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

My concern is more the vibration slowly damaging bearings, splines and seals not so much the annoying drone. Blaine, Dave or those that are mechanically knowledgibe and experiencesd with this specific issue, do you believe this harmonic issue can cause mechanical wear with known balanced driveshafts and good pinion angles? I have too many hours and thousands of dollars in my LJ that I built in the last year from stock.
 
Here is an observation of other LJs with automatic transmissions that I test drove recently that do NOT have the drone above 62 mph. Both of them had spacers between the frame and the skid plate. One LJ had a bent stock skid plate and the other was a Clayton long arm skid plate with 4.88s. They both did have the same drone at 45 mph when the over drive is engaged and the engine is lugging being in a tall gear same as my LJ. Turn off the overdrive it goes away. This is not my concern, it is the high-speed drone that I want to solve/eliminate, just providing more information to the group. Another LJ I test drove recently that did have the drone at high-speed‘s was a 9th degree long arm with no spacers on the transfer case skid. Not suggesting only observing, would love to hear others input on this.
 
My concern is more the vibration slowly damaging bearings, splines and seals not so much the annoying drone. Blaine, Dave or those that are mechanically knowledgibe and experiencesd with this specific issue, do you believe this harmonic issue can cause mechanical wear with known balanced driveshafts and good pinion angles? I have too many hours and thousands of dollars in my LJ that I built in the last year from stock.
It really isn't that type of vibration but it is. It is all due to the resonant frequencies, but a typical vibe like something being out of balance it isn't.
 
Hey guys,
Not an expert, but I am very mechanically inclined, and I am curious if anyone has tried one of the following and what results they found?
1) moving the clock postion of the DS spline between the yokes of the DS? Reason I ask is because years ago I had the tyranny of my 34' motor home replaced, after which I had a vibration come in around 68-70 mph, after many go rounds w the shop they insisted it was the parking brake assembly which on that unit was a drum on the DS, I finally one day said screw it and bought a new 400 drum and installed it myself and low and behold the vibe was gone, so I figured they were right, then months later while doing something I noticed I had the DS off 1 spline from where the factory Mark's were, I then fearing would be hurting something took it out and lined it up and the vibes were back..... now out of curiosity I then put the old drum back on and put it back and one spline off and the vibes were gone again??? Only conclusion we had was the first 26k miles things wore together a certain way, w the new tranny perhaps moving the DS 1 spine out the rest of the drive line closer to that ol sweet spot now between old and new to keep everything in phase???? Keep in mind this DS the splines were very close together so moving it one spline wasn't drastic.

2) Has anyone tried shiming the diff so the pattern is slightly more to the drive side then the coast side? Again reason I ask is 20 years ago in my early 20s I bought a skid steer and was looking to get more oomph out of my truck to tow it so shop installed a new gears (4.56 I believe) and that vehicle around 67 had a slight vibration, took it to the shop and after trying a few things the ol man of the shop told them to go in and shim it just a touch more to the drive side, his theory was the truck was now engaging more and spending less time in coast during normal driving then before, now back then I didnt know much, but I remember the other guys looking skeptical, well he was right I guess, minor vibes were gone....

3) has anyone tried using a chassis mic to isolate the origin of the harmonic distortion? I have a 4 mic system, just hoping I don't need it after my re gear.

What's ironic is those 2 experiences, along w owning several lifted diesels I always say a trucks sweet spot for happiness and MPGs is 67 mph, seems to be true here too.

Just my .02 on past experiences and curious since my LJ is now lifted 4" on 33s and I have to order gears, was going to do 5.13 since its not my DD but now I am wondering if 4.88 is a safer bet?
 
Pzilly good input.

If all the LJ/42rle owners want to call out their build (precisely) and what they have tried I am willing to build a spread sheet to try to point out similarity’s. The recent conversation has been about harmonics, although we have multiple transfer case skid plates included on this forum that still have the harmonic vibration. With that said the tub is still going to be a universal similarity. Although we all know there are LJ/42rle that do not have our drone vibration issue.

A question it is, is speaking in riddles and yoda formation beneficial or intriguing, the future is uncertain.
 
