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Harmonic vibrations in Jeeps: A new theory (please read!)

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psrivats

psrivats

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If you were going to get new gears installed, who would you use?
John Buss at Maddie's Garage in Hillsboro. They were the ones that worked with 6states to resolve my vibes. Great shop. Very close to me and John is the only one I'll let work on my Jeep going forward.

PM me if you want to talk more, happy to share more details.
 
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psrivats

psrivats

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Nope, and they have some serious drama between the two of them on social media last I heard :ROFLMAO:
Better let them sort it out .. no point making a difficult situation more complicated.
 
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RaymondT

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It seems to point to bending ears when installing the u-joints on that inner shaft and rear pinion unless the factory tolerances are just not so good. Did the same shop/person do both of those? It is great that it's resolved.
I think this is entirely likely with a lot of these vibration issues. I have a stock 4.10 ratio which gets the shaft near 3000rpm. When guys regear to compensate for larger rubber, they too get up in this range where the vibes show up. I had both my D/S's balanced but that didn't help. When I measured runout, I found .008-.009" near the pinion on the rear shaft. Hopefully I can measure and tweak it this weekend.
 

S.McArthur

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I quit...I'm freaking done with this damn Jeep!
Yesterday, I took to Firestone just to get the print out and check thrust angle, it was out.
I take it home, use 3 different spots on each side of the axle and get this thing dead nuts on.
I get the axle centered side/side dead nuts on; not 1/16" within each other, but dead nuts center
I set pinion angle perfectly, minus 1 degree

I take test drive, it's perfect, minus a very faint vibration at 65+, I'm freaking happy as hell. I meet up for some tacos at the local Mexican joint and drive home; again, no issues.

Excitedly, I wake up and drive Jeep to work...fxxking bad vibration starting at 45/50mph... how in the hell did something get F'd up overnight while it was parked! It seriously went from nothing to "oh shit your F'd" overnight just freaking parked in the driveway....I'm so pissed.
 

AndyG

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I quit...I'm freaking done with this damn Jeep!
Yesterday, I took to Firestone just to get the print out and check thrust angle, it was out.
I take it home, use 3 different spots on each side of the axle and get this thing dead nuts on.
I get the axle centered side/side dead nuts on; not 1/16" within each other, but dead nuts center
I set pinion angle perfectly, minus 1 degree

I take test drive, it's perfect, minus a very faint vibration at 65+, I'm freaking happy as hell. I meet up for some tacos at the local Mexican joint and drive home; again, no issues.

Excitedly, I wake up and drive Jeep to work...fxxking bad vibration starting at 45/50mph... how in the hell did something get F'd up overnight while it was parked! It seriously went from nothing to "oh shit your F'd" overnight just freaking parked in the driveway....I'm so pissed.
That would almost have to be a tire.

Check your air pressure.
 

S.McArthur

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That would almost have to be a tire.

Check your air pressure.
26psi at all 4 (just now), Firestone paperwork says 33psi, I am using a cheap discount tire gauge though.
Maybe I slung a weight somewhere, but that doesn't make sense, it drove home fine and I didn't hit anything pulling into my hood.
I'm taking it to Discount tire and having them balanced, well, if at least the rear since I have spacers in the front.
 

AndyG

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Tires can do weird stuff , sperated belts, etc.

Keep us posted guy.
 

S.McArthur

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Just dropped Jeep off at discount tire, after sweet talking the guy, they are road-force balancing them for $16/tire.
He did mention whatever half ass attempt Firestone did to “balance” them was trash. Said something about counter balancing the wheel and that’s a half ass way to do it (just tossing weights on).
I also have some cupping in the center of the tread, I’m damn near tempted to just buy new tires.
 

S.McArthur

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Picked up Jeep from Discount Tire, it feels smoother and I can tell there is a difference. But...it's still vibrating, I'll pull the front DS tonight and start running it that way until I'm the rear is set.
For some reason the put 38PSI in 3 of the 4 tires, I've since dropped that to 27PSI. (17/33" Load D) I hope that is a good PSI for those tires.
 

Mike_H

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Picked up Jeep from Discount Tire, it feels smoother and I can tell there is a difference. But...it's still vibrating, I'll pull the front DS tonight and start running it that way until I'm the rear is set.
For some reason the put 38PSI in 3 of the 4 tires, I've since dropped that to 27PSI. (17/33" Load D) I hope that is a good PSI for those tires.
How big are the tires? I'm running 25 psi in my 285/70R17's and it feels really good. They are also LRD
 

S.McArthur

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Anything here stand out to anyone? I've since adjusted that thrust angle and want to go back to confirm my measurements, pretty sure the Firestone people are going to ban my use of unlimited alignments.
And to confirm; front pinion angle is the same as the driveshaft angle. Which doesn't seem to make sense to me, I would think the DS angle is going to be more than the pinion since the pinion flats are...well...flatish.

By using the same method as the rear and the sheet from Firestone (Caster 6.7/6.4), I should be able to roll my angle until its in the 5-5.5.

How big are the tires
33x12.50R17. I'll drop them to 25 and see what happens.
IMG_1114.JPG
 

bobthetj03

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Anything here stand out to anyone? I've since adjusted that thrust angle and want to go back to confirm my measurements, pretty sure the Firestone people are going to ban my use of unlimited alignments.
And to confirm; front pinion angle is the same as the driveshaft angle. Which doesn't seem to make sense to me, I would think the DS angle is going to be more than the pinion since the pinion flats are...well...flatish.

By using the same method as the rear and the sheet from Firestone (Caster 6.7/6.4), I should be able to roll my angle until its in the 5-5.5.


33x12.50R17. I'll drop them to 25 and see what happens.
View attachment 101068
Do you have a digital angle finder?
 

bobthetj03

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Yes, and a big dial manual one.
since you still have a LP D30 up front you can put the digital on one of diff flats, subtract from 90*. That will give you pinion*. Subtract 12* from that number to get your Caster*. Should be dang close to your printout #'s.
 
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bobthetj03

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And for shits-n-giggles, measure your front ds angle to see what that relationship is.
 
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S.McArthur

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Ill work on that this evening, I feel like I've done everything right, I'm in last ditch effort mode asking the dumb question publicly now. I have to be missing something somewhere.
 

bobthetj03

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Ill work on that this evening, I feel like I've done everything right, I'm in last ditch effort mode asking the dumb question publicly now. I have to be missing something somewhere.
You have no idea sir! I'm going on 3 years and still can't get them completely dialed out, and my setup is mild by comparison.