Adjusting Pinion Angle

[AVGeek99]

On a trip back from Moab to CO Springs I noticed some noise coming from my driveline at about 68+ mph while under load, hill climbs. Was fine on the flats. Yesterday I measured my front pinion angle and driveshaft angle. They are well outside of the recommended 2 degree variance. I also checked my caster, which is at 3.4 degrees. I know 3.4 sounds low, but my Jeep drives great. The current low caster is absolutely not an issue, but I don't want to go too much lower.

This weekend I'll be making adjustments to improve the pinion angle. My assumption was to lengthen the UCAs to roll the axle forward. My belief is this will improve the pinion angle, but will also reduce caster further.

I decided to ask MS Copilot for recommendations and was surprised by one of it's responses. It says that shortening the UCAs will increase caster (obviously) but would also improve the pinion angle (i.e. point it up towards the driveshaft). That second part about the pinion through me for a loop. I specifically questioned that recommendation and here is the response from Copilot.

"
Bottom Line
You were right to question it — but the original recommendation is correct:
Shortening front upper control arms rotates the pinion up and increases caster
This is the cleanest way to improve driveline behavior on a no‑FAD JL
"
Is this correct? Or just AI being a little too overconfident?

In case your wondering I have a 2025 JLUR (no FAD), full 3.5" MC GC lift (netted 5.25 actual lift) with MC Driveshaft, OEM rear DS. Aluminum (Rock Hard) bumpers, winch, Teraflex Alpha HD hinged spare tire carrer, 37" STT Pros. The net lift is high becuase I don't really have anything weighting it down. I know I could run larger tires or reduce the lift, and I may do that at some point.

But all I'm really wondering is if Copilot is right that shortening the UCAs will actually increase the pinion angle. It seems very counterintuitive since that will roll the axle backwards.

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[Roky]

On a trip back from Moab to CO Springs I noticed some noise coming from my driveline at about 68+ mph while under load, hill climbs. Was fine on the flats. Yesterday I measured my front pinion angle and driveshaft angle. They are well outside of the recommended 2 degree variance. I also checked my caster, which is at 3.4 degrees. I know 3.4 sounds low, but my Jeep drives great. The current low caster is absolutely not an issue, but I don't want to go too much lower.

This weekend I'll be making adjustments to improve the pinion angle. My assumption was to lengthen the UCAs to roll the axle forward. My belief is this will improve the pinion angle, but will also reduce caster further.

I decided to ask MS Copilot for recommendations and was surprised by one of it's responses. It says that shortening the UCAs will increase caster (obviously) but would also improve the pinion angle (i.e. point it up towards the driveshaft). That second part about the pinion through me for a loop. I specifically questioned that recommendation and here is the response from Copilot.

"
Bottom Line
You were right to question it — but the original recommendation is correct:
Shortening front upper control arms rotates the pinion up and increases caster
This is the cleanest way to improve driveline behavior on a no‑FAD JL
"
Is this correct? Or just AI being a little too overconfident?

In case your wondering I have a 2025 JLUR (no FAD), full 3.5" MC GC lift (netted 5.25 actual lift) with MC Driveshaft, OEM rear DS. Aluminum (Rock Hard) bumpers, winch, Teraflex Alpha HD hinged spare tire carrer, 37" STT Pros. The net lift is high becuase I don't really have anything weighting it down. I know I could run larger tires or reduce the lift, and I may do that at some point.

But all I'm really wondering is if Copilot is right that shortening the UCAs will actually increase the pinion angle. It seems very counterintuitive since that will roll the axle backwards.

I don’t know what AI is saying, I try not to use any of that shit….. with that much height, I’d consider using your control arm drop brackets
That will definitely help especially if you need to decrease angle further….also if you have a fad delete axle, you may consider installing one on an auxiliary switch etc….

 

[Graphite JL]

AI is incorrect, shortening the upper control arms will rotate the pinion down, as it would pull the top of the axle reward. Lengthening the uppers would rotate the pinion up.

