Driveline vibes....how bad?

[GreatWhite]

Hey all,

I put in a 2" lift a few weeks back, and netted about 3" lift (old springs were weak, I guess) anyway....I drove it and it had vibes during acceleration that were never there, nothing major or such, but vibes...

I dropped the xfer case 1" with washers, and vibes are very much gone...BUT...

I still have a 'buzz' in the wheel and floorboards from about 40-60mph during acceleration...steady speed they are gone, decel has no vibes....

Is this minor 'buzz' (no shaking of mirrors or dash or such) a big deal, or should I lower the xfer case even more?

Thoughts?

THX as always!!

[jpotts]

First, I would really consider a Slip Yoke Eliminator. Why lift the thing if you are going to drop the t-case? That is if you plan on wheeling it. If you're not going to wheel it, then leave it as is.

As for the buzz, how loud is it? I wouldn't worry about it too much if it was me - but I'm just happy that my rig doesn't have pieces flying off when I'm going that fast. :D

[GreatWhite]

I lifted it for 31's and light 4x'ing...2.5 or less rated....

It is not audible, but a buzz in the steering wheel/floorboards....a buzz/vibration....

=)

[Digger]

This buzz thing sounds like what I am experiencing. It's not a performance problem, just getting tired of the buzz, I just bought the XJ a couple months ago and don't have enough free time to investigate it myself. It's going into a shop tomorrow. It drives real straight, new tires and balance. Maybe U joints? Have any inputs on this?

[Billy 4 hp]

Simple explanation...

When you lift any vehicle (and vehicles with short rear driveshafts like most Jeeps have more problems with most) you are increasing the angle on the drivelines, the shorter the driveshaft the worst the angle change.

When you "get on the gas" or accelerate, the natural action of the differential is to twist (to be exact, it is actually the pinion gear trying the climb the ring gear rather than turning it), the softer the spring the worst the twist is in most situations. When this twist occurs, the driveline angle change only exaggerates hence the vibration under acceleration.

Any vibration that you can actually feel is a huge indication that the u-joints are at an severe angle and something needs to be corrected. Either drop the transfercase, install a SYE, shimming of the pinion angle (leaf springs) etc... All of these are fixes to correct the extreme angle that the u-joint is now operating at... A driveline that is vibrating due to severe angles is just a ticking timebomb and "dropping a driveline" at freeway speeds can be a scary situation... :eek:

Good luck....

[GreatWhite]

Great explanation Billy, I am/was aware of the basics, but just wonder if I will ever be able to zero out all the vibes, or if a slight hmmm in the steering wheel will always be there....

I don't want to hurt the driveline, but also don't want to chase a phantom that is just a side effect of a lift, that being it will never be as smooth as OEM.

[HiJinx]

Ok, but what if all your u-joints are at good angles and you're still getting vibration from time to time?

[Billy 4 hp]

Ok, but what if all your u-joints are at good angles and you're still getting vibration from time to time?

Common causes:

Mud or debris in the wheels
Mud or debris on the driveline(s)
Buddies playing jokes on you
Something else wrong other than driveline... :D (Loose tranny mount(s) motor mounts, etc)...

[Fire Ball]

The problem with U-joints is they only really do what they are supposed to do in a straight line. As the U-joint angle increases the 2 parts of the shaft actually turn at different speeds for a short portion of the rotation.

For example, say the transfer case shaft is turning 1000 rpm, so is the drive shaft. But as the angle of the ujoint increases, the drive shaft will be speeding up and slowing down as it turns.

1/4 turn 1000 rpm, 1/4 turn 1001 rpm, 1/4 turn 1000 rpm, 1/4 turn 999 rpm.

The net is still 1000 rpm but the slight acceleration and decelerations cause the vibration. This is why some vehicles went to CV joints (Constant velocity) or 2 U-joints in one yoke. This cuts the angle in half.

Adding a SYE shortens the transfer output shaft, thus lengthening the drive shaft and reducing the angle. Also many new drive shafts with SYE's also have dual U-joints at that end to further reduce the drive angle.

[Digger]

Got my XJ back from shop today, the buzz/vibration was being caused by the rear driveshaft. good to go now, they did a rebuild. The buzz noise over 50mph was getting bad, I could feel my councel vibrating when I put my arm on it. Had the shop go over entire jeep, everything is good to go. The powersteering unit was bad and I had it replaced a couple weeks ago, (rebuild) For what I paid for the jeep with low milage, no body damage and all the modifications it has I still think it was a bargain. :)