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Why should you not point the pinion up on the rear differential?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by PetaKane, Nov 13, 2002.

  1. PetaKane

    PetaKane 1/2 ton status

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    Faster u-joint where? I need to point mine up temporarily till I save up enough for a CV shaft. I'm workin with 10" susp lift and 400/208 tranny. Before I had a 700r4/208 and driveline was just fine. After I installed the 400, I had to get the shaft lengthened. I thought that having a longer driveline would help out but it hasn't. Should I try 6* shims to point the pinion up?
     
  2. MJF

    MJF 1/2 ton status

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    If you have a cv shaft, the pinion should be pointed at the tcase. If you have the regular shaft, it should be parallel to the output shaft on the tcase. I think it's supposed to be within 3 degrees or something like that.

    I don't really understand the logic here. It seems like the best situation either way would be to point the pinion up.
     
  3. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> The idea is that the u-joints cancel the vibes out of each other when they are not running straight. Since a CV is essentially two u-joints that split the difference of the angle, the third joint, which is at the axle, needs to be straight wo it won't vibrate. With a regular shaft there is only one joint at each end so the joints must run at equal but opposite angles.

    Running the pinion joint straight without a CV shaft will likely produce horrendous vibes and the joint will die quickly. </font color>
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    </font><blockquote><font class="small">In reply to:</font><hr />
    Running the pinion joint straight without a CV shaft will likely produce horrendous vibes and the joint will die quickly.

    [/ QUOTE ]

    You'd think so...my pinion is about 1 or 2 degrees from pointing directly at the T-case, standard driveshaft, and not only do I not get any vibes but my u-joints last 2+ years before needing replacement. I do not have a 10" lift though...only 4" - 4.5". I just set up my buddy's 89 Blazer the same way with a 4" lift...again no vibes. I've had his truck up to 80 mph too. That truck has a 700R4/241 combo with a slip yoke and the T-case dropped 1".

    I do know what the theory says...but I also know what works in the real world./forums/images/graemlins/smile.gif

    Rene
     
  5. SkulzNBonz

    SkulzNBonz 1/2 ton status

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    I'm with tRusty on this one. My pinion is within a couple of degrees of pointing directly at the t-case. Granted, it's a shackle flip w/ a lowered t-case. Prior to lowering the t-case it vibrated like mad. I get a slight vibe from 35-45 but that's it. My u joint's have lasted the 9+ months since I lifted the truck, which sees DD duties as well as road trips and some pretty hard wheelin'.

    John
     
  6. imiceman44

    imiceman44 1 ton status

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    Well trusty, if you look at the angles at the Tcase, and compare it to the angle at the pinion, if you have only a 4" lift, you will discover that it's not more than a couple degrees off, which would be the eason you're not having problems.
    He has much more lift, and believe me it will self destruct in a short while.
    and you were saying 2 years on your setup.
    Mine was the perfect setup, 2 degrees down from parallel, so that when you put the load on, the pinion lifts up and becomes parallel, after 6 years, the U joints were still like new, and I beat the hell out of my truck.
    It all depends on what you will live with.
     
  7. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> The angles on my front shaft are way off from "by the book" and it does fine, but the rear is by the book and my joints are 3 years old and still going strong. I understand that real life doesn't always work like the theory says it does, maybe you'll luck out and be OK with the angles out of whack until you can get the CV shaft. </font color>
     
  8. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I have about 7 or 8 degrees at the T-case and close to zero at the diff. My point was only that the theory is just that...a theory. Real world can be much different. I would not attempt anything similar with a 10" lift /forums/images/graemlins/eek.gif /forums/images/graemlins/eek.gif

    Rene
     
  9. Mudzer

    Mudzer 1/2 ton status Author

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    This is a different scenerio, but same theroy in my opinion. In the Drag Car world, when designing the rear suspension, a common number is 2 degrees "down". The reason why - when the driveshaft turns the pinion, the ring gear rotates one direction, where the housing natuarally opposes and wants to turn in the opposite direction. This makes your pinion rotate upward. If you have your driveshaft "straight" with the pinion when this happens, you will stress the joint much quicker. If you have a negative pinion angle (or in our case negative to the pinion centerline) the driveshaft and joint should be in line when under a load at speed.
     
  10. K05Aggie

    K05Aggie 1/2 ton status

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    On my truck the pinion is pointed almost straight at the transfer case and I have pretty bad vibrations from about 25mph+. No lift, standard driveshaft.
     

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