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Axle plung on rear trailing arms

Discussion in 'Shop - Suspension & Steering' started by ErikIrvine, Mar 26, 2012.

  1. ErikIrvine

    ErikIrvine Member

    Joined:
    Sep 29, 2004
    Location:
    AZ
    When I setup my rear suspension, the limiting factor for wheel travel was not my CV angle, but axle plunge. I was suprised how much axle plunge I had, at the time I should have measured it, but I didn't. I set it up so the axle barely cleared the CV cups when the axle was parellel to the ground, and enough space at drop out so the droop didn't pull the axle clips off the end of the axles. I think the reason I have excessive axle plunge is because my trailing arm CV cup is not raked back far enough when compared to the transmission CV cup. I attached a picture to help clarify, this dimension is when the CVs are parellel to eachother in relationship to the ground (about mid travel).

    When I get new arms, I plan to order them to the dimensions needed to remove this axle plunge at least some so I can get a little more drop travel. Right now the CVs sit at 22 degrees, and I am comfortable running them higher if the axle plunge was reduced. What do you guys recommend for rake back? If I order the arms 2 inches longer so the rake back is closer to 4 inches, will that reduce the plunge?
     
  2. ErikIrvine

    ErikIrvine Member

    Joined:
    Sep 29, 2004
    Location:
    AZ
    Drawing of current rear setup
     

    Attached Files:

  3. Wilson

    Wilson Active Member

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    Jul 26, 2008
    Location:
    Carlsbad, CA
    You haven't given enough info. to answer the question, would really need more dimensions for your specific parts. Longer trailing arms may help some, but you have to remember that the outer ends of the axles are traveling in an arc and the plunge is a result of both the vertical and horizontal movement as the suspension cycles up and down. Longer trailing arms will change at least the one dimension you have given, and therefore the axle plunge as a result of horizontal movement. Ideally (for minimizing plunge), you'd want to optimize the trailing arm length and inner CV locaton so that the plunge due to horizontal travel cancels the plunge due to vertical travel. I don't think it's practical for them to entirely cancel each other out, so it's a game of working the dimensions so that you can eliminate as much as possible. It's a High School trigonometry exercise in 3 dimensions and everyone's setup is unique so there's no "one size fits all" solution.......
     
  4. ErikIrvine

    ErikIrvine Member

    Joined:
    Sep 29, 2004
    Location:
    AZ
    Thanks for the reply. Not sure which dimensions you need but I am guessing you would need distance from the torsion housing to the centerline of the transmission CV cup, and width of the trailing arms (12 in. wider torsion housing with 1 in. wider x 7 in. longer arms). I added some dismensions to the drawing from earlier. Maybe what you are saying is there are too many dimensions to provide a solution and the only way to figure it out is to measure it on the car.;

    Also, is there a standard set back from Transmission CV cup to trailing arm CV cup? I thought I remember people throwing around a 3 or 4 in; standard for the rake back
     

    Attached Files:

  5. Wilson

    Wilson Active Member

    Joined:
    Jul 26, 2008
    Location:
    Carlsbad, CA
    To minimize plunge, you basically want all the CV's "lined up" at full droop or compression. If the outer CV's end up in front of the inner CV's at any time, you are increasing the amount of plunge the axles and CV's see. The horizontal (fore/aft) distance between CV's should be at a maximum at mid travel and a minimum at full up or down travel.

    Using the dimensions you've provided, here's what I came up with (ignoring the side to side motion that results from your pivot geometry, which you can't really do anything about - unless you want to spend lots of money):

    If you allow the CV's to go to 30 deg. of angle, the outer CV will move approx. 2.9" forward as it moves from mid travel (0 angle) to full travel (30 degrees). So, you'd want the outer CV to be about 2.9" behind the inner CV at "0" CV angle to minimize plunge.

    If you limit your CV's to 25 deg. of angle, the outer CV's will move about 2" forward as they move from mid travel to full travel. You'd want the outer CV about 2" behind the inner at mid travel for this case.

