Drive Line Vibration Driving Me Crazy

[USAF_XJ]

So I have a rhythmic drive line vibration above 70. It all started after swapping 4.56 gears. I currently have a long arm lift of 5.5", SYE, and a Ford 8.8. Everything ran smooth and silky until I got around to swapping in the new gears. I have since gone through 3 drive shafts trying to resolve this issue. First was the stock and that was awful, vibration came in at 45. Then swapped to Adam's drive shafts front and rear and made a good improvement but shifted the vibration to 65-70. Had a local shop balance them and it made a slight improvement. Sold those and went with Tom Wood's drive shafts which I still currently have on. The Tom Woods so far have been the best but the rhythmic vibration is still there above 70. Now I know my lifted jeep really has no right going above 70 but speed limits here are 75 and when I just barley bump over 70 the vibes come, so its very annoying just cruising or passing. I'm starting to wonder if my pinion angles are off so I have attached some pictures of those. Could I simply need to have the Tom Wood's shafts balanced again?

 

[USAF_XJ]

Better photos of the front.

 

[RCP Phx]

Well the rear pinion is high so try moving it down a few degrees.

 

[maxbraketorque]

I agree that the rear pinion angle is high. The rear pinion should be 1-2 deg below the rear driveshaft angle.

Also, if you're unsure of whether its the front or rear driveshaft, its ok to remove the front drive shaft when driving around in 2WD.

 

[Runnin'OnEmpty]

I concur with the too high rear pinion
angle. The slip joints look too clean lol,
are they filled with grease...?

 

[USAF_XJ]

I removed the front shaft and it was really smooth all the way through 80 so its definitely coming from the front.

 

[maxbraketorque]

Cool. Still should address the rear pinion angle. As the pinion shaft points more and more upward, the passenger side rear shock effectively gives up more and more travel, so it bottoms out sooner.

 

[USAF_XJ]

So consensus is that the rear needs to be lowered a bit but the front is fine, so maybe take this front drive shaft to a shop and see if its out of balance?

 

[Runnin'OnEmpty]

Make sure both driveshafts are in-phase,
meaning the u-joints are perfectly parallel
with each other.

You're probably already aware of this, but
I thought it important enough to mention.

 

[RCP Phx]

What transfer case do you have and did you recalibrate (accurately) the speedometer? It could also be in your gear install!

 

[maxbraketorque]

So consensus is that the rear needs to be lowered a bit but the front is fine, so maybe take this front drive shaft to a shop and see if its out of balance?

If you didn't change the speedo gears when you did the R&P swap, then indicated vehicle speed vs driveshaft rpm doesn't change, and if the front driveshaft didn't vibrate at 80 mph before the R&P swap, then it should't vibrate now. If this is the case, then I'm thinking there is something funny with the yoke on the pinion shaft.

However, if you did a speedo gear swap, and if you went from 3.55 to 4.56 gears, then the driveshaft is spinning ~30% faster for a given actual vehicle speed. Drive shaft rpms at a true 80 mph with 4.56 would be the same as at a true 100 mph with 3.55. In this scenario, its plausible that a sufficient level of front drive shaft balance with 3.55 gears is not sufficient for the 4.56 gears.

 

[USAF_XJ]

If you didn't change the speedo gears when you did the R&P swap, then indicated vehicle speed vs driveshaft rpm doesn't change, and if the front driveshaft didn't vibrate at 80 mph before the R&P swap, then it should't vibrate now. If this is the case, then I'm thinking there is something funny with the yoke on the pinion shaft.

However, if you did a speedo gear swap, and if you went from 3.55 to 4.56 gears, then the driveshaft is spinning ~30% faster for a given actual vehicle speed. Drive shaft rpms at a true 80 mph with 4.56 would be the same as at a true 100 mph with 3.55. In this scenario, its plausible that a sufficient level of front drive shaft balance with 3.55 gears is not sufficient for the 4.56 gears.

Speedo gear has been corrected for the current tire and gear setup and has been verified with a GPS. I'll double check the phasing (just to cover all my bases) then I will have it rebalanced if that's not it. If its not a balancing issue I am at a complete loss. The rear is a Tom Woods too and it drive smooth when to the front is disconnected so its balanced correctly not sure why the front would be off since it came from the same place.

 

[RCP Phx]

It could also be in your gear install!

Seriously think about this also!!!!

 

[USAF_XJ]

Seriously think about this also!!!!

I did think about that when this all first started so I took it to a different shop to check the work of the first and they said they were fine and suggested Tom Woods vs Adams driveshaft. I also figured I've got about 5000 miles on these gears and if they were wrong they would have blown by now. I even replaced wheel bearings even though they were fine to begin with.

 

[maxbraketorque]

Have the front drive shaft balance checked again. I'd call Tom Woods. Both my driveshafts are from them, and they were great to work with. I suspect they'll check balance for free and might even cover shipping both ways.

 

[RCP Phx]

I did think about that when this all first started so I took it to a different shop to check the work of the first and they said they were fine and suggested Tom Woods vs Adams driveshaft. I also figured I've got about 5000 miles on these gears and if they were wrong they would have blown by now. I even replaced wheel bearings even though they were fine to begin with.

I was not thinking of the gears themselves but the Chinese bearings that most kits use!

 

[RCP Phx]

You never did say what transfer case? It might also be a front output shaft bearing (or related).

 

[USAF_XJ]

You never did say what transfer case? It might also be a front output shaft bearing (or related).

Transfer case is the stock NP231 with an advanced adapter SYE.

 

[SlowXj]

You can remove each driveshaft individually and run it down the road to isolate which shaft is causing the problem.

Do you have any kind of tcase drop?

What was said above about rear pinion angle is accurate but probably not your main issue

Weird question but how much caster are you running? May be able to take some caster out to improve front driveline angle.

Sent from my SM-S901U using Tapatalk

 

[Anak]

Front output is worth considering.

Given the problem persists across different driveshafts I would be looking for something other than a driveshaft as the source.