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How much is too much stock driveshaft angle

35K views 31 replies 10 participants last post by  MTJeepTJ 
#1 ·
I just got some Rokmen rear lower control arms to help adjust my pinion angle after my 3" lift. Yes, I got lower instead of uppers because Rokmen uppers don't go any shorter than stock so it wouldn't have bought me anything. I've also installed an MML to help with the angle.

I started with ~1/2" over stock LCA length and gave it a ride. The vibrations were reduced dramatically but now I am getting this loud humming from the rear along with a little vibes

LCA Installed :D


Driveshaft angle relative to ground


T-Case Angle


Pinion Angle


It appears that I am about 1-2* off with the pinion. I moved the jeep until the t-case/pinion yokes were flat to the ground and then took the measurements off of them (let me know if you have a more accurate method please!). I'm going to try for another 1/4" on the LCAs later this weekend and see what it buys me. I am curious if the angle of the driveshaft might be too steep with the stock setup? I have a feeling the steep angle might be causing the humming
 
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#3 ·
I understand the side effects of longer lowers, from what I read this will be negligible since I am just extending the lowers and not the uppers at the same time. MML has already been done per the OP. Ultimately I understand SYE/CV is the way to go but I am trying to find a solution to get me by in the meantime. When I am ready I will do SYE/CV/TT/Rear trackbar all at once and do it right.

A link in the link you provided suggests a maximum driveshaft angle of 15*, I'm unsure if this is relative to the TC/Pinion or to the ground. If that is the case then I am already approaching the limit
 
#5 ·
The problem is you've made the angle at the driveshaft's u-joints worse which is why it is vibrating.

The rear pinion angle must (!) be parallel to the transfer case output shaft and yours is not.

This is what your rear pinion angle must look like... it must be the same angle as the output shaft coming out of the transfer case. Exactly.
 

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#10 ·
I can't seem to get this quite right. I used Stu's method of pinion measuring with the socket in the u-joint end cap but I can't seem to take the same measurements twice on either pinion or yoke. Any suggestions on accurate measuring? I'm ready to go buy a digital angle finder if it helps
 
#16 ·
Neither dignital nor sub-degree accuracy is necessary. The below method works fine... this ain't rocket science folks. Heck I don't even need/use an angle finder to set up the pinion angle.

The pic on the left shows how the pinion angle is measured from that rear flat circular spot to the passenger-side of the rear differential cover. That particular location is exactly perpendicular to the pinion angle so holding the angle finder like shown shows the pinion angle directly. The pic on the right is of course showing the drivehshaft angle.
 

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#18 ·
I've been tinkering with it and just can't get rid of these vibes. I went as far to remove the driveshaft and take it for a ride, night and day difference. At least now I know it IS the DS or u-joints. I used the opportunity and aligned the pinion off of the point Jerry pointed out above directly to the output of the transfer case. Using a HF digital angle finder they were within 0.2* yet I still have vibes.. I noticed that the slip yoke has some vertical up/down play, about 0.3* worth when measuring at the slip yoke base, is this normal?

I feel at this point I have a driveshaft balance or u-joint problem, any input?
 
#19 ·
I'm at my wits end with this one now. I've been tinkering with it some more. I tried dropping the t-case skid a good 1/4 to 1/2" along with readjusting the arms and I'm still in the same boat. The weird thing I noticed is that dropping the skid this much only changed the slip yoke angle by ~0.2* I noticed that Jerry's measurement location isn't the exact same from what I am getting directly off of the u-joint (I've seen up to +/- 1*). I've tried matching either location with the slip yoke u-joint with no luck.

With the new angle finder I can get consistently accurate measurements +/-0.1*

I roll the jeep until the slip yoke angle is about 0*


I take the pinion u-joint angle with 2 super magnets back to back attached to the u-joint end cap


Slip yoke angle directly off of the yoke


Wide view of entire setup


Measurement off of back of axle, it says 80.6=9.4*


T-case skid drop


I'm going to try to pick up some u-joints and see if that helps. Finding spicers locally is hell but I found a few shops that carry them but have been closed all weekend.

Any other suggestions?
 
#25 ·
A few updates for those who may care, I did several things and finally got rid of the vibes :thumb::thumb:

First up, I replaced my original transmission mount, this made a huge difference at take off, all of the chatter I had is now gone and it feels completely different



Next up I replaced the two u-joints in my rear driveshaft with two spicer 5-1310x (5-785x equivalents). Man these suckers were hard to find locally



Finally I (sadly) dropped my t-case skid a little over 1"


T-Case output angle is now 9.9*


Driveshaft angle is now 14.10*


Rear Pinion is now 10.00*




Operating angles for each u-joint have now been reduced to about 4.1*! That's a huge difference from before :) She drives great now and this will get me by while I save up for all of my SYE/TT gear (going to run me about $1600 for everything I want). Thanks all for the help
 
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