Bad Vibration after Front Driveshaft Install

[bleda2002]

4hi auto uses the fad to either engage or disengage axle shafts, when the speed sensors sense slip/traction loss in the rear.

To my knowledge, they started not including fads in 2024.

No, the transfer case has clutches that engage/disengage when the sensors detect slip, the fad doesn't really play a part in this other than locking when in any mode other than 2h. This is why jeep recommends not using 4h auto in slippery situations but instead 4h part time so that the clutches stay locked together and don't over heat.

Consequently the 392 also eats transfer cases due to the clutches not handling the power well and a commonish mod is to swap in a regular Rubicon transfer case instead

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[JeepOfTheseus]

4hi auto uses the fad to either engage or disengage axle shafts, when the speed sensors sense slip/traction loss in the rear.

To my knowledge, they started not including fads in 2024.

I thought there is a clutch in the transfer case that engages or disengages, and when you’re in 4H Auto the FAD coupler simply locks the passenger shaft. But even with the clutch disengaged, the locked shaft (along with the driver shaft) causes the driveshaft to spin. Point of the FAD is to save on gas when there’s no way to use it (2H).

 

[WILDHOBO]

No, the transfer case has clutches that engage/disengage when the sensors detect slip, the fad doesn't really play a part in this other than locking when in any mode other than 2h. This is why jeep recommends not using 4h auto in slippery situations but instead 4h part time so that the clutches stay locked together and don't over heat.

Consequently the 392 also eats transfer cases due to the clutches not handling the power well and a commonish mod is to swap in a regular Rubicon transfer case instead

That’s great news actually. Thanks very much. I was under the impression the fad was involved, as the driveshaft always rotates in 4hi auto, even without slip.

 

[WILDHOBO]

I thought there is a clutch in the transfer case that engages or disengages, and when you’re in 4H Auto the FAD coupler simply locks the passenger shaft. But even with the clutch disengaged, the locked shaft (along with the driver shaft) causes the driveshaft to spin. Point of the FAD is to save on gas when there’s no way to use it (2H).

And stop the driveshaft from rotating whatsoever in 2hi, which decreases wear on it and the transfer case.

 

[Soulfa535]

I’ve been looking for this solution for like 4 years. I have a FAD on my 21 JTR, but I have the 4hi auto transfer case, so can run at highway speeds with the driveshaft rotating. I’ve killled two Oem driveshafts in front. I now have a double cardan 1350 up there. I’d owned the shaft for years, but due to the problem, I kept avoiding putting it in. But the day before a trip to Moab I found my new mopar shaft blown again, and had to put it in. It’s fine, but I do get a vibration at higher speeds. I don’t see a cost effective solution. It used to be my daily driver, so it was a bigger deal then. Now I just deal with it as it’s not as of recently. The real solution, as far as I can tell, is a high pinion axle, allowing for good caster and perfect driveline angles. But it drives me crazy that I’d likely lose my 4hi auto functionality with an aftermarket axle without a fad. Rock and a hard place stuff. Let me know if you have other thoughts. I’ll do the same.

Unfortunately, if I install a high pinion axle my dishwasher is probably going to go on the fritz and I like keeping her around . I have a part time transfer case. I'm not sure if that would be any different with the drive shaft spinning full time. Maybe I will be going back to the 02 Tacoma as my daily or at least when it snows, not that we are getting any this year.

Thanks for the info

-David

 

[bleda2002]

Unfortunately, if I install a high pinion axle my dishwasher is probably going to go on the fritz and I like keeping her around . I have a part time transfer case. I'm not sure if that would be any different with the drive shaft spinning full time. Maybe I will be going back to the 02 Tacoma as my daily or at least when it snows, not that we are getting any this year.

Thanks for the info

-David

How long are your front shocks that you needed a drive shaft? Its not really the lift height but the shock length that means you need a new drive shaft. I think a lot of these lift companies are a bit disingenuous saying that xxx lift needs a drive shaft when it should be more based on shock length and axle travel. If you stick to around I think it's 28.5 inch extended length you can keep the stock drive shaft.

 

[Soulfa535]

How long are your front shocks that you needed a drive shaft? Its not really the lift height but the shock length that means you need a new drive shaft. I think a lot of these lift companies are a bit disingenuous saying that xxx lift needs a drive shaft when it should be more based on shock length and axle travel. If you stick to around I think it's 28.5 inch extended length you can keep the stock drive shaft.

I have Falcon SP2 3.3 shocks. The extended length is 28.4". The repezza joint looks like it has taken a couple of hits but isn't totally deformed like some of the pictures I've seen online. The boot is shredded and the grease look a little toasty. I kind of thought this would be a problem before the lift because of the shock length. I didn't know that the drive shafts with with the CV joint at the transfer case had a vibration issue when they spin full time.

Who knows? Maybe I am blowing thing out of proportion and everything will be copasetic with the new shaft. Just want to have a plan B ready if Murphey shows up.

 

[bleda2002]

I have Falcon SP2 3.3 shocks. The extended length is 28.4". The repezza joint looks like it has taken a couple of hits but isn't totally deformed like some of the pictures I've seen online. The boot is shredded and the grease look a little toasty. I kind of thought this would be a problem before the lift because of the shock length. I didn't know that the drive shafts with with the CV joint at the transfer case had a vibration issue when they spin full time.

Who knows? Maybe I am blowing thing out of proportion and everything will be copasetic with the new shaft. Just want to have a plan B ready if Murphey shows up.

Your vibration is most likely related to the amount of caster that you've got dialed in. Anything over about 4.5-5 degrees and you'll get a vibration from the front just because of the angles. Running a non-rzeppa at that angle will be even worse so you may just want to replace the rzeppa and dial back the caster a bit.

