Jeep XJ Control arms and Pinion Angles...

[LKN_xj]

Hey Guys, I have a 1991 XJ with 3.5" RE lift with full packs. I'm running my stock front driveshaft in the rear with a Crown SYE kit and a chrysler 8.25 swapped in. I also have a 4 degree shim in the back to correct the pinion angle for the double cardon shaft.



Interestingly enough, my rear DS angle is 17.5 degrees and my pinion angle is 10 degrees up, making my pinion angle in the rear 7.5 degrees down from the DS. This SHOULD be unacceptable but 1) I can't figure out why I would need more than a 4 degree shim in the back and 2) my stock front DS runs perfect with no vibration in the rear.



I purchased a new double-cardon DS for the rear and can't get it to stop popping. The vendor is being great about it. They're saying my stock DS isn't popping in the rear because it's "broken in" and can accept a greater angle, where the new shaft isn't.



In effort to correct the pinion angle in the rear to bring it more in line with the ideal 1-2 degrees below the DS angle from my 7.5 degrees, I was considering the RC shackle extension bracket to point my pinion up a bit instead of going to a 6 degree shim. But this will give me 1.5" of lift on top of my 3.5", so now I have to level the front.



My question is two fold:



1 - When the back rises 1.5" from the shackle relocator, I need to level the front. Plan was to put 1.5" coil spacers up front to compensate. I have RE longer lower control arms that came with my 3.5" kit, and stock uppers but would be replacing the uppers with adjustable uppers. When I raise the front 1.5", can I still use the lower control arms from my 3.5" lift, along with adjustable uppers to get my front pinion back where it needs to be, or do I now need adjustable lowers and uppers?



I know that was a whirlwind. If there's easier options I am open to that too. Maybe the easier route is just to do the 6 degree shim and call it done.



Thanks!!

 

[TRCM]

do the 6 deg shim and be done with it.

MAKE sure you get the shims that come with a new bolt and actually bolt to the spring, not just sit under it with the bolt head keeping the shim in place. The latter kind often move as the bolt head isn't big enough for the shim AND keeping the spring centered correctly. I just went thru swapping shims out for this very reason.

Also make sure you use new u-bolts.

 

[LKN_xj]

Thanks, sounds easier doesn't it?

Here's the shim I have in there now. It bolts to the pack with a 3/8 bolt.

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[LKN_xj]

do the 6 deg shim and be done with it.

MAKE sure you get the shims that come with a new bolt and actually bolt to the spring, not just sit under it with the bolt head keeping the shim in place. The latter kind often move as the bolt head isn't big enough for the shim AND keeping the spring centered correctly. I just went thru swapping shims out for this very reason.

Also make sure you use new u-bolts.

What if I hypothetically wanted to go up 1.5" in the future? If I did the shackle relocation in the back and a spacer in the front, could I use the LCA's that came with the 3.5 kit if I got adjustable UCAs to adjust the pinion angle?

 

[TRCM]

What if I hypothetically wanted to go up 1.5" in the future? If I did the shackle relocation in the back and a spacer in the front, could I use the LCA's that came with the 3.5 kit if I got adjustable UCAs to adjust the pinion angle?

You want adjustable lower control arms to adjust angle....otherwise you will pull the top of the front axle back towards the body to get it adjusted correctly.

Much kits come with adjustable lowers if needed in the lower lift ranges. Once you get up in height, then adjustable upper/lower come into play.

 

[LKN_xj]

You want adjustable lower control arms to adjust angle....otherwise you will pull the top of the front axle back towards the body to get it adjusted correctly.

Much kits come with adjustable lowers if needed in the lower lift ranges. Once you get up in height, then adjustable upper/lower come into play.

How high is "up in height?" My kit came with fixed lowers and didn't come with uppers for 3.5". I wonder in my case if I'll even need them. I hear what you're saying. For more detail, my front pinion is ALSO 7 degrees below my driveshaft, which is contributing to some vibrations from the front end at high speed. I'm currently driving with the front DS out because of it. My pinion is too low. Raising lift higher would make that worse, but then if I got adjustable UCA's I could elongate them to push the top of the axle forward to bring the pinion back up yea? Maybe I'm thinking of this in reverse.


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[TRCM]

You could do that to bring the pinion up, but then your steering angles would be all out of whack.

Think of this.....draw a line thru both ball joints on the axle. This is your caster angle. Now, if you rotate the front axle forward at the top like you want to, this angle will tilt forward, and when it does, then when you turn, the tires won't pivot on the bottom but on a point somewhere up the tire radius, inducing sideways motion to the front axle while turning and greater tire scrub.

I got 4.5" lift on mine right now, and no vibes from my front D/S at all, so @ 3.5" you shouldn't either....have you checked the u-joints yet ??