    As soon as you change any of the dimensions that you provided, the above numbers go "out the window" (longer trailing arms result in more fore/aft movement through the range of suspension travel, meaning you'd need to fine tune the inner CV location to match the horizontal location of the outer CV at full travel.....). 3 to 4 inches of inner to outer CV horizontal displacement at mid travel is probably about as good as you're going to get. As long as the outer CV never ends up in front of the inner CV, you should be in good shape (your dimensions look like the outers do end up in front of the inners at full travel). I haven't looked at exactly how much additional plunge you are experiencing because of the less than ideal CV to CV locations, but it's probably not very much. This is just one of the limitations of this type of suspension; there is only so much travel possible given your configuration, that's why you see limit straps on race cars.


    There are a few tricks that racing CV's use to allow more plunge - look at the shoulders on the stars; you'll see one side has been machined off to allow more movement before the axle's circlip starts "pulling" on the star......

    Good luck!
     
  6. ChromolyKid

    ChromolyKid Well-Known Member

    Joined:
    Mar 15, 2009
    Location:
    Phelan, CA
    Put the inner stars on a lathe and machine out a 1/8" to allow the axle clips to plunge into the star, you'd be surprised how much extra play this gives you and can actually help keep the clips in place at full droop.
     
  7. Wilson

    Wilson Active Member

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    Jul 26, 2008
    Location:
    Carlsbad, CA
    Sorry, messed up the high school trig.....

    At 30 degrees CV angle, the outer CV actually moves about 5.5" forward with your measurements and there is about 3.3" of plunge (neglecting what happens as a result of your inner pivot arrangement). With a 28 degree CV angle (more realistic maximum if we're talking about 930 CV's), you'd have about 2.8" of plunge with the outer CV moving about 4.5" forward from mid travel. As I indicated though, I didn't take into account the fact that the outer CV's actually move side to side some as the trailing arms cycle from full droop through mid travel and then full compression (a consequence of the inner pivot geometry). That motion tends to decrease the actual amount of plunge typically; your track width changes as the suspension cycles, widest at mid travel and narrowest at full droop or compression. That could be factored in with a few more dimensions and angles given but if you're just interested in the most favorable fore/aft CV locations, it's not that important (and I'm not interested in doing any more trig!).
     
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  8. ErikIrvine

    ErikIrvine Member

    Joined:
    Sep 29, 2004
    Location:
    AZ
    Thanks Wilson and chromoly kid for your replies, both very informative! Tonight I'll drop the rear susp. out and see where the two CVs end up in relation to each other. Also, I have the race prepped CVs from Mckenzies on there but next time I disassemble my CVs I''ll look for an opportunity to machine the shoulder if its still there, I have a low compression type 4 VW so it's not super high HP going through those 930s. Thanks again!
     
  9. Wilson

    Wilson Active Member

    Joined:
    Jul 26, 2008
    Location:
    Carlsbad, CA
    If you've got race prepped CV's from McKenzies, your stars are probably already machined so that there's a shoulder on one side only. Be careful to assemble them with the "no shoulder" side towards the circlip on both inner and outer CV's. Also remember that there are power losses that increase proportionally with CV angle increases; we were only talking about minimizing axle/CV plunge here.
     
  10. ErikIrvine

    ErikIrvine Member

    Joined:
    Sep 29, 2004
    Location:
    AZ
    Thanks again Wilson, I have a worksheet I use during CV rebuilds so I make sure the shoulders are facing the correct direction. Also I'm not too worried about power loss when my wheels are dropped out, I would rather get a couple inches more travel and set my CV angle at 26 vs. 22 deg.
     
  11. 5racer

    5racer Well-Known Member

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    Jun 22, 2003
    Location:
    san jacinto calif

    k.i.s.s
     
  12. joe1369

    joe1369 Member

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    Mar 16, 2008
    Location:
    northridge, Ca.
    You have to use broached cv stars to get the maximum extended length of the axles without pulling the clips at full extension, then the axles cannot be to long either, because they will mushroom the ends from the collision inside of the cv cups at the trans side. Starting point for relationship between inner and outer cv is 3" back at the outside, drop out rotates the cv forward to almost parallel, giving a non-compound angle. Wilson has it right!!!!
     
  13. ChromolyKid

    ChromolyKid Well-Known Member

    Joined:
    Mar 15, 2009
    Location:
    Phelan, CA
    Broached is the word I was looking for, this is what I was talking about. I know some guys leave the wheel side axle clip off but it always scared me so broaching was the next option.
     

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