driveline angles parallel, new u joints, smooth untill 100kmh, balance shaft or sye?

01 xj, just installed new u joints, greased the yoke splines, shimmed my pinion 2.5* down, t case output is pointing down 4* D35 is pointing up 3.5*, 3.5" OME lift + about a 1/4" in the rear from the shims.

the driveline has been super smooth around town, but i took the jeep out on the highway yesterday and as soon as i get to 100kmh the jeep picks up this humm, not like the mild vibe i had before, but a humm that could drown out a conversation pretty quick,not shaking the jeep to death but im sure an hour of that humm would cause some damage

this jeep will be making 3 hour high speed trip bi weekly for the rest of time if i have my way with it, so i gotta be able to run at 100+ kmh without trouble, the only other thing i could do that i have not done is have my drive shaft balanced.. what do you guys think? throw a couple more bucks at a balance job in hopes that it solves my high speed trouble, or is this the point where its SYE or bust?

i tried shims and u joints first just because the shims were free and u joints cost next to nothing..im not really out any money and it did make my around town drive much better than before with no shimming.

i should also add that i have new gears/carriers/bearings installed. just did the 500km break in diff oil change and everything is tight and looking good. no slop in anything that i can feel, i did have a bit of a clunk coming from my factory slip yoke before the d shaft rebuild,but fresh grease made that quiet..maybe the clunk ment my slip was worn out? and the grease is just a bandaid?

i do not have a transfercase drop, and im not willing to install one after reading all the draw backs.

i have read other posts with similar problems, but most of them seem to be guys with SYE's getting the hummmmmmmmm

A loose slip yoke can cause a vibration. At +3.5" lift, you probably should have an SYE, why not get one ? At 2" of lift, my 00 needed a T-case drop.

i will get one if thats what it takes, i just had the goods on hand to get my drive line parellel
figured i would give that a try to see if it -would be the cure..and i honestly thought it was good to go untill i got the jeep running at highway speeds.

either way, its been a good learning experience, learned alot about driveline angles and now have a good grasp on the how and why's that i did not have before going through the paces and trying different setups out.

BUT i would still like to know WHY i have this humm now, possibly even with the driveline being parellel, my U joints are operating at to high of an angle.. not sure, and i hate that, i really like to know why things are doing what they do, i am a millwright, i cant help but want to know everything about everything.

the only drawback i have with the SYE is down time, i can get the sye installed no problem, but if my reading is correct, my dana 35 rear end makes it so i cant just slap in a front auto 4.0 shaft to get my wheels back on the road while i build a proper new driveshaft. it will be down for at least a week while i have something ordered in. so this was worth a shot

The local drive line shop can shorten or lengthen and balance a drive shaft in one day. If necessary, you can drive in 4x4 with the rear drive shaft removed.

The greater your drive liune angles, the more elliptical the u-joint rotations are. The u-joint is speeding up and slowing down which creates the vibrations you feel.

The angles stated mean nothing. Seriously.

The angles (T-Case to DS) and (DS to rear yoke) should be equal an opposite. Post those figures, and it will make sense.

External slip yokes will "Clunk" if they are short on grease on the splines. But don't pack it tight because they have to compress.

Maximum angle of operation of a u-joint is around 5 deg. Allow much less if DD. Operating them at a high angle of operation will cause premature failures and as the limit is reached, can cause "hum" at the speed where all the parts in the system come into harmony.

Vibrations that are speed dependent will hum in a cyclic manner then steady when you reach that magic speed, then will start to hum in a cyclic manner as you pass that speed.

You didn't post anything about tires, tread type, if you "pushed though" the vibrations...
Does the "hum" go away if you "coast" at the speed indicated, or just under power.

i am gonna have to think more about your post before i can give the info your looking for, i set my yokes to run parallel to one another,like is posted on the most of the driveshaft webpages,

thanks for your help though

Boost_Retard said:

but if my reading is correct, my dana 35 rear end makes it so i cant just slap in a front auto 4.0 shaft to get my wheels back on the road while i build a proper new driveshaft. it will be down for at least a week while i have something ordered in. so this was worth a shot

Click to expand...

I think you are incorrect. I installed the RE HnT in my 1998 and used a front driveshaft, and the 211229x yoke. I had the D35 axle and auto tranny. This was done the night before I did my lift and I could drive it around with no problems.

I was under the impression that a front shaft would be too short.
i will just grab one and give it a try then.

and i understand now your asking for the operating angle of the u joint. i will have to check my notes tomorrow as i dont have them with me right now, thanks for the help