Rear driveshaft vibrations improve with pinion 1 degree above the driveshaft angle

[06lj]

I regeared my LJ auto to 5.13 gears this past spring. It spent the summer in the garage and I am back to working on it. I've got about 400 miles on the new gears and took it out this week to test it at some higher speeds. I noticed a vibration that starts around 50mph and gets significantly worse with increased speed. I have narrowed down the vibrations to the rear driveshaft. I have removed the front shaft and had no change. With the rear shaft removed the vibrations are gone. I adjusted the rear pinon from 3 to 0 degrees below the driveshaft angle. The vibrations get worse the lower I set the pinion. At 0 degrees the vibrations were improved but still there. Above 65 they were bad enough I wouldn't want to drive very far. I pulled and inspected the yoke and output shaft but didn't find any obvious problems. I even set up a dial indicator to see if there was any runout on the output shaft. Everything checked out. I had replaced all of the u joints when I did the gears. Out of curiosity/desperation I adjusted the pinion to 1 degree above the driveshaft and had the best results yet. There are still vibrations that can be felt, heard, and seen in all of the mirrors but it is almost tolerable. I'm curious if anybody can enlighten me on why this might be happening?

 

[Rubiconned]

I have no idea, but everything I've read said never to have the pinion angle higher than the driveshaft.

 

[LONGJP2]

There's no magic number.
It is what it is.

 

[AndyG]

There's no magic number.
It is what it is.

Exactly it’s happening because you have not found the sweet spot- Even though these are mass produced there is all kinds of variation in the geometry.

Top notch shops may adjust the pinion angle 60 times ot more before they quit.

And yes you can have all sorts of other factors such as wear at a transmission mount and u joint wear, shaft balance, etc.

 

[06lj]

There's no magic number.
It is what it is.

Thanks

Exactly it’s happening because you have not found the sweet spot- Even though these are mass produced there is all kinds of variation in the geometry.

Top notch shops may adjust the pinion angle 60 times ot more before they quit.

And yes you can have all sorts of other factors such as wear at a transmission mount and u joint wear, shaft balance, etc.

I was figuring as much. It just had me scratching my head as everything I have read has the pinion angle below the driveshaft. Maybe my Jeep is old, cranky and just wants to be different. I was doing research tonight and am thinking of ordering a new shaft to see if I can get rid of the vibrations altogether. I currently have a RC shaft which, I know, is not the best. To it's credit it did work fine with the 3.73 gears. I'm thinking the increased rpm's might be to high for it.

 

[xXDavidCXx]

When was the last time the rear drive shaft was balanced?

 

[06lj]

When was the last time the rear drive shaft was balanced?

I've never had it balanced. I haven't driven the Jeep much in the past few years so the shaft only has about 600 miles of use. I was going to contact Tom Woods and Adam's driveshaft to inquire about what rpm they can balance their shafts.

 

[AndyG]

Thanks



I was figuring as much. It just had me scratching my head as everything I have read has the pinion angle below the driveshaft. Maybe my Jeep is old, cranky and just wants to be different. I was doing research tonight and am thinking of ordering a new shaft to see if I can get rid of the vibrations altogether. I currently have a RC shaft which, I know, is not the best. To it's credit it did work fine with the 3.73 gears. I'm thinking the increased rpm's might be to high for it.

Most of the maxims really just get you in the ballpark- It can be a tedious game

 

[06lj]

Most of the maxims really just get you in the ballpark- It can be a tedious game

So true.
I have never been so happy that I went with double adjustable arms.

 

[AndyG]

So true.
I have never been so happy that I went with double adjustable arms.

I know of one top-notch tech who will not even touch a rig that does not have them.

You know I don’t know if we ever mention that but single adjustment limit you to half turns- double do not.

 

[mrblaine]

Thanks



I was figuring as much. It just had me scratching my head as everything I have read has the pinion angle below the driveshaft. Maybe my Jeep is old, cranky and just wants to be different. I was doing research tonight and am thinking of ordering a new shaft to see if I can get rid of the vibrations altogether. I currently have a RC shaft which, I know, is not the best. To it's credit it did work fine with the 3.73 gears. I'm thinking the increased rpm's might be to high for it.

The rule of thumb is the rear shaft is forgiving when lower, the front is forgiving when higher. I've seen 100% vibe free Unlimited models with rear shaft 2-3 degrees high and the front several degrees low. All I know is set them where it works.

