Bent belly skid and driveline vibrations


srimes

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Howdy, new guy from Oregon here with my first tj. Had an xj before that I mildly buit. Didn't want a "project" jeep so I bought one that was already built mostly how I like, but of course nothing's perfect.

It drives pretty well for 35s and a 6" suspension lift. I've had it at 70 with no drama, except for periodic vibes that become noticeable over 55mph. It feels just like the driveshaft vibes I got in my xj when I lifted it, (which I solved with a h+t sye and angle shims under the leaves). Not really uncomfortable, but noticeable and it does shake the mirror. It makes me uncomfortable because it sounds like somethings wrong and I'm worried about premature wear leading to failure.

The belly skid is well used. Looks like 1" raise at the trans mount.
20210519_162246.jpg


I measured the pinion vs driveshaft as best I could with an angle app on my phone, and it looks like they're off 5-6 degrees. Enough to see with the naked eye.
20210519_162806.jpg



Here are the options I'm considering:
1)Ignore the vibes for a while as I get to know the jeep. They won't hurt anything.
2) Leave the stock skid for now, just adjust the pinion angle.
3) Remove and straighten the stock skid, then adjust the pinion angle.
4) hurry up and get an aftermarket high-clearance skid and all that involves.

What do yall say?
 

eastbloc

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Welcome to the vibe club!

Pinion angle is pinion angle, before or after is your call. But check trany mount to see if it got damaged with skid plate.
If pinion angle withing specs, we typically advise people to remove one of the ds at a time to single out a possible ds problem.
Sometimes one ds at a time is a smooth ride, but both in and vibrator is on.

But first put it into 4hi, and go for a highway drive with both ds in, see if vibes go away.
Pulsating vibes starting at 55mph sound like vibes most of us got after the regear of axles, and it goes away in 4Hi if ds is not an issue. It is a mystery of mysteries, and a money pit to find the cause.

Never hurts to check tire balance.
Check yoke nuts on transfer case for looseness .
 
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AndyG

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Welcome to the vibe club!

Pinion angle is pinion angle, before or after is your call. But check trany mount to see if it got damaged with skid plate.
If pinion angle withing specs, we typically advise people to remove one of the ds at a time to single out a possible ds problem.
Sometimes one ds at a time is a smooth ride, but both in and vibrator is on.

But first put it into 4hi, and go for a highway drive with both ds in, see if vibes go away.
Pulsating vibes starting at 55mph sound like vibes most of us got after the regear of axles, and it goes away in 4Hi if ds is not an issue. It is a mystery of mysteries, and a money pit to find the cause.

Never hurts to check tire balance.
Check yoke nuts on transfer case for looseness .
Pulling the front shaft can isolate the rear also, and remember...vibes ultimately can wreck parts. Chase ‘em.
 
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hosejockey61

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Your skid is the very likely culprit as it raises the output shaft. Happens to me all the time. Go wheeling, beat the shit out of it and get vibes. Go home, pull the skid off and flatten back out by driving over it with a truck. Reinstall and repeat. Mines a rokmen steel skid. The Savvy aluminum is better built but comes at a higher price.
 

Jerry Bransford

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Your pinion angle is obviously too low given your driveshaft angle, it must be at the same angle as the driveshaft.

Obviously the skid plate is bent excessively which has raised the rear of the drivetrain way up. That might be why the pinion angle is looking too low at this point.

I'd replace the skidplate with a straight one and see how the pinion angle looks afterward.

This is how your pinion/driveshaft relationship must look when you're done fixing things.

cv_angle_large.gif
 

pagrey

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I'd bash the skid back, mine looked like that every season until I got a real one. You'll probably laugh at how easy it is, at least the newer ones are pretty weak. Most aftermarket skids will tuck slightly or allot making things worse, that's probably not the first step you want to take. Stock used might be a good option if you don't want to sling the sledge.
 

Jerry Bransford

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I'd bash the skid back, mine looked like that every season until I got a real one. You'll probably laugh at how easy it is, at least the newer ones are pretty weak.
I used the weight of the rear of the jeep to straighten a used flat(er) transmission skidplate I got pre-bent from Gerald Lee at savvy. But it took putting all the weight onto one small section at a time via a single big socket to get it to move. That bend sure didn't move by just putting the weight of the Jeep via a tire though. I think I have a photo of what finally worked to get it straight somewhere on my computer I'll try to post Thursday.
 
