Driveline vibes/rumble on acceleration - JeepForum.com
 
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post #1 of 11 Old 09-04-2019, 11:06 PM Thread Starter
sdg3205
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Driveline vibes/rumble on acceleration

Iím running 2Ē spacers on upcountry springs. I recently added new coil isolators which probably added 1/2Ē of lift. At this point everything was fine. But, the rear axle was starting to tuck in close to the front of the rear wheel wells so I installed IRO rear lower control arms to push the axle back to centre. I also added a BDS upper arm spacer to level out the upper control arm.

Now, under harder acceleration I get a rumble, which must be vibrations. Itís smooth at any speed and under less aggressive acceleration and also smooth and quiet on deceleration.

I take it the lower rear arms has thrown off the pinion angle? The DS has pulled another 1/2Ē out of the transfer case slip yoke, so perhaps the longer driveline has a balance issue?

Anyway, any input would be great.

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post #2 of 11 Old 09-05-2019, 05:18 AM
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Definitely need to check the pinion angle. Upper arm spacers also affect the pinion angle as the upper arm is a fixed length but in a different position in its rotation. So combination of the adjustable lower arms and the upper arm spacer definitely changed your pinion angle.
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post #3 of 11 Old 09-05-2019, 11:20 AM Thread Starter
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The gap between the tire and fender in the front is 6". The gap in the rear is 6.5". I'm going to remove the lower spring isolator which should bring the gap in the rear down 1/2". I should then be able to tweak the lower arms pull the axle forward slightly. This may be all it takes. 1/2" goes a long way (thats what she said!).
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post #4 of 11 Old 09-05-2019, 03:33 PM
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Quote:
Originally Posted by sdg3205 View Post
The gap between the tire and fender in the front is 6". The gap in the rear is 6.5". I'm going to remove the lower spring isolator which should bring the gap in the rear down 1/2". I should then be able to tweak the lower arms pull the axle forward slightly. This may be all it takes. 1/2" goes a long way (thats what she said!).
Still need to measure the driveshaft angle when you are done.

Also, have to plan for the arc motion of the axle. If you center the axle in the wheel well at right height, the axle shifts rearward as suspension travels upwards. Could contact the rear of the fender lip when at compression.
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post #5 of 11 Old 09-05-2019, 03:38 PM Thread Starter
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For sure. I have a gravity angle finder. Just need to find some time.
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post #6 of 11 Old 09-05-2019, 10:59 PM Thread Starter
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Pinion angle looks to be about 10 degrees, with the DS at 14 degrees and the pinion at 4 degrees.

EDIT - I don't know why it's rotating two of my pics 90 degrees to the left, but just imagine the 2 pics below are rotated to the right.
Attached Thumbnails
IMG_2473.jpg   IMG_2472.jpg   IMG_2471.jpg  
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post #7 of 11 Old 09-06-2019, 06:57 AM
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Not sure of where the 4 degrees is being measured off of.

That 12 degrees is just your driveshaft angle relative to the ground. Pinion angle is the difference between your driveshaft angle and the mating surface of the pinion yoke.

https://www.wolferacecraft.com/pinionangle.aspx
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post #8 of 11 Old 09-06-2019, 08:20 AM Thread Starter
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Thatís correct

4 degrees is the pinion itself.

If it makes more sense to you, if the numbers are inverted 90 degrees, the DS is 77 degrees and the pinion is 87 degrees, still 10 degrees.
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post #9 of 11 Old 09-06-2019, 09:06 AM
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What's the angle at the t-case end of the driveshaft? U-joints travel in an elliptical pattern. If the angle at both ends aren't exactly opposite of each other (i.e. +10 at one end and -10 at the other, then you'll have some driveshaft vibration as the joints fight each other). I would try lengthening the lower arms a bit, to rotate the pinion down a few degrees. The closer that you get to matching the angle at the other end, the smoother it will operate.

Here's a vid that shows the effect in action:
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post #10 of 11 Old 09-06-2019, 09:46 AM
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Quote:
Originally Posted by sdg3205 View Post
Thatís correct

4 degrees is the pinion itself.

If it makes more sense to you, if the numbers are inverted 90 degrees, the DS is 77 degrees and the pinion is 87 degrees, still 10 degrees.
Ah, you don't have all of the measurements to determine where your pinion angle needs to be as HarryH3 is pointing out. For conventional, 2 joint driveshafts, you need three measurements. Transfer case output, driveshaft, and pinion yoke. Couldn't if your 4 degrees was on the transfer case or pinion.

If you have a double cardan driveshaft, you would only need the driveshaft and pinion measurements but in that case would aim for zero degree difference.

When I setup my rear driveshafts, I aim to have the pinion angle 1-2 degrees down in relation to the transfer case output. So in HarryH3's example, if the transfer case output angle is +10 degrees, then the pinion angle is -11 degrees. This is to count for axle twist under acceleration.
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post #11 of 11 Old 09-06-2019, 09:49 AM Thread Starter
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Quote:
Originally Posted by Oktavius View Post
Ah, you don't have all of the measurements to determine where your pinion angle needs to be as HarryH3 is pointing out. For conventional, 2 joint driveshafts, you need three measurements. Transfer case output, driveshaft, and pinion yoke. Couldn't if your 4 degrees was on the transfer case or pinion.

If you have a double cardan driveshaft, you would only need the driveshaft and pinion measurements but in that case would aim for zero degree difference.

When I setup my rear driveshafts, I aim to have the pinion angle 1-2 degrees down in relation to the transfer case output. So in HarryH3's example, if the transfer case output angle is +10 degrees, then the pinion angle is -11 degrees. This is to count for axle twist under acceleration.
Gotcha! i'll check that out and report back. thanks guys!
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