before the rusted bolts started to break loose.


If your exposed threads are stripped, you can't use the locked nuts, and you won't find pliers or vice grips that will get a strong enough hold. At this point you'll either have to cut off and drill out the old stud, or weld something to the stud and then work it back and forth until it starts to break loose (don't forget more PB Blaster and heat).

 

Those studs are a royal pain. I had to weld nuts on to them to get them out.

 

I had a similar issues but mine was on a 3" lift with SYE. I was running a front shaft on the rear and i had replaced all the ujoints in it. I ended up going with shims bc the drive line angle was off. That didn't fix it so i swapped the front shaft to the rear and it was gone. I assume the driveshaft had just became unbalanced or something.
Maybe your DS has the same issue. I think they are like $25 at the local pick n pull here for a shaft.

 

nice big pipe wrench on the stud and divine intervention.

 

oh an days of soak with blaster/wd 40 morning an nite

 

from your pic, I'd start with 4 deg shims on the rear axle (thick part of wedge facing front).

Or a 1" TC drop with 3 deg shims on the axle (thick part of wedge facing front).

These are starting points, and may need adjusting.

As the previous post recommended, I would change out the U-joints too if it has a lot of miles on it since it may be worked-in to the old geometry.

Also, be aware of how much spline engagement you have on the driveshaft at the TC.