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Rear suspension subframe alignment?

Discussion in 'VT - VX Holden Commodore (1997 - 2002)' started by immortality, Sep 5, 2018.

  1. immortality

    immortality Moderator Staff Member

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    Does anybody know how to check and adjust the rear subframe alignment? Fitted a new diff mount today and I'm sure the subframe is out of alignment.
    [​IMG]
    As you can see the mount is as far back as it will go and the diff is still pushing the mount backwards and it's also not straight. The wheel gaps also suggest the passenger side is further back than the drivers side wheel.
     
  2. EYY

    EYY Well-Known Member

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    If you flip the part that bolts directly to the rear diff cover does that give you the clearance you need?

    If it’s actually out of alignment something may be bent? Do you have the adjustable arms?
     
  3. immortality

    immortality Moderator Staff Member

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    Nahh. The angle of the pic is deceiving but it's even on both sides.

    edit: I think the subframe front mounting bushes are probably a bit stuffed, they way the rear suspension springs sit/load up it would effectively push everything rearward.
     
    Last edited: Sep 5, 2018
  4. MYVESSV8

    MYVESSV8 Well-Known Member

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    off the shelf part or aftermarket, any better than whats out there, next why? (i like this sort of mods/improvements)
     
  5. immortality

    immortality Moderator Staff Member

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    We've had it on for a week now, I'm amazed at how much more stable the rear end is. Unfortunately I also replaced the rear shocks at the same time so I can't say with 100% confindence how much is just down to the rear mount. What I can say is it's the best this car has ever handled, the rear end is a lot more firm. The old shocks were still working but in my opinion well past their best. I also changes from Munroe GT gas to KYB Excell shocks.

    Aftermarket part? Sure is, Harrop was probably the first to come out with something to firm up the rear end which was their Harrop Sports Cover and replaces the whole rear cover on the diff and incorporates a new diff mount with urethane type bushes. I didn't think I needed the sports mount as it's not a real high performance build. The Harrop was designed to reduce axle tramp I believe and these mounts seem to be known as "anti-tramp" mounts.

    We all know that for suspension to work properly the mounting points need to be stable and rigid (note it's common to fit a front strut brace on a Commodore or any other car for that matter to improve steering). So with the Commodore IRS the rear control arms are mounted to the rear subframe with the springs and shocks located against the chassis. When you hit a bump the rear control arm absorbs that load via the spring/shock but because the rear subframe mount (the diff mount) has a fair amount of movement in it so the subframe moves up and down as well. The old rubber mount that came off had witness marks at both the top and bottom of the housing so under load the diff was moving up and down about 16mm (it doesn't sound like much).

    It was getting so bad that combined with the extra power/throttle response and the poor handling in the rear the missus didn't like driving it when wet. I'm quiet happy steering with my right foot but apparently the missus isn't.
     
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  6. afstruct

    afstruct Well-Known Member

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    Don't know if this helps
    Photos from harrop's instructions/ install booklet that came with my sports diff cover
    20180909_094800.jpg
    20180909_094553.jpg

    And yes the harrop diff cover has pretty much got rid of my axle tramp.
    I haven't got the alignment checked.
    I did mention it to pedders but they picked a heap of other stuff needed fixing.
    I went away and have since done rear CVs front struts and most bushes in the front.
    I went to O'Neils to get wheel alignment, he seemed to know nothing about the subframe alignment and just did a normal wheel alignment.
    Might try pedders again or have to try Holden.

    From this link https://www.google.com.au/url?sa=t&source=web&rct=j&url=https://www.ls1gto.com/forums/showthread.php?t=179799&ved=2ahUKEwiA1q7i0azdAhWSd94KHSVoAJ4QFjAWegQIABAB&usg=AOvVaw3Oso0OjrStgWkoWuK5asN9

    I've been doing rear suspension upgrades and believe this IS possible as a DIY. The manual has illustrations of the alignment tool and how it fits into holes in the body near the subframe bushings, and into smaller holes in the subframe itself, further aft & maybe a little inboard. The body holes are 19mm and are easy to find, they're between two big elongated holes in a pseudo "frame rail". My subframe has two sets of holes in the angled arms, I'm not sure which set the alignment tool uses, but I don't think it matters...I'll come back to that. I threaded a tape measure from the body hole, above the exhaust pipes, below the driveshaft (doesn't touch), and over to the subframe holes on the other side. Then switch to the opposite set of body holes and subframe holes and re-measure. It's not a perfect measurement because the tape has to go slightly up & over the exhaust, but it does the same amount of up & over both ways, and the measurement is mostly lateral - just compare measurements to see if the subframe is centered. Here's what I got after putting in new subframe bushings and diff insert:

