Getting the driveline right

I have a 98 XJ 4.0 auto with ~5" of lift. I had a 1" t-case drop and 6* shims, things ran pretty smooth. I put in a H&T SYE with front DS at the beginning of the summer, took out the shims and t-case drop. Had to shorten the front drivehaft, a front XJ shaft wont fit mine for whatever reason. Thats a different mystery.

I can't remember what my angles measured out to, but a 4* shim should have put my diff right ~ 2* below the case output according to the writeups Ive read. I put the 4* shim in, vibes like crazy. Took it out, much better but still vibes up around 50. Thought maybe I should have divided the measurement by 2, since the pinion going up means the t-case angle going down too. Tried a 2.5* shim, just as bad as the 4* shim.

Ive taken all the shims out, and tried 2 washers under the x-member, it's slightly better than nothing at all, but not much still vibes around 55 and up. Its more of a noise than a vibration.

Any thoughts on how to get the perfect driveline angle? This is the only thing that keeps my jeep from going down the road as smooth as stock, giving it one more try before I find a driveline shop.

Thanks

You need an angle finder, if you intend on dialing in your shaft angles. Throwing various shims at it without knowing what affect each change is having, is a slow, painful, and probably ineffective way of going about it.
Constant velocity shafts (double Cardin u-joint at one end) require the pinion vs drive shaft angle to be ~1* low at the diff, if you have leaf springs in the rear; linked suspension could be "0", +- .5*. The 1* low angle is to allow for the upward movement of the rear pinion under acceleration--and at sustained high-speed driving, due to spring wrap; the linked suspension doesn't experience the same axle wrap, so the angle could be 0*, but you should have approximately .5* u-joint angle to insure u-joint bearing lubrication.
At 5+ inches of lift, the front u-joint angle of the rear shaft is probably nearing critical mass (around 15*??). At some point, vibration is unavoidable until the u-joint angle is reduced by lowering your transfer case.
The thing is, at 5" of lift (true lift, not "kit" advertised lift), it will take trial and error, and compromise, to achieve angles resulting in "acceptable" levels of vibration.
Read Tom Wood's write up for drive line angles: http://www.4xshaft.com/driveline101.html

maybe the new rear driveshaft is causing the issue, if you tried all those shims, not too much more you can do, but as xjbubba says use an angle finder and set the pinon vs driveshaft angle up.

As I said, I read all the writeups and did the angle finger. I had 18-19 degrees measuring with a socket on the pinion side of the u-joint and I had 25 degrees on the driveshaft side, after jacking it up and turning the shaft by hand to make sure they were both measured on the same plane. I put in a 4 degree shim, thinking this would put me at ~23 on the axle side and maybe bring my shaft side down slightly, giving me my 1-2* difference. Didn't work. Is there any other way to measure, or am I measureing incorrectly? This is the method I used: http://www.stu-offroad.com/axle/pinion/pinion-1.htm

Funny thing about the rear end of this jeep, it seems to be shorter than most somehow. I did the IRO SYE, and could not cut following thier instructions it would have put my cut at the t-case, with no ouput shaft left to bolt to. A stock front driveshaft from a 4.0 auto 231 fully compressed is still too long to fit. :dunno:

That style of kit is known to be a problem.The yoke is a "slip" style and doesn't press on to the splines.Also I've found most stock front shafts have to much play in the slip-yoke.The 96-97+ years always have more issues due to the new t-case change.I lifted my 2000 right after coming home from the dealer(had 27 miles on it).I tried this style kit and could never get rid of all the vibes.I immediately went the a JB Conversions kit,now 10yrs(and 5 lift kits later) its still smooth a silk.Heres a pics of my old Oscar's kit(it did include a new driveshaft though,not a XJ front)!

Seems you have more than one issue going here.
As RCP says, it's possible your TC mod and selected drive shaft are the issue.
To help eliminate them as the reason for your vibration, concentrate on shaft angles first.
To that end, I'm not sure you precisely followed the directions given by the post you cited; namely his admonishment: "---You can NOT do this adjustment when your vehicle is up in the air, supported by the frame, on a vehicle lift.---. I say that because you stated: "---I had 18-19 degrees measuring with a socket on the pinion side of the u-joint and I had 25 degrees on the drive shaft side, after jacking it up and turning the shaft by hand to make sure they were both measured on the same plane You need to have the truck setting level, on the ground with full weight on the suspension.
To measure the pinion angle, I always use the two round "bosses" located on the rear of the differential case, one on each side, near where the axle housing enters the case. These are used for a case spreader, and are perpendicular to the axle center-line, so they are a good proxy for measuring the pinion angle, and you don't have to futs with getting the rear u-joint properly oriented.
Adjusting the drive shaft-to-pinion angle is an iterative process--installing a shim affects the drive shaft angle as well as the pinion angle--so predicting what the resulting drive shaft-to-pinon angle will be if a given shim is installed is difficult. You measure, install a shim, and re-measure. You keep doing this until you get the prescribed angles. Then test drive, then--you get the picture.
The right way to do it is to cut the spring pads loose on the axle tubes, rotate the diff with full weight on the suspension, including a full tank of gas, and whatever "weighty thing" you normally carry around. You make the adjust, re-weld the spring pads, then test drive.
Now, if you went from a standard, two single cardan u-joint drive shaft, to the double cardan shaft, you need a shit load of pinion rotation; I doubt 4* would even start to get you there. If that's the case, I HIGHLY recommend you rotate the spring pads on the axle housing.
If you haven't done so, I recommend you read the Tom Woods' tech info at: http://www.4xshaft.com/index.html ; Tom Woods is the drive shaft guru worth paying attention to.

So the equipment itself may be the issue. Always a possibility. Thats why Ive been playing with angles first. I lifted the vehiceto spin the driveshaft, then put it back down on the ground. I did not take any measurements with the vehicle in the air.

I will try taking measurements from the different places to see what I get. I tried a 6* shim also, as it's what was in the jeep with no sye. Almost smooth at coast, but horrible with any load on the driveline. Thats why I figured 4* would do it, after taking measurements confirmed my thoughts.

Maybe I was just measuring in places not quite perpendicular, always possible and hopefully more likely than bad components.

Heres a thought. I had to cut down a front XJ driveshaft, it was too long. Maybe I cut off too much, and the amount of engagement is too little. If the driveshaft is extended too much, can it vibrate? It seems to quiet as I hit dips in the road, which would compress the shaft...

I haven't had time to put the shims in and remeasure, been a rainy month.

I have ~3 1/8" of the slip section showing, there is a total of 3 3/4" of slip shaft before the splines are exposed.

How much compression should be left on a driveshaft sitting at ride height?

98XJSport said:

Heres a thought. I had to cut down a front XJ driveshaft, it was too long. Maybe I cut off too much, and the amount of engagement is too little. If the driveshaft is extended too much, can it vibrate? It seems to quiet as I hit dips in the road, which would compress the shaft...

I haven't had time to put the shims in and remeasure, been a rainy month.

Click to expand...

After you cut it, did you get it balanced? If not, that very well may be your issue. Just like tires, modified d-shafts need balancing too.

I can't find anyone locally to balance it, so no. But it didn't wobble on the lathe we cut it on, for what that's worth.