Driveline Vibration

I've searched around a lot and it seems like everyone's drive line issue is different so I decided to just make a thread. Issue came out of nowhere. One morning I noticed a screechy sound similar to when a brake pad is done and you're grinding the metal tab but it wasn't nearly as prominent. Very slight vibrations at 35-40 mph. Ignored it for a few days. Fast forward few days, I'm driving at highway speeds and I'm getting terrible vibrations 75-85 mph. Drove through them as conservatively as I could for the 5 hour trip I had to make. Now I am getting a worse vibration at 35-40. If I put it in neutral it goes away, it becomes worse when I'm engine braking. Can't seem to find any consistent rpm range it exists at.

Replaced my carrier bearing and no difference at all. Rear diff seems to have excess backlash but I'm fairly sure it has been that bad since I got the truck and it drove perfectly smooth until now.

Truck has 5100's at highest setting and All Pro 3" leafs. I've driven it with the lift for about 4,000 miles with no issues at all until recently.

I read the front diff bearings may be the issue and to test it in 4wd. While in 4wd the vibration seems to diminish slightly but is still present.

Rear boots are most likely toast but have been since I got the truck 10,000 miles ago. Going to remove the rear shaft now and see if that changes anything.

Any guidance will be helpful. Is there are more conclusive way to test front diff bearings?

 

how did the U-joints look?

 

The driver side front diff bearing issue should not be "load" related but more related to just rotation unless the bearing is really really gone but at that point I would think the diff seal would be on strike. The diff vibe should go away completely in 4wd as the bearing is not turning in 4wd. You might have 2 issues.

A bad u-joint at the shaft ends or in the DC joint can cause a vib as well as the DC "centering ball".
Get that shaft off and check for smooth movement and no play in all the joints thru the full range of motion. Are your drive line angles good, diff pinion pointing at or just below the DC by around 1-2 degrees? Bad angles could exaggerate and/or hasten a bad u-joint.

 

Think I traced it to the u-joint that connects to the rear diff pinion flange. Has a bit of play in one of the needle bearings. Looking at my drive line I can see how the lift I installed may have indeed caused that particular joint to prematurely fail. The DC feels fine, no play at all and smooth motion, same with the front joint.

I've read up a bit on drive line angles and such but I'm still a little confused on what ideal angles are. From what I've read with the DC style shaft it seems like you want the diff pinion to be parallel with the rear shaft segment and the trans flange to be parallel with the front shaft segment, contrary to a one piece shaft where the trans and diff need to be parallel in a 2 piece you want all of the "bend" to occur within the DC? This sort of contradicts my intuition which is to evenly distribute the angles between all joints and also renders any sort of center bearing drops not beneficial? The reason I bring this up is because I installed a make-shift center bearing drop of about 5/8" with my new bearing. Also, what are your thoughts on diff shims?

I took out the rear shaft and drove the truck around a bit and the 35 mph vibe is completely gone but there is now a vibe at 55-60 that occurs in 3rd and 4th gear. Is this what you meant that I might have 2 problems @Dirty Pool? Haven't checked the front shaft yet but I'll let you know.

Thanks again for all the insight @Dirty Pool !

 

Also if your shaft is too rusted and pitted it can theow it off balance

 

Your understanding of the angles is correct. Straight line from transfer to support bearing, the DC having all the "bend" in the rear section and no angle at the diff end u-joint. This is due to the rotation speed canceling effect between two u-joints in phase. The front section of the rear shaft has one joint so it needs to be straight to run smooth. The rear section has 3 joints. The DC joint handles the angle with its u-joints canceling out each others rotation speed differences. This leaves 1 joint at the diff with nothing to offset it, so it needs to be straight like the front section.

All that is theoretical. In the real world you want a slight angle, usually 1-2 degrees where there is a single joint. This is not enough to cause stress or "felt" vibration but is does several beneficial things. The slight movement of the joint helps to "move" lube around the needles and spreads out the wear, preventing a continuous "point" load on the caps, trunions and needles. Further, all leaf springs will have some degree of spring "wind up" or "axle wrap". By having the rear diff pointing "under" the DC by the 1-2 degrees you achieve to above items and allow the diff to be closer to a zero (stronger, smoother) angle when the power is on and the axle rotates a bit pointing the pinion up some. Of course different springs will wind up different amounts but usually the "ballpark" 1-2 degree thing works fine.

Support bearing drops.
This process "can" provide symptomatic relief for an excessive pinion angle vibration but it is at the expense of the angle of the joint at the trans end and the rubber in the support bearing. Again, this is going to be a matter of degree (pun yes) and a happy trade off sometimes can be had. Tapered shims between the spring and axle are the best way to achieve proper pinion angle.

You mentioned a slight reduction in the vibes with the 4wd test for the front diff bearing so my thought was there might be two issues, front diff bearing a little tired and a rear drive line issue. Just tossing it out there.

Sounds like you may have found at least something promising with the rear joint.

 

One thing to keep in mind with tapered shims. They rotate the axle housing, usually in the pinion up direction. This means the fill plug on the back side is going down. Hence using the fill hole to determine the correct amount of oil is out the window. Go by recommended capacity. Even there you will note different capacities for the same rear used in different models (reg/extra cab, 5/6 lug) due to different pinion angles relative to horizontal.

 

Just an observation and correct me if im wrong here. I thought the front diff bushing was only an issue in the '01-'04 models??

 

It applies to all ADD diffs.

 

Good to know, thanks my man!!