    Forward set of subframe holes: 36-15/16 (to near edge of subframe hole), L>R and R>L, 37-1/2 (to far edge of subframe hole), L-R & R-L

    Aft set of subframe holes: 36-even (to near edge of subframe hole), L>R andR>L, 36-9/16 (to far edge of subframe holes), L-R & R-L

    I had the tape hooked into the 19mm body holes, and used one hand to make sure it was seated all the way in (not slipping down any). The attachment is a rough diagram of what I was measuring, except I have a 2005 that doesn't have mufflers in the way. I don't think this would work on a 2004, sorry first-year guys!

    I'm thinking I've got my subframe on center, within no more than a sixteenth, based on a total of eight measurements. What do you think? I've torqued my TTY bolts, but haven't done the "yield" step yet. If someone finds the fatal flaw in this approach, I will take it in to the dealer, but I think I've already got it centered.

    I might add the handling of mine seems good
     
  7. immortality

    immortality Moderator Staff Member

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    Cheers for posting :)

    That first page looks like it came direct from the Holden Dealership service manual.

    What they've done is what I was planning on doing, just measuring between the various mounting positions on the frame the factory jig uses.

    If I knew it was right then it would be easy enough to make a jig (hmmm, I know someone who's had theirs checked by Holden, wonder if he'll let me borrow his car for a few hours to make a jig?)
     
  8. afstruct

    afstruct Well-Known Member

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    Same. I might get mine checked and then make a jig as guessing I'm keeping it long enough that I'll one day remove subframe and strip and repaint it and do my diff.
     
    Last edited by a moderator: Sep 9, 2018
  9. MYVESSV8

    MYVESSV8 Well-Known Member

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    where did you get it from
     
  10. immortality

    immortality Moderator Staff Member

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    I got this one from TradeMe here in NZ, I've actually had it for about 2 years, was intended for my Senator (which is still in the garage in a 1000 bits...) so I decided to throw it under the Calais instead.

    Tuff Mounts sell one too Tuff Mounts Billet IRS Mount
     
  11. Nut Kracker

    Nut Kracker Kamikraze

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    Pretty sure you'll need a special alignment tool.
    If yo PM me your email address, I'll send you the rear suspension pdf file, from the factory service manual.
     
  12. afstruct

    afstruct Well-Known Member

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    I went to pedders today and after talking to 3 different people, yeah we have that alignment tool.
    So I bit the bullet and it's booked in for next Friday
    Hopefully they don't do their normal pedders thing and just do alignment, then I hopefully know it's right and make a jig for next time.
     
  13. vc commodore

    vc commodore Well-Known Member

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    My experience with pedders is, they will do their normal thing...Find a million problems that don't exist
     
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  14. Deuce

    Deuce Super Stock

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    Mine was checked at the dealership after harrop hat was fitted. It was all good.
    Have swapped diffs since then and assume nothing moved.
    Did you want me to measure anything for you to compare against? Or does it need to be a 3rd generation chassis?
     
  15. afstruct

    afstruct Well-Known Member

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    Yeah, hopefully just do alignments .
    I took it to them last December to get it done and they did that mile long list thing.
    Only 2 I can remember were worn strut top mounts ( what Commodore with a few kms on it doesn't ) and CVs which I had suspected and were stuffed.
    March this year did CVs and every bush in front suspension except sway bar D bushes .
    Found the drivers side strut was stuffed ,so new struts.
    So ,yeah ,hopefully good enough to just do alignments.
     
  16. immortality

    immortality Moderator Staff Member

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    Yeah, I have that document :)

    I might make a jig cause I know someone who's car has been checked :)

    One day I'm gonna spends soem time under your car......
     
  17. Deuce

    Deuce Super Stock

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    I'm reasonably free mon-wed this week. (Back to work thurs).
    More than happy to give you access to view.
     
  18. immortality

    immortality Moderator Staff Member

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    No rush.

    I think I'm gonna measure all the holes and make up a few bits at work to make a jig first :)
     

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