Full-time four-wheel-drive driveline vibration with lift

[stumblinhorse]

I wouldn’t say thousands of Jeeps are running a 3.5 inch lift with full Auto and no 2WD or FAD without issue. The front driveshaft always spinning on our 392s is what causes the vibrations. That is not the case with any other JL. I think someone said earlier here or in another thread the driveshaft is spinning on JLs even with the FAD but that is not true (see below video). Jeep really screwed us with this transfer case / no FAD combo.

As I said earlier, 2024 JL ordered with 4H auto will be the same set up as the 392 No FAD and connected all the time.

And agree with @guruman shift on the fly means everything is turning…. No way shifting at speed is possible without stuff already turning equally. But 5mpg is a bit of an exaggeration. Everything would have locking hubs and we would still have V8s if it simply needed locking hubs to get low 20’s mpg in a 392…

 

[EBYCREEK]

Wonder how hard it would be to purchase the FAD motor and install it on the 392. I'm sure somebody on here could figure this out. Or setup a manual cable to lock and unlock the front FAD.

 

[7.62x392]

There's probably a dozen 392s on here doing it.

JKs have the same limitations with pinion and drive line angles. I've driven my 5.5 inch lifted JK for many many miles at speed in 4wd without vibration, again worse angles the a JL with 3.5 inches of lift.

And you may be right about the drive shaft not spinning, but I cannot get my head around how you could shift on-the-fly at speed, if it's not. It cannot be engaging the FAD as the axle shafts would be spinning in opposite directions. So how then does the 100 pounds of steel (transfer case, drive shaft, ring, pinion spider gears and one half of the axle) get from zero to highway speed in the amount of time it takes to move the lever from 2h to 4h?

This has always been the dirty little secret of shift-on-the-fly, all of the driveline had to be spinning all the time anyway. It's why some folks would swap in manual locking hubs, so that they could gain an extra 5 or so MPG when in 2h.

I wouldn’t necessarily call the recommended maximum shift from 2H to 4H of 45 mph highway speed. The re-engaging of the FAD probably works no different than the way the synchronizer does in your transmission to bring idle gears up to speed in the fraction of a second it takes to shift. With the spider gears unlocked and disconnected in the diff and allowed to spin in the opposite directions, the pinion gear, and therefore driveshaft will not spin (as the disconnected shaft on the passenger side of the axle offers much less resistance than the pinion gear which is connected to the driveshaft and T case). Not to mention if the pinion gear and driveshaft were spinning it completely defeats the purpose of the FAD - to cut down on emissions by reducing rolling resistance.

I can’t speak for the JK, but there was an aftermarket rzeppa shaft available for lifted JKs (as noted by someone else above) which would also stop the vibrations. No such option for the JL. The JK and JL axles and tcase are also different so there may be different geometry and other factors working there as well to keep the vibrations down.

I know I’ve seen enough examples with the 392, myself included, to know there is an issue here with carden shafts coupled with taller lifts on stock axles causing vibrations due to the pinion angles and lack of a FAD. Granted, once I corrected the caster it’s much more subtle, but I do still hear / feel it from time to time, especially when decelerating in 4cyl mode over 55 mph when there is the least resistance on the driveshaft from the transfer case allowing the shaft to vibrate more.

 

[EBYCREEK]

Everybody on the forum needs to email TeraFlex and get them to build us a driveshaft. I emailed them a couple of times and I'm sure if there was big enough demand they would build one. I'm lucky and have no vibration on my 2021 392 running the 3.5" MC Gamechanger lift. But I'm getting tired of removing the skid plates all the time to grease the front driveshaft.

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[7.62x392]

As I said earlier, 2024 JL ordered with 4H auto will be the same set up as the 392 No FAD and connected all the time.

And agree with @guruman shift on the fly means everything is turning…. No way shifting at speed is possible without stuff already turning equally. But 5mpg is a bit of an exaggeration. Everything would have locking hubs and we would still have V8s if it simply needed locking hubs to get low 20’s mpg in a 392…

There’s actually information on another forum about this. When you shift into 4HI while moving, the computer will send power to the front shaft a split second before re-engaging the FAD collar which gets everything moving in the same direction by forcing the pinion to turn which allows you to “shift on the fly”.

 

[Granny Gear]

I feel fortunate. I am in the camp of having the 3.5" MetalCloak Game-Changer lift and no vibration issues (knocking on wood). My Jeep (2021 non-XR) is currently running the MC 6Pak shocks, and Nitto 37” Trail Grapplers.

When the lift was done, I also changed out steering components (track bar, drag link, tie rod, & steering stabilizer) and chose to go with MC drive shafts as recommended (1310).

I also changed to Dynatrac HD ball joints, RCV axles, MC sector shaft brace, MC Axle Side Steering Stabilizer Mount, and moved to forged arm RockJock Antirock sway bars.

I live less than an hour away from CloakWorks (in Northern CA), so I chose to have them complete the installation. With the changes I mentioned, the Jeep drives great with absolutely no driveline or steering issues. A big improvement over the OEM set-up. I am extremely pleased how well the components are working together.

 

[7.62x392]

I feel fortunate. I am in the camp of having the 3.5" MetalCloak Game-Changer lift and no vibration issues (knocking on wood). My Jeep (2021 non-XR) is currently running the MC 6Pak shocks, and Nitto 37” Trail Grapplers.

When the lift was done, I also changed out steering components (track bar, drag link, tie rod, & steering stabilizer) and chose to go with MC drive shafts as recommended (1310).

I also changed to Dynatrac HD ball joints, RCV axles, MC sector shaft brace, MC Axle Side Steering Stabilizer Mount, and moved to forged arm RockJock Antirock sway bars.

I live less than an hour away from CloakWorks (in Northern CA), so I chose to have them complete the installation. With the changes I mentioned, the Jeep drives great with absolutely no driveline or steering issues. A big improvement over the OEM set-up. I am extremely pleased how well the components are working together.

I’ve heard people have had better luck with the 1310 shafts due to them being a lot lighter. I’m running the Adams 1350s.

 

[EBYCREEK]

I'm running the MetalCloak 1310 driveshaft also with no vibration. I installed the lift myself and set the control arms as per the instructions. So far drives straight and no vibration. It drove so good after the lift installation I've never had it aligned. Currently have about 7,000 miles on the lift.

 

[guruman]

There’s actually information on another forum about this. When you shift into 4HI while moving, the computer will send power to the front shaft a split second before re-engaging the FAD collar which gets everything moving in the same direction by forcing the pinion to turn which allows you to “shift on the fly”.

I don't buy it. Again, a hundred pounds of steel does not start rotating 45mph in a fraction of a second. At least not smoothly.

The XJ and the ZJ both had an axle disconnect, and manual non-computer controlled transfer cases (both select-trac and command-trac)... with shift-on-the-fly....

 

[7.62x392]

I don't buy it. Again, a hundred pounds of steel does not start rotating 45mph in a fraction of a second. At least not smoothly.

The XJ and the ZJ both had an axle disconnect, and manual non-computer controlled transfer cases (both select-trac and command-trac)... with shift-on-the-fly....

It’s not hundreds of pounds of steel it’s a drive shaft and carrier that is geared and made to spin that maybe collectively weigh 100 pounds, and that’s being generous. The whole point of the FAD is to keep the moving parts to a minimum to save gas and improve emissions. To have the pinion spinning completely defeats the purpose of having that system in the first place.

You don’t have to believe it but they discuss how it works right in that video. This is also right off the Adams website.

 

[Tank2112]

To get fuel milage advantage of 2-wheel drive....you need manual locking hubs and a true 2wd transfer case.
Like a RocTrac 241 or Atlas.

Not sure about longevity of stock transfer case, with Tazer forced 2wd and manual locking hubs?

Manual locking hubs can disconnect wheels from axles/ring&pinion, just need transfer case to disconnect front drive-shaft output yoke.

 

[guruman]

It’s not hundreds of pounds of steel it’s a drive shaft and carrier that is geared and made to spin that maybe collectively weigh 100 pounds, and that’s being generous. The whole point of the FAD is to keep the moving parts to a minimum to save gas and improve emissions. To have the pinion spinning completely defeats the purpose of having that system in the first place.

You don’t have to believe it but they discuss how it works right in that video. This is also right off the Adams website.

I've never been a fan of the old "I saw it on the internet so it must be true" trope. Especially when it's someone trying to sell something.

But, I'm also not dumb enough to believe that I cannot be wrong, so it's possible.

But what I do know, is that previous Jeeps with front axle disconnects and shift-on-the-fly transfer cases did not have fancy computer controlled systems to spin up the front driveshaft. And spinning the front driveshaft up to speed in a fraction of a second is no small engineering feat, which is exactly why every previous generation of Jeeps, even those with disconnecting axle shafts, relied on keeping the front driveline moving all the time for the shift-on-the-fly to work.

We are also talking about a manual shifted mechanism, there is no fancy electronic sequence... you pull the lever and gears engage immediately.

So there's what... a switch in the shift mechanism that tells the transferase to spin up the front driveline when you move the lever a bit (but before actually leaving the 2wd position)? And non-392 JLs have the clutch mechanism on the front output as well to facilitate this? But I'm reading that the non-392 JL have there NV241(OR), but the 392 is a MP3022... I'm pretty familiar with the 241 cases from way back. I do not think they had a clutch/actuator/whatever for spinning up the driveline.

My curiosity is killing me... I promise I'm not just trying to be argumentative.

Either non-392 JLs have some mechanism to handle the start up of the front driveline (which they appear NOT to have based on the NV241)

or the front driveline is kept spinning... ?

Here's a diagram of the NV 241 the NV241OR is similar, but with a lower 4 to 1 low range gears:

And here is our MP3022:

Without all of the extra bits in the MP3022, I just do not see HOW the nv241 can start the driveline spinning on demand, in a fraction of a second.

I guess for the OPs issue it does not matter if I'm right or wrong, as several members have chimed in that they are running the exact lift setup without driveline vibrations using replacement driveshafts.

But dang it, I'm not going to be able to sleep until lI figure this out.... Time to go test drive a 3.6 JL maybe......

 

[guruman]

...
Not sure about longevity of stock transfer case, with Tazer forced 2wd and manual locking hubs?

...

In my bit of research into these cases, I did discover that the MP3022 in our 392s are the same as in the 4xe, which I has a 2wd setting.... so the same case is OK running in a 4ex in 2wd long term.

The bigger problem would be longevity of the rear driveline taking the 470hp output all the time by itself. Which, I think, is why they eliminated the 2wd for us. Not to mention the toll it would take on the rear tires...

Supposedly the front power transfer relies on throttle input in our 392s. So at low throttle it's basically 2wd, but at full beans it's 4wd. Someone also suggested that it takes info from the wheel sensors as well.

 

[stumblinhorse]

In my bit of research into these cases, I did discover that the MP3022 in our 392s are the same as in the 4xe, which I has a 2wd setting.... so the same case is OK running in a 4ex in 2wd long term.

The bigger problem would be longevity of the rear driveline taking the 470hp output all the time by itself. Which, I think, is why they eliminated the 2wd for us. Not to mention the toll it would take on the rear tires...

Supposedly the front power transfer relies on throttle input in our 392s. So at low throttle it's basically 2wd, but at full beans it's 4wd. Someone also suggested that it takes info from the wheel sensors as well.

Pretty much the exact same setup as the 392 is in all the quadra-trac 2 JGC WK2s(MP3022/3). I have one. But obviously not the FAD, since they are IFS. It is full time 4WD in those and works the same. Ratio etc is the same.