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pagrey

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I used the weight of the rear of the jeep to straighten a used flat(er) transmission skidplate I got pre-bent from Gerald Lee at savvy. But it took putting all the weight onto one small section at a time via a single big socket to get it to move. That bend sure didn't move by just putting the weight of the Jeep via a tire though. I think I have a photo of what finally worked to get it straight somewhere on my computer I'll try to post Thursday.
My 04 bent easily with a 7lb sledge, some heat from a torch and quite a few well placed hits. I've done it a handful of times. I guess it's probably best to say try it and if it doesn't work look for a new one. Sounds like yours wasn't easy. I also think the impact of a hammer might work better than simply putting dead weight on the skid? I feel like I could have driven all over the skid with a 2 ton rig and done nothing to the skid unless I dropped it hard from a jack or something.
 

Jerry Bransford

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My 04 bent easily with a 7lb sledge, some heat from a torch and quite a few well placed hits. I've done it a handful of times. I guess it's probably best to say try it and if it doesn't work look for a new one. Sounds like yours wasn't easy. I also think the impact of a hammer might work better than simply putting dead weight on the skid? I feel like I could have driven all over the skid with a 2 ton rig and done nothing to the skid unless I dropped it hard from a jack or something.
Mine didn't budge until I focused all the TJ's rear weight onto 1-2 square inches at a time via a big 1/2" drive socket. The hydraulic bottle jack I used allowed precise control of the weight, it ended up perfectly straight with no BFH or sledgehammer needed.
 
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hosejockey61

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I had a dodge truck with a Cummins drive over it. Put one side of the skid up on the curb. I put a 4x4 under it in case it was going to try to bend too far back but it does it perfectly.
 
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srimes

srimes

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Thanks yall. I do have a bfh but no torch. No votes for just adjusting the pinion angle? I'd like a higher clearance skid at some point (thinking barnes 4wd) and it seems like this would be a free "test" to see if it my driveshaft can handle it. I found posts about 4" lift with a belly up but not 6. Just want to make sure I'm not near the limits of the cv joint with the stock wheelbase.
 
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srimes

srimes

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My guess is driveline bind will become an issue at full droop. I suppose you can test it.

What's the best way to test? Lift the rear until both wheels are off the ground and turn the driveshaft by hand? If only I had a 2 post lift or fork truck. I doubt my farm jack has enough travel and even if it did I know the wicked thing would take the opportunity to try and kill me.
Come-along the rear bumper to a tree branch? Should be stable at least...

Maybe if I take it to les schwab for a tire rotation/parking break adjustment they'll let me sneak back and check it out while it's on the lift
 

hosejockey61

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What's the best way to test? Lift the rear until both wheels are off the ground and turn the driveshaft by hand? If only I had a 2 post lift or fork truck. I doubt my farm jack has enough travel and even if it did I know the wicked thing would take the opportunity to try and kill me.
Come-along the rear bumper to a tree branch? Should be stable at least...

Maybe if I take it to les schwab for a tire rotation/parking break adjustment they'll let me sneak back and check it out while it's on the lift
That's exactly it. Jack up the rear end until the axle hangs free. The rotate the driveshaft and feel off it binds at all.
 
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srimes

srimes

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I took the jeep to get the rear breaks adjusted, thinking I'd also be able to check for bind a full droop. Unfortunately the shop didn't have a frame lift available. The guy looked at the DC joint and said it's binding where is now and that I should get spacers to lower the skid plate like most lift kits have. Nice guy and the breaks were better after adjustment but that's not what I'm looking to do with the skid plate.

So I decided to try checking it myself using the mostly useless, always dangerous farm jack. Chocked the front tires and lifted from the receiver until I could ture both rear tires by hand. On drug on the gravel but still turned. I didn't want to go any higher than necessary less it tip over.

I took video while I checked:

I was able to turn it, so I guess it isn't binding even though it looks super tight. Plus when I rotate the pinion up to dial out the vibes the driveshaft angle will reduce a little bit.

So I should be good to run the skid as is for now, until I got for a high-clearance replacement. No need to lower transfer case.
 

Jerry Bransford

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Mine didn't budge until I focused all the TJ's rear weight onto 1-2 square inches at a time via a big 1/2" drive socket. The hydraulic bottle jack I used allowed precise control of the weight, it ended up perfectly straight with no BFH or sledgehammer needed.
I just happened to run across the photos of how I did the above. These were taken after it was straight, it had about a 3" bend when I got it.

DSCN0510.JPG
DSCN0511.JPG
 
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srimes

srimes

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I adjusted the pinion angle. Best I can measure it's about 0.8*.

1623251334769.png


Vibes are the same. I rotated the tires and no change there.

Vibes are definitely more pronounced under load. Going up hill the vibes are much worse than coasting down hill at the same speed.

In 4wd the vibes are less. Not sure what to make of that. May be reduced load on the rear driveshaft.

I guess next step is the pull the rear driveshaft to confirm that's the problem and inspect as best I can.
 
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