Rear pinion UP!

Discussion in '80-Series Tech' started by -Spike-, Aug 18, 2006.

  1. -Spike-

    -Spike-

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    As some of you know, I recently damaged my rear driveshaft while wheeling. I decided to raise the rear pinion, putting my driveshaft in a 'broken back' configuration- both u-joints angling down from the front of the truck to the rear axle, instead of the t-case output shaft and pinion shaft being parallel as is normal. This would raise the pinion end of the driveshaft, hopefully keeping it off the rocks.

    I have Slee's 6" lift, which comes with adjustable rear upper control arms, making this mod fairly easy.

    I first removed the damaged driveshaft and had it re-tubed with a .156" wall tube, and balanced. The stock(?) driveshaft was .107" wall. I then measured between the t-case flange and the rear pinion flange and cut a piece of 1/2" square tube slightly longer than that measurement at 39". I placed this straight edge to span across the top of both flanges, simulating the driveshaft but easier to measure angles from. I used an angle finder similar to this one http://images.rockler.com/rockler/images/36915-01-500.jpg to find the relative angles between the flanges and the straight edge. I then cranked the upper control arm adjusters (lengthening the arms) to raise the pinion until I got matching angles at both flanges. This took some trial and error, as both angles change as you raise the pinion. I then dialed the pinion down just a bit to compensate for the fact that it would normally raise under power due to axle wrap. Then I installed the driveshaft and test drove it on both city streets and on the highway up to 80 MPH. I got no vibrations or noise at full throttle, cruising, off the gas or engine braking.

    My rear t-case output shaft is at 4 degrees from level, the driveshaft is at 10 degrees, and the rear pinion shaft is at 15 degrees, indicating that I have a 6 degree angle in the front u-joint and 5 degrees in the rear u-joint. I estimate them to equal out under power- pure theory here. I have no idea if the 15 degrees at the rear pinion is going to be a problem- ideally it was at 4 degrees originally with parallel t-case and pinion shafts. If you see me post in the future for help on replacing pinion bearings, you'll know I had issues. I will be looking into an oil slinger setup (that Tools R Us mentioned is in the front diff, and I'm hoping can be fitted into the rear) if the bearing goes out or I get around to re-gearing, whichever comes first. By my calculations, the u-joint angles are less than they were with the 6" lift (approximately 7 or 8 degrees), but my wife's stock 80 seems to have even less of an angle at the u-joints at around 3 degrees. Even 8 degrees is no where near the operating limit, so all that is probably not relevant.

    I figure I gained about 2-3 inches of clearance at the rear pinion flange.

    I have no idea if anyone else has done this, if so please post up. The driveshaft guy agreed that this was the way to go for rock crawling rigs- apparently he's seen a few trashed driveshafts- and indicated that it was a regular practice on 4 wheelers. He didn't think it would be a problem. I asked him if the u-joint yokes should be in-phase or out of phase and he said that the way I had them was the correct orientation- what I assume is in-phase, with the yokes in line with each other. (I have a hard time remembering which is which, and I don't know the complete history of my rig so I don't know if the driveshaft was factory correct when I got it).

    -Spike
     
  2. Tools R Us

    Tools R Us

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    Good deal! :cheers:

    I have never seen a good argument for why the stock front is out of phase. The vibration modeling software the I played with predicted it would vibrate more out of phase. It probably has to do with harmonics unique to that setup?
     
  3. Tapage

    Tapage Club 4X4 Panamá SILVER Star

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    Not the same overall setup .. coz I use springs .. but I do that in my 60 series .. So a buddy need to use a CV rear to do the same thing as you .. coz it have a lots of vibration on the HWay ..
     
  4. Nay

    Nay

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    Very cool - good solution. I think you will be ok @ 14 degrees. What you can do is park the rig on a slight downhill and then add some gear oil just to increase capacity a bit.

    By lessening your actual u-joint angles the joints will be stronger as well. U-joint wear does increase once you are past 3 degrees, but 6 is nothing.

    Setting 1 degree low on the pinion end is a typical solution - I don't think it makes any difference because a degree is within tolerance either way, and a four link should have no axle wrap under normal conditions unless you have worn bushings.

    Nay
     
  5. Tools R Us

    Tools R Us

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    I have found newer bushings to flex more than older still intact ones, they do wrap more than you would think, big bushings.
     
  6. -Spike-

    -Spike-

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    A follow up: Took the truck on the same trail I trashed the driveshaft on. I didn't hit it on the obstacle this time, but the other 5 trucks did. Yeah, 3 80's, one 4Runner, and one Land Rover, all with candy stripes and dents on their rear driveshafts. They named the obstacle Barber Pole Rock.

    Thinking back, I think one of the 80's avoided damage.

    On the bad side, there's a slight noise and vibration on both acceleration and deceleration above 30 MPH. I'm going to try two things to correct this: One, rotate the pinion up the final degree to match both u-joint angles at rest, and two, if that doesn't do the trick I will have the driveshaft re-balanced with the u-joints out-of-phase. I'm wondering if the stock front driveshaft at stock height isn't out-of-phase because it is set up in the broken-back configuration from the factory- I will be measuring my wife's tomorrow to confirm or deny this theory. Just thinking logically about it, if the pinion is pointing at the t-case with a 6 inch lift and caster correction, it must have been pointing above the t-case at stock height with correct caster. Hopefully I'll know tomorrow, if I decide to go out to the shop and do some measuring.

    -Spike
     
  7. bhmmapping

    bhmmapping

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    bringing this back to life to see how it has held up, anyone else done this?
     
  8. -Spike-

    -Spike-

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    Absolutely no issues in ~4 years. I believe the vibe was in the front shaft, which I replaced back then.
     
  9. TRDCRUISER95

    TRDCRUISER95

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    Can you put some pics ? Thanks man!
     
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