OK. Here are some data points:

06' LJ Rubicon, 4.0L motor, 42RLE Transmission, NV241OR transfer case, approx 5" actual lift - RE long arm with RE crossmember/skid plate, 35" tires (actual 34.6"), equipped with 4.88 gears at acquisition, no vibrations/harmonics. After re-gear to 5.38:1 vibrations/harmonic barely detectable at 60 mph, like turning a switch "on" at 67-68 mph (calibrated speedometer). Vibrations/harmonics did not change in character/intensity as speeds increased over 67-68, substantially reduced below that speed, undetectable below 60 during deceleration. Removal of front double cardan driveshaft = no vibrations at any speed. Installed new Tom Wood front driveshaft because center joint worn and chirping. Repeated tests - no change other than chirping being gone. Change in front pinion angle = bad vibration if front pinion angle lower than front driveshaft angle, gradually reduced as pinion angle matched to driveline angle and reduced slightly more with front pinon 1 degree higher than driveline angle. However, caster was reduced so much at that angle that steering was slow to return to center. The maximum I could adjust the front pinion angle was 1.5 degrees above driveline angle before lack of return to center became objectionable. Stopped experimenting at that point and addressed problem by converting to manual hubs. Issue resolved.
 
I was reading an article on double cardan drivershaft design today to educate myself and I came across this tidbit.

Double Cardan joints require a centering element that will maintain equal angles between the driven and driving shafts for true constant velocity rotation. This centering device requires additional torque to accelerate the internals of the joint and does generate some additional vibration at higher speeds.

Is that bloody centering ball the cause of these vibes even if the shaft itself is straight and true? The problem is amplified by the factory skid's natural frequency and the design of the tub with minimal sound deadening? And is this why the problem can be resolved with locking hubs in the front?

@mrblaine any thoughts?

Is there anything that could be done reg the centering ball in the joint? Better quality and the? Or is this an inhernt limitation of the joint itself given the angles in a lifted Jeep?
 
Last edited:
I was reading an article on double cardan drivershaft design today to educate myself and I came across this tidbit.

Double Cardan joints require a centering element that will maintain equal angles between the driven and driving shafts for true constant velocity rotation. This centering device requires additional torque to accelerate the internals of the joint and does generate some additional vibration at higher speeds.

Is that bloody centering ball the cause of these vibes even if the shaft itself is straight and true?

@mrblaine any thoughts?
It certainly can be. It isn't common for the centering ball to lose strength and let the joint flop around but it does happen. The steeper the driveshaft angle, the more likely it is to wear.
 
  • Like
Reactions: psrivats
It certainly can be. It isn't common for the centering ball to lose strength and let the joint flop around but it does happen. The steeper the driveshaft angle, the more likely it is to wear.

Thanks. I keep learning something new everyday. The angle part of definitely makes sense.
 
And now you are introducing a 2nd drive shaft with a centering ball for the rear. Top that with a really short drive shaft.....
 
  • Like
Reactions: b0xcar
To add another data point with so many differences than most it may eliminate a bunch of possibilities.

06 LJ rubi 42rle and 241 case, still has the large damper on the back, 5.13 gears

3.5" Metalcloak springs with spacers to level the back

3link/4link suspension

TMR trans crossmember with a rubber trans mount

TMR belly up skid, only connected to the frame, doesn't touch the crossmember or trans moun

JK Rubi rear axle JK prorock 44 front (front is high pinion)

1350 based JE reel drive shafts

Jeep has had a harmonic vibration starting at about 110km/h and gets really bad at 120km/h

The vibration is there with 2 sets of 35" tires and 1 set of 37" tires. There has been no significant change in the vibration between any of those tires. I have never done anything to resolve it as I don't usually drive faster then that anyway. The amount of slop in the slip doesn't seem to have change anything and there's been no change from new till now. I have also rebuild the front shaft (Spicer joints) due to joint failure, at this time I also replaced the centering ball. No changes there ether. Front drive shaft angle is not perfect in order to compensate for caster.
 
2006 Rubicon LJ 42RLE with balancer, 3.5 Metalcloak lift, 34's, no vibes UNTIL 488 gears, vibes start around 63mph, Now front shaft disconnected and searching for an answer
 
2006 Rubicon LJ 42RLE with balancer, 3.5 Metalcloak lift, 34's, no vibes UNTIL 488 gears, vibes start around 63mph, Now front shaft disconnected and searching for an answer

The quick answer is locking front hubs if you are vibe free without front driveshaft. You can try heavier skidplate and sound deadening the tub but those are not guaranteed to make the vibes disappear completely. This is assuming your pinion angles, ujoints, and driveshafts are all in good working order without any issues.
 
