wristwister

OK here's the deal on FrankenJeep II (95 XJ, Dana 35 LSD rear). We had a 3" lift, lifted springs up front, blocks above axle in back, long shocks all around. Everything seemed fine, ran nicely. But ... it was riding a little low in the rear, especially after installing that heavy XRC rear bumper. Plus, I hated the idea of those 3" blocks on the back, plus the stock rear springs seemed a bit sprung. So ... we bought a set of Rubicon Express 3.5" lift rear springs. We just mounted them up, got rid of those blocks, everything looks great although now it's maybe 1" or so higher in the rear than the front but that's fine. Took her out for a road test and ... thump thump thump, some sort of noise and vibration at slow speed. It feels like it's something with the wheels/tires, but all is perfect there, nothing rubbing, nothing out of sorts.

So the question is, what could have happened from simply raising the rear a couple inches with goods springs? Is it possible that couple inches did something to drive shaft alignment. If the Jeep is sitting slightly rear high, does that cause some issue? HEEELP! My kid and I want to take her XJ on a road trip this weekend, but we need to fix this thump thump thump! Thanks

5-Speed

Sounds like time for a sye to me. I have the same leafs as you and mine lifted more then the advertised 3.5" for sure. What I did to help was remove the 2nd leaf from the bottom. Dropped it down about an inch. In your case, that might be enough to get rid of the noises for now but just know, a sye is the right fix.

Rogue4x4

Winner for thread title of the year, lol

Rogue4x4

All kidding aside, have you measured the angles on your driveline? Should be within 3* of each other. SYE is the best answer, but you may need to shim the rear axle even with a SYE.

As you're learning, there's a lot involved with lifting, there's no one perfect way to do it, it takes time, patience, and money.

Quick fix would be a TC drop, but it's a bandaid. If you want to verify, it's a cheap way to get by for now, but may or may not work, but its cheap, easy, and reversible. Downside is hard on your mounts.

SexyDexy

When I installed 3" rear springs on my daughters XJ, all I needed to was space the cross member down about 1/4" (with washers) to get rid of the vibrations. Only space it down as far as you need to...no sense going to a 1" drop from the start and losing more belly clearance than you need to.

wristwister

Thanks for the advice all. It sounds pretty unanimous that my problem is likely driveshaft alignment. I've read up on the SYE and that does indeed seem like a good solution, although we've been hemorrhaging money on this thing (why didn't you guys warn me about that before we started?) so we may pursue a less costly fix.

Ronin, good suggestion on the shimming. I understand the need for the same drive shaft angle at Xfer case and axle. The lift block that used to be there was slightly canted, probably to maintain this angle. I might look into this wedge thing.

I'm hesitant on the TC drop thing, I've been reading about some downsides to that. Although I'll keep that in my bag of tricks if needed.

Dexy, which cross member are you talking about?

Thanks again all

SexyDexy

The cross member that supports the trans. Its the same that gets spaced down with 1" T/C drop kit. I am just suggesting to start with 1/4"-1/2" drop instead of jumping to the full 1" drop.

This is an example of someone's 1" T/C drop

TRCM

JEEP

Just
Empty
Every
Pocket


If you only hear it at low speeds, chances are you're pretty close, as the more power you put on it, the more the rear axle will angle up from the torque and align itself somewhat.

Jeepin'_Aint_EZ

It's also a joy trying to remove those bolts.... just a heads up. Don't plan on it being a smooth.

wristwister

Oh HELLS ya! My daughter learned all kinds of new words as I wrestled with the breaker bar and long extension! But I got those bolts off, didn't break the hidden welded nuts free (my biggest fear), and got 'er done. Cleaned up the bolts and used plenty of anti-seize putting the new springs on. I used a technique I saw on a Rough Country video; putting a jack under the breaker bar and jacking. It worked, but I'm guessing Craftsman came very close to having to honor their lifetime warranty on that breaker bar!

SexyDexy

I did that too and it was a bit sketchy...my 1/2" HF breaker bar now has a slight bend to it, but it worked. lol

Turbo X_J

Did you torque the U-bolts with the weight of the Jeep on the ground?

wristwister

No, I didn't! Never thought of that, but it sure makes sense to re-torque them on the ground. Thanks for that.

wristwister

OK folks, I forgot to mention that in addition to the RE springs it also has 1" lift shackles on the back. Tonight I removed those shackles and went back to stock. When we took her out, at first we heard the same thump thump thump noise at very low speed, but very minor, hardly noticeable. It went away when we sped up a bit, then it went away altogether at slow speed too. Also, I live on a hill, and when I first braked hard going down a very similar noise appeared, but by the time we got to the bottom of the hill it went away too. Took her out for a longer drive, all seems to be OK.

So what happened here? I assume it's still a drive shaft alignment issue and lowering the lift by an inch or so helped the situation. Then what, do these things just kind of wear into each other and work themselves out?

SexyDexy

My theory is that when you first install the leafs, they are likely over 3.5". As you drive it for a bit, the leafs will settle closer to 3.5"...so the pinion angle is likely going to be closer to the angle of the T/C output shaft angle, hence fewer/no vibrations.