The 8.25 4.56 gear swap... vibration issue.

[srmitchell]

We just swapped out my 8.25 with 3.55's for another 8.25 housing that has 4.56 gears.

I am running a RE hack n tap with a jy shaft. I checked the shaft yesterday, all is ok. U joints are 1.5 years old, and well greased.

I had a slight vibration at 65+ before I swapped gears. It is a droning pulsating vibration that was never bad enough to cause concern.

Now with the new gears, the shaft spins a whole extra rotation per wheel revolution, so the vibration started at 45, and got HORRIBLE past 55+. at 75 mph, it was deafening.

Keep in mind that my speedometer is set up for a jeep on 3.55's and 31's, so now that its on 4.56 and 32's, I have no idea how fast Im going.

So I ordered 4 degree shims (because I had never shimmed the axle) and I just installed them today. The pinion angle points right at the tcase, and the shaft seems to be straight inline.

30 minutes ago, I went for a speed run, and it seems that the vibration starts later, (maybe at 55 or 60) but it is still BAD.

-It still vibrates, bad, but it starts at about 55. (better.)
-However, there is a pretty consistent vibe that increases with throttle.
-There is a new vibe when decelerating from around 80-65.

My jeep sounds and feels like s***. I was always really happy with how smooth my rig was, but it just feels like poo.

Since my driveline angle is very good, I'm thinking its the driveshaft itself, or the RE flange has slop.

Also, before you ask, the vibe still occurred with my old tires, so this isnt a wheel balance issue. (I think)

I need ideas, because I have to drive back to school in a week.

Thanks!
-Columbus.

Pinion.

 

[xjcody]

Check the play in the axle yoke? Might be worn pinion bearings.

 

[the_weirdo]

It looks like you might be pointing too high from that picture. Can you measure the angles? Did the guy you got the axle from have vibes?

edit: here's what I am seeing. red (the pinion line) should point below blue (the driveshaft line) by 1-2*. It looks like it points over to me by a degree or two.

 

[srmitchell]

Damn. Maybe I should have gone 2 degrees?

 

[Ivan]

Two degrees shims will rotate you pinion the wrong direction.

Actually you need to go to 6 degrees, to rotate your pinion down a tad more. As mentioned in few posts above, your pinion is too level with your driveshaft, sitting static. Under power, your pinion wants to rotate up, which now puts your pinion angle above your driveshaft.

By having the pinion angle slightly below in the static postion, under load when the pinion then rotates up, the pinion should be in the correct orientation, that is, in line with your driveshaft.

That's the reason I have the extra 4 degree shims myself... I had to bump up to 6 degress of shim, and my set up is very similiar to yours...

 

[tigerchief]

Two degrees shims will rotate you pinion the wrong direction.

Actually you need to go to 6 degrees, to rotate your pinion down a tad more. As mentioned in few posts above, your pinion is too level with your driveshaft, sitting static. Under power, your pinion wants to rotate up, which now puts your pinion angle above your driveshaft.

By having the pinion angle slightly below in the static postion, under load when the pinion then rotates up, the pinion should be in the correct orientation, that is, in line with your driveshaft.

That's the reason I have the extra 4 degree shims myself... I had to bump up to 6 degress of shim, and my set up is very similiar to yours...

:confused1 :gag:

 

[Bryan C.]

By having the pinion angle slightly below in the static postion, under load when the pinion then rotates up, the pinion should be in the correct orientation, that is, in line with your driveshaft.

:confused1 :gag:

Seems pretty much spot on to me. :dunno:

One other thing you might want to try is removing your front driveshaft to eliminate it from the equation at this point. I will also suggest removing the rear and driving on the front only. This is just a step to help you isolate the source of each vibration you are feeling.

 

[blondejoncherokee]

get tires balanced. mine get outta balance sometimes after only 2k.
check pinion u joints front and rear for any discoloration or journaling.
also, how soft are your rear springs?

 

[srmitchell]

My pinion was already slightly downward. I used the shims to point it toward the 0 degree mark.

 

[tigerchief]

Seems pretty much spot on to me. :dunno:

What you quoted, is ok.

I might be completely retarded tonight, but re-read this part and explain it to me (this after OP put in 4* shims, and when pointed out that was too much, said he should have gone 2* instead):

Two degrees shims will rotate you pinion the wrong direction.

Actually you need to go to 6 degrees, to rotate your pinion down a tad more.

 

[srmitchell]

Agreed. Im confused.

I am adding some 3/4 inch shackles tomorrow, so maybe if I remove the shims it will set me at the right angle.

 

[tigerchief]

No, you aren't confused. Keep the pinion 1 or 2 degrees below the driveshaft angle, and you're good to go.

Take a read after installing the shackles since it'll move your axle forward a bit (which will help with the pinion angle).

Yvan might have tried to make a funny or is new to crack.

 

[330LICIOUS]

hey sean, from the photo, i'd try going with 2* shims also.

also, have you tried driving w/o the front d/s, to see if the vibes are coming (partly) from the front pinion angle, perhaps?

also, you have aftermarket long arms with skid right? check that the front AND rear d/s inline direction are centered. on my TNT skid, i can move it 1/2" toward the driver or passenger side, making the t/c shift left and right, the same amount.

and lastly, you can get your d/s re-balanced.