  • Like
Reactions: Chris
2006 Rubicon LJ 42RLE with balancer, 3.5 Metalcloak lift, 34's, no vibes UNTIL 488 gears, vibes start around 63mph, Now front shaft disconnected and searching for an answer

Yes, as @psrivats said, the quick answer (but not cheap) is manual locking front hubs.

The long answer is that you'll have to spend countless hours and lots of money trying to figure out what's causing the harmonic vibration. I think @psrivats is onto something with his possibly, but I'm going to keep my lips sealed until Dave (the guy working on his Jeep) has some new information to report back.
 
The quick answer is locking front hubs if you are vibe free without front driveshaft. . . .

^^ This. Its not the cheapest option but it instantly resolves the issue if your vibration/harmonic disappears completely when the front driveshaft is removed. I used the Yukon YA-WU-08 kit that requires changing to the CJ 5x5.5 lug pattern. The Yukon YA-WU-07 kit utilizes the Warn hub design for the Ford Explorer and allows you to retain your TJ 5x4.5 lug pattern.

If you are thinking about a manual hub conversion, confirm that your front driveshaft is perfectly balanced and spend some time before you spend your money adjusting your front pinion angle in an effort to dial down the vibrations. The pinion angle must be equal or greater than your front driveshaft angle, not lower. At each step do a test drive to observe how the character of the vibrations has changed, if at all, and if your steering still returns to center. [As the front pinion angle is increased the caster angle will decrease.] When you are at the point where you still have "just enough" return to center that is effectively as far as you can go. If the vibrations are still there, and you are sure that your front driveshaft is balanced, then it makes sense to start giving serious consideration to manual hubs.
 
Last edited:
  • Like
Reactions: psrivats
I think @psrivats is onto something with his possibly, but I'm going to keep my lips sealed until Dave (the guy working on his Jeep) has some new information to report back.

I'm looking forward to the answer.

Carnac.jpg
 
  • Like
Reactions: Chris
With my LJ I also have gone through 2 Tom Woods front shafts and 1 rear, Countless front and rear pinion angle adjustments, 3 different wheel/tire combinations with Hunter road force balancing, I run Metalcloak double adjustable aluminum control arms along with their 3.5" lift springs. I did have to notch my stock skid plate as my front driveshaft was making contact. I had also previously removed the complete interior and installed sound deadening material in the complete tub. I have two friends that also have LJ's, 1 has slight vibes with 488's the other none but runs stock gears with a Rubi Crawler. I have never tried disconnecting the rear and only run on the front but may. Wonder if going to a Conventional Two Joint Drive Shaft would have any effect. It might be worth the investment to look into it but I am close to pulling the trigger on the manual hubs.
 
Last edited:
From Shawn Wood today @ Tom Woods Shafts-———-I had asked them if going to a Conventional Two Joint Drive Shaft might help-—--His reply-—————

I do not think a 2-joint shaft on the front will make things better. From the factory your vehicle would have run a double cardan (cv) style shaft on the front. This is necessary due to the angles, once you lift the jeep it makes it even more necessary.

What size of tires are you running? Speed could be a factor, if you are geared exceptionally low for your tire size the shaft might just be spinning too fast. Sometimes there might be an issue with whatever the drive shaft attaches to. For example, if there is a little bit of play/slop in one of your yokes then it will cause the entire drive shaft to rattle around and cause a high speed vibration. So I recommend that you give everything a shake test, scoot under your jeep and give the shaft a good hard shake side to side or up and down. Everything should feel solid, if you can feel movement then there is something loose that needs to be tightened up. I answered his questions and then-——

If it all feels tight then maybe it is worth replacing the yokes at the transfer case and differential. I have encountered it before where one of the yokes is not machined perfectly on-center.

Speed is for sure a factor here. At 60mph your shaft is spinning at about 2,900 RPM. This means that at 60 your shaft is spinning as fast as most people’s shafts are at 70. At 70 your shaft is spinning at the equivalent speed of about 82.