28.4 should also be fine with the rzeppa again assuming a caster of about 5-5.5 degrees. I am at 28.45 and 3 wheeled it quite a bit on all kinds of crazy shit and still on the stock shaft at 65k miles.

The wife also has a 4xe and we have to run around 5 to keep it from humming. Also the same 28.45 length front shocks and plenty of 3 wheeling and still on stock front as well at 50k

 

[WILDHOBO]

I have Falcon SP2 3.3 shocks. The extended length is 28.4". The repezza joint looks like it has taken a couple of hits but isn't totally deformed like some of the pictures I've seen online. The boot is shredded and the grease look a little toasty. I kind of thought this would be a problem before the lift because of the shock length. I didn't know that the drive shafts with with the CV joint at the transfer case had a vibration issue when they spin full time.

Who knows? Maybe I am blowing thing out of proportion and everything will be copasetic with the new shaft. Just want to have a plan B ready if Murphey shows up.

I deformed both collars and then both boots grenaded. My shocks are longer than that at full extension. But if need to measure again.

 

[WILDHOBO]

Your vibration is most likely related to the amount of caster that you've got dialed in. Anything over about 4.5-5 degrees and you'll get a vibration from the front just because of the angles. Running a non-rzeppa at that angle will be even worse so you may just want to replace the rzeppa and dial back the caster a bit.

28.4 should also be fine with the rzeppa again assuming a caster of about 5-5.5 degrees. I am at 28.45 and 3 wheeled it quite a bit on all kinds of crazy shit and still on the stock shaft at 65k miles.

The wife also has a 4xe and we have to run around 5 to keep it from humming. Also the same 28.45 length front shocks and plenty of 3 wheeling and still on stock front as well at 50k

My problem was trying to find a happy medium between spring bow and a bad driveshaft angle. I don’t care if my caster isn’t perfect. But spring bow is an issue for me. Whenever my springs are straight, my driveshaft angle sucks.

 

[bleda2002]

My problem was trying to find a happy medium between spring bow and a bad driveshaft angle. I don’t care if my caster isn’t perfect. But spring bow is an issue for me. Whenever my springs are straight, my driveshaft angle sucks.

Even if you just use the upper control arms to adjust caster you get that much bow? Which company springs and which way are they bowing? I had to use coil correction seats on the wifes

 

[Zachanadandy]

is it this shaft? If so -
This end - must be run straight, little to no angle. That's a single cross or single cardan u-joint and those can't be run with any appreciable angle.

I can't believe driveshaft makers put a single joint at the differential end of the shaft. It just boggles the mind. That end should have a CV joint since yo have to run the shaft uphill to the transfer case, and those with lifts often run heavy caster, meaning the pinion is tipped down, while the transfer case is lifted up farther from the pinion of the front axle.
What the heck is up with that?

A properly designed front shaft should have a double joint, or a CV, at the pinion end to take care of the extra caster, or downward tilt of the pinion, and the increased angle to the transfer case due to lifts.
IMO, it's a pissy design.

I've written about this before, even included youtube videos of shafts running at the angles some Jeep people need to run - with a single joint. It shows exactly why it's a crap design and why they vibrate or jerk when turning at angles above about 2 or 3 degrees. 1 degree is best, above that and you start phasing in RPM changes that cause vibrations.

I've never seen a driveshaft with the CV/double cardan at the axle end either factory or aftermarket. The pinion should be pointed directly at the output flange on the transfer case so the CV at that end deals with the angle and the axle end u-joint is square and happy. Before OEMs started using the rzeppa joints that's how all the front driveshafts were built and setup from the factory. The rzeppa at each end allows the driveshaft to handle wonky/mismatched angles at each end which is why they all run them now. The problem being they aren't capable of enough travel for long travel suspension setups. The issue in the JL/JT is the separation angle between the inner Cs and the pinion. You have to compromise caster in order to get a good pinion angle with a ujoint/cv driveshaft especially when running 3"+ of lift. The 2024 and up models with no FAD run into this issue even worse because they will vibrate in 2wd.

 

[WILDHOBO]

Even if you just use the upper control arms to adjust caster you get that much bow? Which company springs and which way are they bowing? I had to use coil correction seats on the wifes

I’ve been through three spring vendors so far. Not because of this problem, but because I like tinkering. Currently I have rock krawler fronts, and I have their correction matching pads. I have all 8 adjustable arms. Teraflex IR. I have as close to perfectly straight springs as I could currently. But the other side of the coin is driveshaft angle. My caster isn’t actually not too bad. I’ve been over the angles many times. Unless I’m missing something, a high pinion axle housing is the silver bullet. But silver is trading at a pretty high number.

 

[bleda2002]

I’ve been through three spring vendors so far. Not because of this problem, but because I like tinkering. Currently I have rock krawler fronts, and I have their correction matching pads. I have all 8 adjustable arms. Teraflex IR. I have as close to perfectly straight springs as I could currently. But the other side of the coin is driveshaft angle. My caster isn’t actually not too bad. I’ve been over the angles many times. Unless I’m missing something, a high pinion axle housing is the silver bullet. But silver is trading at a pretty high number.

The price of axles(as with so many other things) went absolutely retarded. The big issue is that jeep specs the danas with only 6 degrees of offset, they should really be looking for 8-10 which would let us keep near 6 degrees and good drive shaft angles high or low pinion.

 

[WILDHOBO]

The price of axles(as with so many other things) went absolutely retarded. The big issue is that jeep specs the danas with only 6 degrees of offset, they should really be looking for 8-10 which would let us keep near 6 degrees and good drive shaft angles high or low pinion.

No argument there. I’ve settled on the fact that I can just not engage 4hi auto going quickly, very often, and not for long durations. Oh well. First world problems.

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