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  #1  
Old October 21st, 2015, 06:35 PM
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driveline angles question

Attached is a picture representing the angles on my truck.

The front works well. I'm happy with it.

After some recent work however, the rear has developed a low speed vibe. It's worst at slow speeds, improving steadily up to 35mph, after which it is pretty much undetectable.

1. My engine driveline is not level to the ground. It's angled down towards the rear diff at exactly 2 degrees.

2. My rear pinion is angled up at exactly 4 degrees.

3. The rear driveshaft (stock) is at angle of exactly 20 degrees positive from horizontal (as measured from the rear pinion)

Does anyone have any precise technical recommendations on what to do about these three measurements?

Is the engine driveline supposed to be perfectly horizontal?
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  #2  
Old October 21st, 2015, 06:38 PM
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Are your trans mounts shot?
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  #3  
Old October 21st, 2015, 06:43 PM
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Quote:
Originally Posted by grnrvrs View Post
Are your trans mounts shot?
I don't believe they are. There are two. Both are composed of two rubber pucks sandwiched together, along with a threaded rod connecting the transfer case to the frame.

Both appear fine and of standard dimensions. Not much to go wrong with them, I presume.

BTW the picture of the engine is a Duraqtorq w/ MT82, but I'm running a 200tdi with an LT77.
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  #4  
Old October 21st, 2015, 06:48 PM
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What was the "recent work"?
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  #5  
Old October 21st, 2015, 06:58 PM
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Terrafirma inverted relocation cones installed.
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  #6  
Old October 21st, 2015, 07:00 PM
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FWIW - When I installed a +2" on my 90 I immediately had a noticeable increase in front vibration not only under the front but has the whole dash vibrating more than "normal" - All at or below 20MPH. Also noticed my RPM's where running higher. Changed back to stock height and extra vibration gone and RPM back to ideal.
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Old October 21st, 2015, 07:03 PM
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Quote:
Originally Posted by sonoronos View Post
I don't believe they are. There are two. Both are composed of two rubber pucks sandwiched together, along with a threaded rod connecting the transfer case to the frame.

Both appear fine and of standard dimensions. Not much to go wrong with them, I presume.

BTW the picture of the engine is a Duraqtorq w/ MT82, but I'm running a 200tdi with an LT77.
On my series rig, the rubber on my trans mounts deteriorated to the point of collapse, which changes the prop shaft angles and introduced vibes. It was easy to see/fix.

Assuming the chassis side of your engine mounts are stock, as opposed to 'custom' welded mounts, collapsed trans mounts could explain why it tips rearward.

x2 on the recent work q.
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  #8  
Old October 21st, 2015, 07:38 PM
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You could add some washers to the trailing arms to rotate the rear pinion down to be level with the TC.
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  #9  
Old October 21st, 2015, 07:50 PM
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Quote:
Originally Posted by fishEH View Post
You could add some washers to the trailing arms to rotate the rear pinion down to be level with the TC.
^ this


But check your mounts first. Get in the with a crowbar and see if you can see them move. I had one separate a while back and it looked fine from the outside but was very much not OK.

Do the U-joints have any play?
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  #10  
Old October 21st, 2015, 08:56 PM
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I have found this info drive lines from Tom a Woods to be suited to this.http://www.4xshaft.com/

------ Follow up post added October 21st, 2015 06:00 PM ------

I just read my post. Boy did I mix up words and add some. Sorry. Wine just kicked in. Still good info to follow on geometry.
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  #11  
Old October 21st, 2015, 10:21 PM
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I will check all of the above, thanks.

One important question: if the transfer case mounts are brand new, should the engine drive line angle be perfectly horizontal??
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  #12  
Old October 21st, 2015, 10:27 PM
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No. Typical is 2-4 deg.
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  #13  
Old October 22nd, 2015, 07:34 AM
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X2 on the tcase mounts. I had one fail about a year ago. My vibes are back so time to recheck.
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  #14  
Old October 22nd, 2015, 07:41 AM
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Quote:
Originally Posted by fishEH View Post
You could add some washers to the trailing arms to rotate the rear pinion down to be level with the TC.
If the drive line angle is below the horizontal by 2 degrees, what should the pinion angle be? Some reading shows me that the pinion should then be 0-2 degrees up from the horizontal. But I have no idea if that is really true.
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  #15  
Old October 22nd, 2015, 09:28 AM
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The two flanges should be parallel, but a few degrees is not going to be causing you vibes. You need to look elsewhere for the problem.
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  #16  
Old October 22nd, 2015, 09:29 AM
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There are two trains of thought. First, equal and opposing angles with a stock driveshaft. The reason is the u-joint actually speeds up and slows down when the rotating angle is anything but 0. By making angles at the TC and pinion this rotational speed is equalized reducing wear and vibration. The other approach is using a double cardon joint at the TC and pointing the pinion to create a 0 degree angle to the driveshaft. This works where the rear axle cannot rotate in acceleration/deceleration. If you were using leaf springs that pinion angle is usually slightly upwards a degree or two the account for that rotational force. This a very simplified explanation but it will work for you.
Side note, most drivelines have some angle difference to promote u-joint lubrication. In the general operation there should be enough suspension deflection to cause angle variation to also promote this lubrication.
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  #17  
Old October 22nd, 2015, 09:50 AM
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  #18  
Old October 22nd, 2015, 09:56 AM
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You shouldn't need a rear DC driveshaft if you're running 3" of lift or less. The stock two ujoint driveshaft should be sufficient provided the pinion and transfercase angles are matching.
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  #19  
Old October 22nd, 2015, 10:34 AM
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Tcase slop I believe can be a culprit as well.
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  #20  
Old October 22nd, 2015, 11:29 AM
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Sonoronos, what is the condition of your rear a-frame ball joint and bushings?
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