How do you know you have vibes from the front D/S....have you been driving at high speed in 4wd ??

Are you sure it isn't a balljoint or an axle shaft u-joint ? or maybe tire/balance ??

Check out this chart for suggestions on what is needed vice suggested for the various lift heights. It's not perfect, but very close. I say not perfect, in that I don't have some of the stuff they say is needed for 4.5", and I have no issues or problems with mine.

 

[LKN_xj]

You could do that to bring the pinion up, but then your steering angles would be all out of whack.

Think of this.....draw a line thru both ball joints on the axle. This is your caster angle. Now, if you rotate the front axle forward at the top like you want to, this angle will tilt forward, and when it does, then when you turn, the tires won't pivot on the bottom but on a point somewhere up the tire radius, inducing sideways motion to the front axle while turning and greater tire scrub.

I got 4.5" lift on mine right now, and no vibes from my front D/S at all, so @ 3.5" you shouldn't either....have you checked the u-joints yet ??

How do you know you have vibes from the front D/S....have you been driving at high speed in 4wd ??

Are you sure it isn't a balljoint or an axle shaft u-joint ? or maybe tire/balance ??

Check out this chart for suggestions on what is needed vice suggested for the various lift heights. It's not perfect, but very close. I say not perfect, in that I don't have some of the stuff they say is needed for 4.5", and I have no issues or problems with mine.

I simply know the connection of the front driveshaft causes a vibration. Could be anything I suppose - I just know there's a direct correlation between driveshaft and vibration at speed. There's also a correlation between vibrations and lift versus no lift so there has to be a pinion angle issue OR an issue caused by lift geometry changing. I've checked ball joints but haven't checked u-joints. Before the lift, no front vibes (high frequency, not tire issues).

I agree that at 3.5 inches I shouldn't have many issues, but that said I still have a 7 degree difference between rear drive shaft and rear pinion after a 4 degree shim.

Regardless of this I'm rebuilding a lower mileage HP 30 in my garage so I'll very soon eliminate u-joints, ball joints and hubs as an issue so that's exciting!

Thank you very much for the caster explanation from the line drawn through the ball joints. That is a good visual reference. Would you also be more prone to death wobble with the pivot point changed?


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[LKN_xj]

Is there a standard pinion angle such that caster should be perfect so you can figure out what your caster is based on pinion?

For example pinion should be 3 degrees up for a caster of 7 degrees or whatever. Then you adjust based on what problems you're having?

I guess this would assume an unbent virgin axle...I see ideal is +7 from vertical with a range from 5.25 to 8.5.


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[LKN_xj]

This thread says its 1:1 per spec. So zero pinion = zero caster I believe. Since my pinion is pointed up 3.5 degrees or so, my caster would be +3.5 which is below spec? Seems like I need to raise the pinion 2-3 degrees and change caster that same amount.

http://www.naxja.org/forum/showthread.php?t=1077166


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[TRCM]

I simply know the connection of the front driveshaft causes a vibration. Could be anything I suppose - I just know there's a direct correlation between driveshaft and vibration at speed. There's also a correlation between vibrations and lift versus no lift so there has to be a pinion angle issue OR an issue caused by lift geometry changing. I've checked ball joints but haven't checked u-joints. Before the lift, no front vibes (high frequency, not tire issues).

I agree that at 3.5 inches I shouldn't have many issues, but that said I still have a 7 degree difference between rear drive shaft and rear pinion after a 4 degree shim.

Regardless of this I'm rebuilding a lower mileage HP 30 in my garage so I'll very soon eliminate u-joints, ball joints and hubs as an issue so that's exciting!

Thank you very much for the caster explanation from the line drawn through the ball joints. That is a good visual reference. Would you also be more prone to death wobble with the pivot point changed?


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Could also be an issue with the tcase front output.

Are you sure the tcase is disengaging completely ??

 

[LKN_xj]

Could also be an issue with the tcase front output.



Are you sure the tcase is disengaging completely ??

I'm not sure how to tell if it is. It only comes on at 55 or so. The transfer case is duly rebuilt NP231, probably 200 miles on it and that's it.


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[TRCM]

Not sure.....but front driveline vibes when the D/S is in, means either it is the D/S, the tcase, or the axle (gears/pinion bearings or axle u-joints), as everything else is moving with the D/S out also

 

[LKN_xj]

Not sure.....but front driveline vibes when the D/S is in, means either it is the D/S, the tcase, or the axle (gears/pinion bearings or axle u-joints), as everything else is moving with the D/S out also

I'm getting ready to find this out - I'm rebuilding a front Dana 30 now with backlash that is in spec and it will have new joints. I have a brand new DS to put in and my Tcase is newly rebuilt.


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