 

[Roky]

I’m kinda glad A I is full of shit sometimes, the day it gets everything perfect, that’s gonna render us old mechanics obsolete …...🫤

 

[yokramer]

I’m kinda glad A I is full of shit sometimes, the day it gets everything perfect, that’s gonna render us old mechanics obsolete …...🫤

Dont worry it wont be anywhere near correct any time soon and will get all the rubes that use it so you have more work until then.

 

[EbyCreek]

On a trip back from Moab to CO Springs I noticed some noise coming from my driveline at about 68+ mph while under load, hill climbs. Was fine on the flats. Yesterday I measured my front pinion angle and driveshaft angle. They are well outside of the recommended 2 degree variance. I also checked my caster, which is at 3.4 degrees. I know 3.4 sounds low, but my Jeep drives great. The current low caster is absolutely not an issue, but I don't want to go too much lower.

This weekend I'll be making adjustments to improve the pinion angle. My assumption was to lengthen the UCAs to roll the axle forward. My belief is this will improve the pinion angle, but will also reduce caster further.

I decided to ask MS Copilot for recommendations and was surprised by one of it's responses. It says that shortening the UCAs will increase caster (obviously) but would also improve the pinion angle (i.e. point it up towards the driveshaft). That second part about the pinion through me for a loop. I specifically questioned that recommendation and here is the response from Copilot.

"
Bottom Line
You were right to question it — but the original recommendation is correct:
Shortening front upper control arms rotates the pinion up and increases caster
This is the cleanest way to improve driveline behavior on a no‑FAD JL
"
Is this correct? Or just AI being a little too overconfident?

In case your wondering I have a 2025 JLUR (no FAD), full 3.5" MC GC lift (netted 5.25 actual lift) with MC Driveshaft, OEM rear DS. Aluminum (Rock Hard) bumpers, winch, Teraflex Alpha HD hinged spare tire carrer, 37" STT Pros. The net lift is high becuase I don't really have anything weighting it down. I know I could run larger tires or reduce the lift, and I may do that at some point.

But all I'm really wondering is if Copilot is right that shortening the UCAs will actually increase the pinion angle. It seems very counterintuitive since that will roll the axle backwards.

Did you adjust your control arms to the MC Game Changer recommended length?
https://metalcloak.com/jl-wrangler-3-5-game-changer-suspension-system.html

 

[AVGeek99]

I don’t know what AI is saying, I try not to use any of that shit….. with that much height, I’d consider using your control arm drop brackets
That will definitely help especially if you need to decrease angle further….also if you have a fad delete axle, you may consider installing one on an auxiliary switch etc….

AI is incorrect, shortening the upper control arms will rotate the pinion down, as it would pull the top of the axle reward. Lengthening the uppers would rotate the pinion up.

Thank you both for confirming what I thought was total bullshit.

I've just been dabbling with AI a little lately. In can be good at speeding up research. It crawls all these forums and other websites and then gives you an organized easy to read and understand answer to your question. The problem is, there's a lot of bad info on the interwebs and it isn't smart enough to know what is right and what is wrong. Everything else in a few pages of it's explanations and recommendations was accurate and made perfect sense.

But looking at more of the response in detail I think I figured out how it's getting it wrong. It probably saw people talking about adjusting the rear pinion. Where if you rotate the rear axle back (by lengthening the rear UCAs) it does point the pinion up. It's just not smart enough to realize it has the opposite affect in the front.

Thank you again both!!

 

[AVGeek99]

I don’t know what AI is saying, I try not to use any of that shit….. with that much height, I’d consider using your control arm drop brackets
That will definitely help especially if you need to decrease angle further….also if you have a fad delete axle, you may consider installing one on an auxiliary switch etc….

@Roky Part of your response brings up another question to something Googles Gemini AI mentioned when I did a google search a few days ago. It recommended drop brackets as well and said that the drop brackets will flatten the CA angles, which is obvious. But it also said that by itself would improve both caster and pinion. At the time I dismissed that second part as AI not knowing what it was talking about.

When you say drop brackets will "help especially if you need to decrease angle further", were you referring to CA angles or caster and pinion angles?

My understanding is that my pinion angle issue is purely a function of my ride height being so high. Drop brackets will flatten the CAs, which will improve ride/handling, but the pinion angle can only be addressed by rotating the axle by adjusting the lengths of the control arms. And improving the caster angle will reduce my caster further.

