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O.K. Mechanical Guru's

Discussion in 'Chevy Trucks' started by repo_man62, Sep 19, 2007.

  1. repo_man62

    repo_man62 Senior Member
    Messages: 502

    My theory... if a "joint" is in a bind, even if it's new and well greased, if it's not as straight as it possibly can be, especially under torque and stress, IT WILL BREAK...right? I'm looking at this from a draftsmans perspective...not a mechanic. I know enough about mechanics to keep my truck from breaking me.
    So here's the skinny... a buddy of mine keeps breaking u-joints on his Blazer.Here's what he has done... converted from automatic tranny to 4-speed... and lifted it 3".He also added a extra leaf spring on each side in the front. He also has put(I'm waiting on the response from this) Air shocks on the front AND BACK. He DID NOT pay attention to the way he put the blocks in the rear. I THINK the rearend is tilted downward too far creating the FIRST bind. Then when he accelerates, the torque forces the rearend down and the front of the shaft upwards, thus causing it to break the u-joints. Am I right or wrong?
     
  2. mike psd

    mike psd PlowSite.com Addict
    from pa
    Messages: 1,028

    you are correct on where your going he didn't do this homework when can to the swap and the lift . home brew lifts are never good thing that why there are kits that are engineered. typical manuals are nomrally shorter then autos . also sounds like thre driveline angle aren't never correct . shims can connect the angle but if never took a measurement before you started your done unless have factory service manual . also i'm guessing its a 4x4 does he have a fix yoke t-case or slip yoke . slip yokes can never run steep angles vs fix yokes . crappy u joints fail like butter in cases like this . also sounds like having some axle wrap issue aswell
     
  3. ght1098

    ght1098 Member
    Messages: 86

    I don't think that he is out of luck not knowing the measurements. The angles of the transmission and the differential need to match. if the tranny is at 90 deg to the ground, then the diff needs to be the same or the drive shaft will bind. Same goes that if the tranny output is -1deg the diff needs to be +1deg. It doesn't matter what the angle is (within reason) as long as they match.
     
  4. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    I'd bet it's binding the U-joint during braking, not during acceleration if the lift blocks are in backward. Like you said Bob, if the blocks are in backward, the pinion will be pointing more toward the ground at rest than it should be. Now, during a brake application the pinion will rotate further down,(pinion rotates down during braking, not acceleration) thus binding the joint even more. Now on many vehicles, this wouldn't be enough to cause enough of a bind to physically break the joint but on a short wheelbase Blazer with a 4-speed, the drive shaft is pretty short so it just compounds the problem.
     
  5. Wicked500R

    Wicked500R Senior Member
    Messages: 394

    That is wrong...I have built quite a few trucks in my day...I have always pointed my diff yokes directly at the transfer case. If both the tranny and diff are parallel to the road, Thats asking for trouble, and on a lifted vehicle, They will bind when the suspension flexes and most likely break. You want the driveshaft to go straight into the differential, you don't want any angles down there....regardless of how the tranny is...
     
    Last edited: Sep 24, 2007
  6. snowpoe

    snowpoe Member
    Messages: 41

    I have issuse on my 90 blazer from doing a shackle flip in the back to gain 4" lift

    It has been a while since I read up on this on coloradok5 so I may be wrong

    I think the only time you want the pinion yoke facing the transfer case yoke is if you have a double carden joint on the case side

    I think with the stock single joint shaft that the angles need to be near the same at both joints
    But the pinion yoke side needs to be down 1-2 degrees at rest so when you are driving it will then be the same from the tourque from driving down the road & slight axle rap

    If they are off there will be vibration at speed

    You have to get a degree angle tool & measure from I think the frame or the ground & then measure the yokes at the t-case & the pinion & make the differance up buy buying a degree shim & installing it in the under the leaf pack

    I lowered my t-case cross member & I needed a 8 degree shim which I have not put in yet.
    I got a steel one with a hole in it that is held on by the leaf pack pin (so it does not come out) I think I got it on ebay

    he better do a search & read up