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Shaky Front End in 4WD While Turning

Discussion in 'The Garage' started by bpiccioni, Mar 16, 2003.

  1. bpiccioni

    bpiccioni 1/2 ton status

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    Hey Everyone

    I have an 87 /forums/images/graemlins/burb.gif 3/4T 350TBI/TH400/208/14BSF/GM10B. When I put it in 4WD, Hi & Lo, and then turn the wheel to near lock, I experience a heavy lurching from teh front end. This seems to be more pronounced in 4Lo than 4Hi, but happens in either range. Much worse on pavement than off road.

    I actually got out and observed this while my brother was doing some sharp turns in a parking lot (walking speed) and it appears that the wheel on the inner side of the turn (example - left wheel in a left hand turn) actually stops spinning after a cpl turns, locks up, skids on the pavement for a few inches, which must be causing the lurching feeling, and then starts to turn again.

    Could this be a 'feature' and not a bug? Maybe front LSD or locker? I havent been inside the front diff yet but everything in the front end seems to be in decent shape, plus I recently lubed everything.

    Ben
     
  2. sled_dog

    sled_dog 1 ton status

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    this is a straight axle 4wd. Nothing special they all do it. My dad's new 2002 Ford F250 does it, my 84 K5 does it. Scared my mom big time first time she drove in the snow. You generally should slowly turn and not too sharp in 4wd.
     
  3. Blazinaire

    Blazinaire 1/2 ton status

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    That is why fulltime cases has a diff in them. When making a sharp turn, the front end has to travel further than the rear but the transfer case will not let the two axles differentiate in speed from each other causing the rearend to have to "push" the front end around and that is the "lurching" you are feeling.
     
  4. HarryH3

    HarryH3 1 ton status Author

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    What you're experiencing is the reason that front wheel drive vehicles have a constant velocity joint at the steering knuckle, instead of a single u-joint. A single U-joint moves in an elliptical pattern, speeding up and slowing down slightly on each half revolution, when transferring power at an angle. The more severe the angle, then the move severe the elliptical travel. On a driveshaft you have another U-joint at the other end of the driveshaft which moves 180 degrees out of phase with the first U-joint. The travel of the two together cancel each other out.

    Since each front axle shaft has only a single U-joint, the tires seem to "hop" in a sharp turn. This is the elliptical travel of the U-joint, with the effect multiplied by the height of the tires.
     
  5. HarryH3

    HarryH3 1 ton status Author

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    Here's a pretty decent techincal explanation of how a conventional U-joint operates: http://www.tpub.com/basae/117.htm /forums/images/graemlins/thumb.gif Click on "Next" at the bottom of the page for info on how a constant velocity joint operates. /forums/images/graemlins/cool.gif
     
  6. tomseviltwin

    tomseviltwin 1/2 ton status

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    There's nothing wrong with this, It's similar to when you are locked in back and it chirps when you turn. The front and rear tires are traveling different paths, therefore the outside wheel moves faster creating a bind in the driveline. Do not put your truck in 4wd on pavement of where there is no loss of traction. The lurching is the drivetrain overcoming the bind by slipping a tire, generally not a good thing
     
  7. tomseviltwin

    tomseviltwin 1/2 ton status

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    OOps, said the same thing as Blazinair /forums/images/graemlins/blush.gif I didn't read completely
     
  8. HarryH3

    HarryH3 1 ton status Author

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    Even rigs with a center diff do this crow hopping, so driveline bind isn't a very big part of the equation. My '79 Dodge 4WD van with the good ol' NP 203 would crow hop pretty good when the wheel was cranked hard. I've driven K5's with 203's that exhibited the same symptom. /forums/images/graemlins/cool.gif
     

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