Work in what order to fix driveline vibes?

[SurplusFan98]

Did an RE 3.5" Superflex lift on my 97 XJ with the full leaf packs. Naturally, I have driveline vibrations now. I know I need an SYE kit, rear axle shims, and a double cardan axle to fix it properly. I want to use the front axle off of a ZJ as my new rear axle if I can since they can be easily found for cheap in my area. I also want to have as little downtime as possible where I can't drive the jeep. So here's my question:

In what order should I make these changes to keep my disassembled time to a minimum?

My thinking is that I should probably go ahead and find shims that put the rear axle at a proper pinion angle first. The work to do that doesn't take long and I could technically still drive the already shimmed axle for a couple of days before installing my SYE.

When the SYE comes in, I can get the measurement from the back of the new housing to the flange. Since my pinion angle is already correct, I could do a little math, and that should tell me what size driveshaft to go and pull before I take everything apart for the SYE install. That would reduce my disassembled time to just however long it took me to do the SYE install and reassemble using the new (to me) drive shaft. Maybe a single Sunday afternoon...

Can anybody see any reason why that logic isn't sound?

 

[xcm]

I want to use the front axle off of a ZJ as my new rear axle if I can since they can be easily found for cheap in my area

?
What in the what now?

 

[SurplusFan98]

I was led to believe based on some reading that, post lift, I could use the front drive shaft from a particular ZJ configuration as the rear driveshaft in my XJ. The ZJ ones are generally longer, and that would give me a cheap double cardan in the back without having to order a custom driveshaft from somewhere like Adams @ $350 or so. I found charts like this one that give collapsed shaft lengths based on year/engine/tranny/t-case.

So a front drive shaft out of a 97 ZJ 4.0 with an automatic tranny is 31.250", and they go up or down in length based on the changes to the configuration. If this drive shaft worked in my scenario, I can get one in my area today for $25.

Have I misunderstood?

 

[RCP Phx]

Shimming requires the SYE kit to be installed in order to get the right angle shims!

 

[xcm]

Have I misunderstood?

Apologies, you called it an axle. I later figured out what you meant.

 

[75SV1]

I've only done the Hack-n-Tap. I am doing one on a 241 TC case and 2" lift. I think your logic is sound to an extent. I don't know what length between the yokes your setup will give. Lift, axle and possibly other stuff will determine that. I am running Ford 8.8 axles with the adapter or conversion to the Jeep U-joint. It adds about 3/4", compared to an 8.25.
Anyways, whatever length between the yokes you measure, it should be 1" less in the compressed state. Thanks for the chart. I have run the stock front axles as the rear for probably 5 years now. A few time wheeling, too.

 

[SurplusFan98]

Shimming requires the SYE kit to be installed in order to get the right angle shims!

Upon further reading, you are absolutely right (and it totally made sense when I stopped to think about it). The SYE lengthens the drive shaft enough as to significantly effect the drive shaft angle. Seems like this is a job you can't really make more efficient. Better to just do it right the first time.

Apologies, you called it an axle. I later figured out what you meant.

LOL! Good catch.

 

[maxbraketorque]

If you are looking for a quick fix, another method to reduce vibes is to put spacers on the transmission mount to drop the trans by about an inch.

 

[SurplusFan98]

Revisiting this for some further advice. My SYE install is finished and I'm trying to figure out axle shims to correct the pinion angle, but I feel like I'm not understanding fully and I don't know if I need to try and grab a replacement DS first. Here's what I have:

At the transfer case yoke face - 84* -> 6*


At the rear axle yoke face - 86* -> 4*


Mocked up DS angle using string from yoke face to yoke face - 74* -> 16*


I also measured for yoke face to yoke face at ride height and got an axle length of right at 34.5".

So my question is again about order of operations here to get the most accurate fit. I could go get a junkyard DS now @ 33" collapsed length and that should be pretty good, but when I adjust pinion angle by shimming I'm pretty sure that would shorten the DS slip a little and I don't know how to estimate how much. On the other hand, if I try and adjust pinion angle first, I can't account for the double cardon joint and feel like that will effect the pinion angle as it sits back a little from the face of the yoke.

I'm also concerned about how much angle it seems I should be taking up. I read other forum posts where guys have installed my lift and needed shims that were 4-5*, but my calculations seems to indicate something larger. If my op angle at the axle is 12*, then I would need something more likely in the 6-8* range, and my reading makes that sound high for a 3.5" lift. That makes me question if my understanding of this is correct.

Any advice?

 

[RCP Phx]

The transfer case output doesn't matter, only the driveshaft angle to the pinion angle.
https://4xshaft.com/blogs/general-tech-info-articles/driveshaft-angles

 

[SurplusFan98]

The transfer case output doesn't matter, only the driveshaft angle to the pinion angle.
https://4xshaft.com/blogs/general-tech-info-articles/driveshaft-angles

OK, cool, thats pretty much how I had interpreted it too. So, to simplify my above question, should I

a) go grab a double cardan shaft from the yard based on my 34.5" measurement from above (I'd pull a 33" collapsed from a 97 4.0 ZJ), mount it, and then recheck my 2 angles and correct

--OR--

b) try to go ahead and correct the pinion angle with shims first, get as close as I can, recheck my driveshaft length, and then go pull one based on that measurement.

I'm trying to solve for 2 problems simultaneously, and I don't know which is more forgiving. I'm leaning toward option a.

 

[RCP Phx]

You would be guessing without a driveshaft, also you need the pinion about 1-1.5* lower for axle wrap.

 

[RCP Phx]

Do realize that a 2* shim is going to net you about 3.5* of total change in degrees.

 

[techno1154]

I would go with the "A" question.
Put a drive shaft in first then measure. I ran a front drive shaft in my XJ for several weeks on paved road only until I got one of the correct length.

 

[RCP Phx]

Something else to keep in mind is that anything more than about a 4* shim (and make sure you use good steel ones) will require the shock mounts to be rotated to get them back in alignment. If that is needed I would recommend moving them "up and out" for better clearance!

 

[denverd1]

Something else to keep in mind is that anything more than about a 4* shim (and make sure you use good steel ones) will require the shock mounts to be rotated to get them back in alignment. If that is needed I would recommend moving them "up and out" for better clearance!

need to be moved anyway. drag on everything in stock form