 

[06lj]

The rule of thumb is the rear shaft is forgiving when lower, the front is forgiving when higher. I've seen 100% vibe free Unlimited models with rear shaft 2-3 degrees high and the front several degrees low. All I know is set them where it works.

Thank you. I appreciate the insight.

 

[Unique Username]

I feel your pain. I struggled with this for a long while after the 5.13 install. I moved that driveline angle all over the place and had it in every location twice. I finally settled with the best I could get. Fast-forward a year later I removed the 3/16 UCF plate and installed the Savvy skid I put the rear driveline at ZERO and its way better than it ever was. Slight vibe at 65mph under load and no vibe without load. I am scared to adjust the front an lock in the hubs only to find out I don't need the front hub kit.

 

[blansky]

So true.
I have never been so happy that I went with double adjustable arms.

Pardon my ignorance but what is a double adjustable control arm. Is it both upper and lower rear or is it a loweradjustable with two adjustment points.

When I raised my 2005 LJ to about 3 inches I started to get a lot of vibration and after putting on adjustable ones on the back lower, it was still there and at different speeds depending on where the shop did the adjustment. So now with 3 different adjustments it's not too bad but I would prefer it perfect. Any suggestions. Thanks.

 

[GregBelleville]

Pardon my ignorance but what is a double adjustable control arm. Is it both upper and lower rear or is it a loweradjustable with two adjustment points.

 

[blansky]

View attachment 566667

Thanks. So I'm guessing that instead of being just able to adjust to just half a turn one a single adjustable, that you can fine tune it down to a quarter turn with the double adjustable. Sorry. I didn't want it to be a math question.

With that much adjustability a person should be able to maybe tweak it to perfection, with no vibration at all.......hopefully.

 

[mrblaine]

Thanks. So I'm guessing that instead of being just able to adjust to just half a turn one a single adjustable, that you can fine tune it down to a quarter turn with the double adjustable. Sorry. I didn't want it to be a math question.

With that much adjustability a person should be able to maybe tweak it to perfection, with no vibration at all.......hopefully.

Not a 1/4 turn. If you rotate the arm 1/10th of a degree or a miniscule amount, the length change is also very small. A quarter turn is the same though as a half turn with a single as you figured out. For the lowers, that moves you to a full turn if you want to keep the fittings out of the rocks.

 

[blansky]

Thanks. I'm still coming to terms with the vibration issues and why the Jeep Shop I used (Blackstone, Santa Clarita) which is highly recommended, wasn't able to solve the problem when the adjustable control arms (lower) were installed. In 3 attempts it went from vibrating at 5 MPH, to bad at 45-55 MPH to the last time being noticeable at a couple of different ones, 5-7ish and around 50ish but not terrible.

I didn't realize it was such a crap shoot getting it perfect. I ordered a gauge to see what the angle actually are at now. I'm sort of wondering how much half a turn to a full turn would make in an adjustment and how many times a person would have to keep toying with it to get it "right"

 

[mrblaine]

I didn't realize it was such a crap shoot getting it perfect. I ordered a gauge to see what the angle actually are at now. I'm sort of wondering how much half a turn to a full turn would make in an adjustment and how many times a person would have to keep toying with it to get it "right"

First step, remove the front driveshaft. Then start adjusting the rear shaft angle. We've had some where we adjusted them 30-40 times in very small increments. Being able to pop the jam nuts loose, give a quick turn to the DA uppers is priceless when you get a problem child.

 

[freedom_in_4low]

First step, remove the front driveshaft. Then start adjusting the rear shaft angle. We've had some where we adjusted them 30-40 times in very small increments. Being able to pop the jam nuts loose, give a quick turn to the DA uppers is priceless when you get a problem child.

I'd be lying if I said I wasn't tempted to expedite this process even further by running the process with one upper arm removed so each adjustment took half the time. I'm on the ledge and all I need is the word of someone I respect to either push me off or back away.

For simple testing purposes, no wheeling, no hard braking (though it's the rear axle anyway, so...) Probably remove the passenger and keep the driver UCA installed since it's mount is beefed up for double duty with the track bar...aside from damage to a bracket my other concern is how well 1 stock sized UCA will locate the axle under load to maintain 80mph, and if I get the vibes adjusted out will they just come back because things don't fall into the same place with both arms installed. Or will the synergy DDBs in the LCAs flex enough that the axle housing goes wonky with thrust angle from the offset of the UCA and I end up chasing my tail.

Thinking through a legitimate time saver or ravings of a madman? Sometimes it's hard to know the difference. I'm great at thinking of what can go wrong, much less good at estimating their likelihood.