 

[AVGeek99]

Did you adjust your control arms to the MC Game Changer recommended length?
https://metalcloak.com/jl-wrangler-3-5-game-changer-suspension-system.html

Yes, the MC recommended lengths were my starting point when I installed the lift. And then I adjusted from there. When I first installed it my caster was a little under 6, but when I drove it the front DS was making a lot of noise starting at about 45MPH and got worse as speed increased. I then dialed back the caster, improving the pinion angle, and significantly reduced the vibration, to a point where it was barely audible from about 48mph to about 61ish and then it went away as speed increased beyond 61. When I measured my caster at that point it was like 2.9 degrees. I didn't look closely at the pinion angle because the vibration at that point was very minimal. I've since put about 10k miles on the Jeep. The caster has now settled to 3.4 degrees when I checked it a couple nights ago.

On the drive out to Moab it was perfectly fine. I ran 6 trails in 4 days and then drove home. It was on the drive home that the vibration started getting bad starting at 68mph and up. And the vibration only occurred while the driveline was under load, primarily on the climbs in the rockies.

 

[Roky]

When you say drop brackets will "help especially if you need to decrease angle further", were you referring to CA angles or caster and pinion angles?

I was talking about if you need to decrease the pinion angle further….or get it more in line with ds. Have you considered getting 2.5” springs ?

 

[AVGeek99]

I was talking about if you need to decrease the pinion angle further….or get it more in line with ds. Have you considered getting 2.5” springs ?

Yes, I have considered going down to 2.5 springs. My only concern about that was my shocks might be too long. When I ordered my shocks I got longer ones that were right at the compressed and extended limits recommended by MC for the 3.5GC. So I was thinking if I went to 2.5" springs I might have to get new shocks, which I don't want to do. But I sent MC an email last night with my question and they responded today confirming that I won't need new shocks if I went to 2.5" springs.

Also, initially I didn't want to go to 2.5s becuase I like the height of my Jeep as is, I never scrape. But as I think about it more, I should only lose an inch of height. But it should go a long ways to help me improve my pinion angle issue. Its really the simple answer.

The other option you mentioned about adding a FAD had me intrigued, but I could find virtually no info about doing that. I do like the idea of having a FAD, but I don't know if I like the idea of adding an electronic component that it didn't come from the factory with. Just worried that it might cause other electronic issues or errors, and that's the last thing I want to deal with.

I have one other quesiton about the drop brackets. I actually do have the brackets for the front, but I never installed them. I wanted to see how the Jeep rode and handled without them first. It rides and handles great so I just never installed them, didn't feel the need.

My question about the drop brackets is this. I know the primary reason for their existance is to correct the suspension geometry when you dont' have adjustable CAs. But if you have adjustable CAs you don't need them because you can correct the geometry by adjusting the CA lengths. In your response you say you were "talking about if you need to decrease the pinion angle further". Are you saying that drop brackets enable you to decrease the pinion angle without further reducing caster?

My understanding is that caster and pinion angles are directly related to one another. When you increase caster you're rolling the axle back, pointing the pinion further down. when you decrease caster your rolling the axle forward, pointing the pinion further up. Do drop brackets somehow make them simpler to balance and optimize?

 

[AVGeek99]

I guess it's just best to quit f!@$ing around. I just placed an order for 2.5" springs from MC. This weekend I'm going to install drop brackets. Will install the springs next weekend if they are delivered by then.

On the outside chance the drop brackets solve my issues I may just return the springs.

 

[Roky]

Yes, I have considered going down to 2.5 springs. My only concern about that was my shocks might be too long. When I ordered my shocks I got longer ones that were right at the compressed and extended limits recommended by MC for the 3.5GC. So I was thinking if I went to 2.5" springs I might have to get new shocks, which I don't want to do. But I sent MC an email last night with my question and they responded today confirming that I won't need new shocks if I went to 2.5" springs.

Also, initially I didn't want to go to 2.5s becuase I like the height of my Jeep as is, I never scrape. But as I think about it more, I should only lose an inch of height. But it should go a long ways to help me improve my pinion angle issue. Its really the simple answer.