 

[Ivan]

What you quoted, is ok.

I might be completely retarded tonight, but re-read this part and explain it to me (this after OP put in 4* shims, and when pointed out that was too much, said he should have gone 2* instead):

Even if the picture shows the pinion being too high, replacing the 4 degree shims with 2 degree shims will only make the situation worse.

Replacing the 4 degree shims with 2 degree shims will rotate the pinion up even more, above the driveshaft angle. Driving load will torque the pinion up even further.

Replacing the 4 degree shims with 6 degree shims will rotate the pinion down, which should set it up to the proper 1 - 2 degrees below the driveshaft...

Measure the driveshaft and pinion angles are the only way you are going to know what degree shims you really need. Eye ball engineering isn't going to cut it here.

Draw it out on a piece of paper, ie spring / axle pad relationship to the varying degree of the shims.... it will make more sense. Remember, the spring doesn't move, the shims rotate the axle which affects the pinion angle.

 

[Ivan]

My pinion was already slightly downward. I used the shims to point it toward the 0 degree mark.

What you had orginally is what you needed. Setting your pinion angle at the 0 degree mark in the static position is what you don't want. Remember, you vibes are coming while your driveline is under load. Under load, your pinion rotates up even further. You'll want to compensate for this by have your driveshaft 1 - 2 degrees below your static driveshaft angle.

I also agree with Bryan.... remove the front driveshaft and see what happens. Remember, you changed out your suspension, which affected all your driveline angles, both front and back. If your vibes go away, then you have to adjust your front pinion angle to correct this. Front end caster adjustment is a fine line though, because it will also affect how your steering reacts. Not enough caster will make your steering squirrely but reduce vibes. Too much caster will tighten up your steering, but increase vibes. You'll just have to find the setting that will balance out the best of both worlds.

This part of the reason why some folks with extreme drive line angles swap in Warn locking hubs....So that they can unlock the hubs while on the road, and not have the front drive driveshaft spinning.

Best way to really do this is to take it to an aligment shop to see what your true caster setting are. If they are sharp enough, they'll know how to adjust them. That's the reason I bought a life time alignment contract when I first bought my XJ. That way any time I mess with the front end, I can take my XJ to the shop and the alignment machine is going to give me better spot on measurements, than I could achieve with a magnetic angle finder.

However, you can also do this yourself by placing an angle gauge on the flat rounds on either side of your front diff. You'll see two round spots with an indent. Place your angle finder there to see what your caster setting is at and report back. Not quite as precise as an aligment shop, but it gets the job done.

Read the following link on how you can check and adjust caster yourself.

http://go.jeep-xj.info/HowtoAlignment.htm

 

[srmitchell]

Ivan, I think theres a bit of confusion. My pinion was low, and the vibration was already there back when I was on 3.55's. When I regeared, the vibration was the same, but way worse and started way sooner.

I added a 4 degree shim, with the thick part at the back of the axle, small part at the front. We have determined that this would be too much.

You are saying that I should add a 6 degree shim, which would send the angle a whole 2 degrees above zero, making the pinion slope past center. Not good.

The only thing I can guess is that you installed a 6 degree shim with the thick part at the front of the axle. (Thin at the back.) Couldnt I just install a 2 degree with the thick part at the back, making the pinion 1-2 degrees below 0?

I am going down to Tim's shop today. We will install the shackles and check the angle. Josh has a spare driveshaft, so we might play with different setups and see what works.

 

[Slonopotam]

By having the pinion angle slightly below in the static postion, under load when the pinion then rotates up, the pinion should be in the correct orientation, that is, in line with your driveshaft.

That's what I thought. May be leafs are flexing.

 

[Ivan]

Ivan, I think theres a bit of confusion. My pinion was low, and the vibration was already there back when I was on 3.55's. When I regeared, the vibration was the same, but way worse and started way sooner.

I added a 4 degree shim, with the thick part at the back of the axle, small part at the front. We have determined that this would be too much.

You are saying that I should add a 6 degree shim, which would send the angle a whole 2 degrees above zero, making the pinion slope past center. Not good.

The only thing I can guess is that you installed a 6 degree shim with the thick part at the front of the axle. (Thin at the back.) Couldnt I just install a 2 degree with the thick part at the back, making the pinion 1-2 degrees below 0?

I am going down to Tim's shop today. We will install the shackles and check the angle. Josh has a spare driveshaft, so we might play with different setups and see what works.

You are thinking this backwards...

Fat end of the shim towards the back. By adding a 6 degree shim, your pinion then rotates down, not up. By rotating down, your pinion will be slightly lower than your driveshaft which is where you want it to be in the static position. This will account for the pinion shift up, when you place a load on it while driving.

Example... Take your shim and set it on the ground. Step on it with your foot, with your heel on the fat end. Which way does your foot now point? Yes, down.

Same theory with you suspension, except the ground is your leaf spring, and your foot is the pinion....

 

[the_weirdo]

Ivan, you have it backwards. You are describing what happens for spring under axle. It is opposite for spring over axle. See the picture below. The blue triangle is the shim. If the shim gets smaller (flatter), the pinion (red line) points down.

Edit: front of the vehicle is to the right.

 

[tigerchief]

:smoker: