4.0, 42RLE, NP241OR High Speed Vibrations After Re-Gear to 4.88 or Deeper

Mr. Bills

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The purpose of this thread is to provide a clearing house of information directly relevant to the the onset of high speed vibrations in a TJ/LJ Rubicon with 4.0l engine, 42RLE transmission and NP241OR transfer case after re-gearing to 4.88 or deeper gear sets..

Discussion of similar phenomena in jeeps with different transmissions, transfer cases or axles is a topic for another thread.

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My jeep: 2006 LJ Rubicon with 4.0l engine, 42RLE automatic transmission, 241OR transfer case, purchased from the prior owner with "metric 35" Mickey Thompson Baja ATZ P3 tires [315/75] and 4.88 gears. Currie Correctlync Steering, Currie Antirock Off-Road Swaybar, 5" lift.

My issue: After re-gearing from 4.88 to 5.38, I had a vibration or harmonic in or near the front end that began at exactly 67 mph and got worse as speed increased, enough so that if felt like it might vibrate my fillings loose when I tried to pass the 18 wheelers on the Interstate at 73 or above.

Posts on various jeep forums suggest that I am not the only one with this issue after re-gearing to 5.13 or 5.38. It appears to occur primarily (or only) in TJ's with the 42RLE automatic transmission and NP241OR transfer case.

My observations:

Immediately after the re-gear I noticed chirping from double cardan joint in the factory front driveshaft. The driveshaft was removed. I ordered a custom Tom Woods front driveshaft in lieu of rebuilding the u-joints in the factory unit. While waiting for a replacement no vibrations were noted.

The new driveshaft arrived in undamaged packaging and was installed. No vibrations were noted for several weeks because the gears were still new and during initial break-in I had been instructed to keep the speeds under 60. It wasn't until a few weeks after the new driveshaft was installed, after ring and pinion break-in was complete, that I drove faster than 65 mph and first noticed the vibration,which begins at exactly 67 mph and feels a lot like hitting a bad section of rain grooves in a concrete freeway. The vibration can only be felt through the steering wheel and not through the body, floor or shift levers. The vibration does not affect steering, which tracks straight and does not drift. The vibration does not increase much with increases in speed, if at all. Its there or its not.

The vibration did not change in mph at onset, character or intensity or any other way after any of the following:
a. Tire pressure adjustments
b. Re-balancing and rotating tires
c. Re-alignment (all within specs except caster, which was low as expected with a lifted rig)
d. Comprehensive professional inspection of all steering components, which are all tight, in good repair and well lubricated.

I had a telephone conversation with @mrblaine, who was aware of similar issues with other jeeps with 42RLE, 241OR and deep gears. He suggested that I check the front pinion angle and raise it if it was low.

My front pinion angle measured 10.5* My front driveshaft angle was 11.5*. The front [pinion angle was adjusted after which the front pinion angle and front driveshaft angle were both 11*.

I have only had time for an initial test drive. The vibration still comes on at exactly 67 mph but the intensity is considerably less after the pinion angle adjustment. I can now get up to 85 mph (as fast as I could go for the conditions that day) and the vibration isn't so bad that I want to immediately get off the throttle. However, there is still enough vibration to be an irritant at 67-75 mph cruising speeds.

I have a 300 mile round trip planned this coming weekend, which will give me ample time to record observations about how the pinion angle adjustment has changed the vibration. Its easy enough to avoid, just set cruise control at 65, but I would like to improve the situation after my return.

My plan is to raise the front pinion angle about 1/2* and observe any changes. Maybe the vibration will subside, maybe it will get worse, maybe it will stay the same.

If pinion angle adjustments can't fix this, I am open to any and all suggestions as to what to try next. I know I can "fix" it with money, i.e., converting to manual hubs, but I would like to try all the adjustments possible before I give up and go that route.

Your similar experiences?
What worked?
What didn't?
 
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Been there done that. 3 Tom Wood shafts, two Denny’s driveline shafts,one Carolina driveline shaft. Re did the gears three times, rebuilt the transfer case, all new parts, two sets of tires, three sets of control arms with different types of bushings, adjusted the pinion angle THOUSANDS of times from -5 to +6 in 1/4 degree increments. Literally spent every night and weekend in the garage making 1/4 turn adjustments and driving it, make 1/4 turn and drive it. Put hundreds of miles up and down a 2 lane by my house doing this. After spending 3x what the hub kit cost and months and months trying to fix it I did the hub conversion. Just drove 700 miles round trip to a Jeep jamboree running 75-80 the whole way and it rolls down the road just as good as my wife’s 17 grand Cherokee.
 
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I am no expert but the one common thread in all these vib threads are lifted rigs and low gears. Is it possible that the increased drive line speeds on such short drive lines is the actual issue? Would be an interesting "test" to run a stock height, highlined Jeep with 35's with deep gearing an see what happens. Of course it does vary from rig to rig so who knows how accurate that "test" would be.
 
I am no expert but the one common thread in all these vib threads are lifted rigs and low gears. Is it possible that the increased drive line speeds on such short drive lines is the actual issue? Would be an interesting "test" to run a stock height, highlined Jeep with 35's with deep gearing an see what happens. Of course it does vary from rig to rig so who knows how accurate that "test" would be.

This is the way I see it. Vibrations come on at higher speeds so it isn't hard to conclude they are driveshaft RPM related. When you regear, you increase the driveshaft speed (when comparing the same MPH before and after the regear) so you increase the need for a perfectly set up driveline. Most people worry about engine RPM on the highway after going to deeper gears, but I see driveshaft RPM as a more important metric.

I can't see how it would have anything to do with the type of transmission or transfer case but I could be wrong on that. In my view, it probably has something to do with your pinion angle, slip joint, or driveshaft u-joints considering the driveshaft will be spinning 10% faster after regearing from 4.88 to 5.38 (10% is fairly substantial when considering how fast it spins).

I've fought my share of driveshaft issues with my TJ. My fix comes down to constantly greasing the driveshafts. I started greasing more often after a few failed slip joints and a failed CV joint. At least in my opinion, if you really want to cruise down the highway at 80 mph smoothly in a lifted TJ, locking hubs are a must. I've had my pinion angle looked at by at least 4 Jeep specific shops at this point and I still get some vibration above 75 MPH. Just one of those issues with a high, short-wheelbase vehicle.
 
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I was told by a Dodge Chrysler master tech it is caused by the angle of the front Cv joint at the transfer case. Spicer rates the joint to be vibration free up to a set rpm, the taller gears increase the DS speed enough to exceed that rpm. Does he know what he is talking about? Who knows, but he seemed to know exactly what I was talking about. I had already done the hub conversion so I never looked much into his claims.
 
I was told by a Dodge Chrysler master tech it is caused by the angle of the front Cv joint at the transfer case. Spicer rates the joint to be vibration free up to a set rpm, the taller gears increase the DS speed enough to exceed that rpm. Does he know what he is talking about? Who knows, but he seemed to know exactly what I was talking about. I had already done the hub conversion so I never looked much into his claims.

The best driveline is a perfectly flat driveline. Can't have any vibrations without any angles (if everything is balanced). Though, I will say that all TJ's with the same lift height will have similar CV joint angles at the transfer case. If the joint angle was causing problems, then every TJ owner with a lift taller than you would be having even more issues. I believe the more important angle is the angle of the driveshaft going into the axle.

Also, what did your hub conversion entail? Are you running a 30 or 44 in the front?
 
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I was told by a Dodge Chrysler master tech it is caused by the angle of the front Cv joint at the transfer case. Spicer rates the joint to be vibration free up to a set rpm, the taller gears increase the DS speed enough to exceed that rpm. Does he know what he is talking about? Who knows, but he seemed to know exactly what I was talking about. I had already done the hub conversion so I never looked much into his claims.

Which leads me wonder if a TT could improve those angles?
 
Like I said, the guy sounded pretty sure of what he was claiming. At that point I already did the hub kit so I didn’t get too far into it. I did the 5x5.5 kit so I had to do a little more to make everything work. It’s a 30 but it’s sleeved, trussed, and the C’s are boxed, chromolly axels, I don’t bounce it around and I am only running a 33” tire.
 
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Like I said, the guy sounded pretty sure of what he was claiming. At that point I already did the hub kit so I didn’t get too far into it. I did the 5x5.5 kit so I had to do a little more to make everything work. It’s a 30 but it’s sleeved, trussed, and the C’s are boxed, chromolly axels, I don’t bounce it around and I am only running a 33” tire.

Well, I'm not sure how much I agree with that guy no matter how sure he sounded. But besides that, I'm considering a similar hub conversion for mine, but from the ones I can find online, it may be cheaper to just throw a 44 in. I'm only on 33' as well so the 30 would be fine. If you don't mind me asking, how much did the conversion run you?

Also, I'm not educated on tummy tucks. But in my brain, wouldn't that move everything up making the driveline angles worse? I've also been considering a tummy tuck and if they made my driveline angles better I might have to order one right now!
 
Well, I'm not sure how much I agree with that guy no matter how sure he sounded. But besides that, I'm considering a similar hub conversion for mine, but from the ones I can find online, it may be cheaper to just throw a 44 in. I'm only on 33' as well so the 30 would be fine. If you don't mind me asking, how much did the conversion run you?


$1200.00 or so if I remember correctly. It’s been about a year now
 
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Well, I'm not sure how much I agree with that guy no matter how sure he sounded. But besides that, I'm considering a similar hub conversion for mine, but from the ones I can find online, it may be cheaper to just throw a 44 in. I'm only on 33' as well so the 30 would be fine. If you don't mind me asking, how much did the conversion run you?

Also, I'm not educated on tummy tucks. But in my brain, wouldn't that move everything up making the driveline angles worse? I've also been considering a tummy tuck and if they made my driveline angles better I might have to order one right now!


The front output shaft on the Tc points up from the factory. The TT actually moves it down 1/2-1 inch which would decrease the cv angle. Again not a clue if the guy was right. His Jeep didn’t vibrate, claimed it did prior to the TT.
 
The front output shaft on the Tc points up from the factory. The TT actually moves it down 1/2-1 inch which would decrease the cv angle. Again not a clue if the guy was right. His Jeep didn’t vibrate, claimed it did prior to the TT.

I didn't know that the front output pointed up. Thats interesting. Makes me want a tummy tuck even more because my front driveshaft does cause some vibrations. I think I'm going to leave that for after I get my new frame installed.
 
Now you guys have me wondering if re-gearing to 5.38 is a wise idea... I get ZERO vibrations with my 4.10 gears, 42RLE transmission, NV241OR transfer case, and 4.0 engine. Not to mention I've got an extreme driveline angle due to the 4" lift and the tummy tuck (it's so extreme that if I tilt the pinion back anymore it's going to cause the track bar to smack the fuel tank!
 
Now you guys have me wondering if re-gearing to 5.38 is a wise idea... I get ZERO vibrations with my 4.10 gears, 42RLE transmission, NV241OR transfer case, and 4.0 engine. Not to mention I've got an extreme driveline angle due to the 4" lift and the tummy tuck (it's so extreme that if I tilt the pinion back anymore it's going to cause the track bar to smack the fuel tank!

Getting manual hubs would get rid of the front driveline to worry about. In terms of the rear, that may be more of a challenge. I think @Jerry Bransford runs a similar setup so he may to be able to give some insight into what works and what doesn't. I'm considering the Savvy under armor skid system and I'm worried about my driveline angles with only 3" of lift and 33's.

I'd try it out and see what happens. If it were me and that ended up with vibrations, I'd chop the fenders and get some smaller springs. But I think your jeep may be too nice to do that. The 5.38's would be worth almost any amount of tweaking it takes to get this to work right.
 
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Getting manual hubs would get rid of the front driveline to worry about. In terms of the rear, that may be more of a challenge. I think @Jerry Bransford runs a similar setup so he may to be able to give some insight into what works and what doesn't. I'm considering the Savvy under armor skid system and I'm worried about my driveline angles with only 3" of lift and 33's.

I'd try it out and see what happens. If it were me and that ended up with vibrations, I'd chop the fenders and get some smaller springs. But I think your jeep may be too nice to do that. The 5.38's would be worth almost any amount of tweaking it takes to get this to work right.

It certainly would be interesting to hear from people running deep gears like 5.38 and how many of them have vibrations while how many don't.

On my old TJ Rubicon with 4" of lift and 33" tires, I never had any vibrations at all after the re-gear to 4.88. Then again, that one also didn't have a tummy tuck.

Makes you appreciate the LJs and their much longer driveshaft.