The other option you mentioned about adding a FAD had me intrigued, but I could find virtually no info about doing that. I do like the idea of having a FAD, but I don't know if I like the idea of adding an electronic component that it didn't come from the factory with. Just worried that it might cause other electronic issues or errors, and that's the last thing I want to deal with.

I have one other quesiton about the drop brackets. I actually do have the brackets for the front, but I never installed them. I wanted to see how the Jeep rode and handled without them first. It rides and handles great so I just never installed them, didn't feel the need.

My question about the drop brackets is this. I know the primary reason for their existance is to correct the suspension geometry when you dont' have adjustable CAs. But if you have adjustable CAs you don't need them because you can correct the geometry by adjusting the CA lengths. In your response you say you were "talking about if you need to decrease the pinion angle further". Are you saying that drop brackets enable you to decrease the pinion angle without further reducing caster?

My understanding is that caster and pinion angles are directly related to one another. When you increase caster you're rolling the axle back, pointing the pinion further down. when you decrease caster your rolling the axle forward, pointing the pinion further up. Do drop brackets somehow make them simpler to balance and optimize?

The drop brackets aren’t for helping you with the pinion angle and caster in your specific situation….. their help will be lowering the angle of your control arms to help with stability. You may have to shorten your lowers if you use the drop brackets to keep them from rolling the axle back worsening the problem…..

When you’re sitting that high on a short arm lift, and the ds spins all the time, the pinion/caster is a give and take, compromise you have to deal with…….. FAD or locking hubs are the best options for the hybrid dailydriver/offroader imo…. These two options don’t eliminate the vibration, they just move it from 2WD to 4WD….. getting a slight vibration at 60 Offroad isn’t a problem for me ….. I’m not going to going that fast Offroad, if I was, I wouldn’t be concerned about the vibration, I’d be way to busy trying to keep it wheels down…

 

[AVGeek99]

The drop brackets aren’t for helping you with the pinion angle and caster in your specific situation….. their help will be lowering the angle of your control arms to help with stability. You may have to shorten your lowers if you use the drop brackets to keep them from rolling the axle back worsening the problem…..

When you’re sitting that high on a short arm lift, and the ds spins all the time, the pinion/caster is a give and take, compromise you have to deal with…….. FAD or locking hubs are the best options for the hybrid dailydriver/offroader imo…. These two options don’t eliminate the vibration, they just move it from 2WD to 4WD….. getting a slight vibration at 60 Offroad isn’t a problem for me ….. I’m not going to going that fast Offroad, if I was, I wouldn’t be concerned about the vibration, I’d be way to busy trying to keep it wheels down…

This makes perfect sense!!

I too like the idea of FAD. Gives you a lot more freedom to run decent caster even at higher ride heights. My height was still a bit of an issue on my 2021 back in MN becuase we got enough snow that I would use 4 hi at 50-55 or a little more. This caused quit a bit of vibration when my caster was near 6 so I was typically at 4 +/-.

On my 2025 I've been running with caster around 3 and it's been fine until my trip to Moab. That was my first wheeling trip with it and it's about 7 hours of a lot of montain driving.

Do you have any links about people that have installed FADs. I searched an only found a single thread on a Wrangler 392 forum but there weren't any details about install.

 

[Roky]

This makes perfect sense!!

I too like the idea of FAD. Gives you a lot more freedom to run decent caster even at higher ride heights. My height was still a bit of an issue on my 2021 back in MN becuase we got enough snow that I would use 4 hi at 50-55 or a little more. This caused quit a bit of vibration when my caster was near 6 so I was typically at 4 +/-.

On my 2025 I've been running with caster around 3 and it's been fine until my trip to Moab. That was my first wheeling trip with it and it's about 7 hours of a lot of montain driving.

Do you have any links about people that have installed FADs. I searched an only found a single thread on a Wrangler 392 forum but there weren't any details about install.

There was a couple guys on here that did it…. I think @EbyCreek was one of them, but not sure…… I’ve been on this board a long time, but I still suck at looking shit up, ….. I remember guys talking about using the Tazer to keep pcm happy…. Maybe make a thread, see if anyone chimes in….

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