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Rear driveline knocking under load

mpv99tj

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May 4, 2021
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11
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Central Kentucky
A little history: Stock height no sounds. Had a 3.15in RC & 33x12.5's lift installed and stared getting some knocking in the rear in slight inclines (my driveway has about a 30 degree incline, knocks) under power was fine on the road even up hill. I was told I had a lower spring cup rusted through and needed to be welded (stock Dana 35), didn't mind as I was upgrading soon to 4.10's from 3.07's. Fast forward, 8.8 rear and upgraded Dana 30 front installed and now still have the knocking and it seems worse and now also have a vibration that comes in about 40mph and up. No SYE yet but planned soon. I've been reading a lot about driveline angles and wonder if that is the problem. Stock Dana 35 should have been fine as I understand it with the 3.25in Lift and the 33x12.25's. Was not expecting the knock. I believe that the shop that installed the axles set the angles up wrong, appears to be set up for SYE when there is none. Comments? Advice? See pic below of current setup.

20220503_174648.jpg
 

flyinfish

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Aug 31, 2019
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906
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Oregon
Without a double cardin style drive shaft you don't wan't your pinion pointed at the output shaft on transfer case.
You should get a SYE and a double cardin style drive shaft from Tom Woods or other reputable driveshaft company.
 
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mpv99tj

mpv99tj

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May 4, 2021
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Central Kentucky
That's what I was thinking too, shop set it up wrong. Just wanted more input before I changed it. Just received my SYE, will be ordering DC driveshaft soon.
 

flyinfish

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Until the new shaft you need to lower your pinion back toward horizontal so the output shaft and pinion are parallel.
 

flyinfish

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Install the SYE and then you can aim the pinion at it and measure. Then you can call someone like Tom Woods with a correct measurement. Make sure your rear end housing is centered on the bump stops so you know your axle is located correctly.

edit: Make sure they know you have a flanged pinion versus a yoked pinion so you get to correct end.
 
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Shawn at Tom Wood's

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Ogden, UT
The knocking could be because the angles are wrong but it could be something else entirely. Usually bad angles on a drive shaft will cause a "shudder". The shudder is essentially caused by the u-joint oscillating and the speed of the drive shaft constantly changing slightly as it rotates. If that shudder is shuddering against some loose parts or some excessive slop in the slip yoke I could see how that could cause a knock.

Not to pick apart the shaft in the photo but I see where a new weld yoke has been welded in one end but I do not see any new balance weights. Though they could be on the back side of the shaft where I can't see. I also see a stock balance weight still on the tube. This leads me to believe that the shaft was not re-balanced after being modified which would certainly cause some of the high speed vibrations you are getting above 40mph.

I think you've probably got a multifaceted issue. A little bit of bad angle, a little bit of bad balance. You can read more about how to diagnose some of these things here https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations?_pos=1&_sid=d6ec76a03&_ss=r.

I agree with everyone who's told you that you should do an SYE and new drive shaft but in the meantime you might want to try rotating the pinion down a few degrees. 3-4 or so but it almost doesn't matter exactly how much. If bringing the pinion down helps, or rather if it affects the knocking at all by making it better or worse, you can be confident you are not barking up the wrong tree and that your issue is most likely angle related. If it doesn't have any affect on the knocking, you may want to look closely at some other peripheral things. I hope/assume you have adjustable upper control arms as I can see that the lower control arms are still stock. If not, you may be wasting a couple hundred bucks on adjustable control arms to see what happens when you bring the pinion down and would be better off forgetting about pinion adjustment and just doing the SYE and double cardan shaft.
 
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mpv99tj

mpv99tj

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May 4, 2021
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11
Location
Central Kentucky
Thanks for for everyone's input. As it turns out lower control arm bushings are worn out (the only ones I haven't replaced) new Core 4x4 ordered. Exhaust was causing all the knocking. Somehow it broke loose. Driveshaft seems to have no problems. However I have a TeraFelex short SYE and a Tom Woods Driveshaft on the way.