Sub Frame Alignment

Discussion in '1970 - 1973 Specific' started by Rich Gershman, Sep 30, 2020.

  1. INSPECTOR57

    INSPECTOR57 Veteran Member Lifetime Gold Member

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    You can check the subframe alignment using the diagram from this site:
    https://nastyz28.com/tech/framechk.html

    The specs for the '73 will be correct for your '70 -
    Park the car on a level concrete area & use a plum bob to mark the reference points on the ground. Ideally the 'H' measurement will be the same from either angle. Note that the intermediate point on the front subframe is NOT at the mounting bolt but taken at the alignment hole near the mount.

    (Btw, some of the vertical dimensions for the '80 seem a bit off o_O )

    Gary
     
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  2. Rich Gershman

    Rich Gershman Member

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    If the sub frame turns out to be not true, can it be straightened while in the car with the fenders and hood in place?
     
  3. flht99b

    flht99b Veteran Member Gold Member

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    You are potentially in for a lot of work. Basically everything forward of the door will be impacted by moving the subframe in relation to the body. Before you enter that nightmare, measure the wheelbase on each side, based on what you said previously will be different than 108" on each side IF the subframe is installed incorrectly. But if the wheelbase is the same side to side, you have other problems. Hopefully, resetting the subframe and realignment of fenders, hood, header panel, radiator support, etc will get you back to a square platform to begin with. Seems to me the alignment rack should have shown a big issue with the thrust angle. Also, why would the subframe being installed incorrectly affect castor, castor effectively is the angle measurement from ball joint to ball joint vertically.
     
  4. Rich Gershman

    Rich Gershman Member

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    So this is what I thought, 1) when you add shims to the rear of the upper control arm you pull the arm ball joint toward the rear of the car increasing positive caster which also re-positions the wheel toward the rear of the wheel fender opening, 2) prior to my visit to the alignment shop, when I measured both sides for wheel base, the right side was at 108" and the left side was 107.625" approximately. I chalked this up to installing control arms during the restoration and not putting any shims in place leaving that up to the alignment shop... my thinking was that the left upper control arm naturally had too much positive caster and the alignment would reduce the excessive positive caster thus putting the wheel where it belonged at 108" wheel base on the left side. Oh how wrong I was. I am not an expert this is why I consult you guys. Am I wrong in thinking that whether you have negative or positive caster that it won't affect wheel position or wheelbase.
     
  5. flht99b

    flht99b Veteran Member Gold Member

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    Positive caster moves the upper ball joint/top of spindle towards the rear of the car which moves the wheel/tire assembly to the rear making it closer to the rear of the wheel well. If the alignment shop shows you currently have negative 4.x degrees caster which I question as using stock suspension components you normally can't get 4 degrees of positive or negative camber, stock components and shims just don't have that range so I question their results. The wheelbase side to side discrepancy is the first issue to resolve. Try and start as a baseline with no shims on upper control arms. Measure the car diagonally, something is mis-aligned or possibly bent which needs to be resolved first.
     
  6. Rich Gershman

    Rich Gershman Member

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    I agree that something is wrong. I will take your advice and start measuring. Can I get your opinion on this; some members say that you can't re-position the subframe without taking the fenders and hood off and other say you can. Again I'm no expert here but it seems to me that if you loosen all 6 mounting points ( rear, middle at firewall and core support ) and loosen the steering column that you could shift the subframe around slightly without affecting the alignment of the fenders and hood. For example technically speaking if you could take the weight off the nose of the car and remove the differential / spring assy, could you not lift the whole car off of the subframe body with fenders and front nose including core support, just as you could lift a C1 to C3 Corvette body including front clip off of the frame.
     
  7. flht99b

    flht99b Veteran Member Gold Member

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    Never tried to move the subframe with the entire nose bolted on. If the car is finished painted I understand the desire not to remove or even loosen everything. However, you may end up damaging the paint on those parts anyway. If you try to move the subframe the core support will need to secured somehow (bolts loosened or removed) so it remains where it currently is. If core support moves, so do the fenders. If you can figure a way to suspend the core support (just enough to take the weight off where it bolts to the subframe), you might get away with it. Ideally, body lifted at pinch welds on a 2 post lift, everything bolted and tight except steering column and core support to subframe bolts, again you might get away with it without incident.
     
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  8. berg2695

    berg2695 Veteran Member

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    You really don't have to worry about sheet metal. Once you loosen all 6 body mounts the frame is free to move small amounts in the horizontal plane. Trust the gauge holes in the frame and body. If there is still something wrong, then you can start measuring suspension points etc. Get two hefty 5/8" bars and a helper to align the gauge holes. For me, a long 1/2" drive socket extension worked well. Good luck!
     
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  9. xten

    xten Veteran Member

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    How are panel alignment and gaps now? anything look uneven?
    We had a 67 Firebird in the shop that had the same issue. It caused all kind of body alignment problems. Car was painted apart and then thrown together. But it was much worse than yours. It was "restored" by a shop that went belly up and was getting pad locked by the state, so owner had to get it out of there. It was so far off, the shop drilled new holes in the frame horns to mount the rad support. After taking the rad support mounting bolts out and loosening the other 4 and aligning the subframe, we loosened everything from the doors forward and removed front wheelhouses. Sheet metal is going to have to be realigned anyway IF there isn't enough wiggle room for what you need. If there isn't enough loosen hood and leave it up, periodically and carefully checking gaps. Do not open doors during this process. It will make one side tight and other side will have a big gap between door and fender. With these gaps 1/8" is a lot. Tight side could chip paint on front of door, so definitely don't open that one till fender is pulled forward.
    Good advise from flht99b and berg2695.
    Good luck and be careful.
     
  10. Rich Gershman

    Rich Gershman Member

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    In response to Xten's post. The doors, fenders, cross nose piece and hood are fitting as good as any 2nd gen car I've scrutinized. So I'm very pleased with that outcome. Since yesterday I've measured the pinch weld to frame rails and they seem to be parallel on both sides. Its difficult because the pinch weld seems to be uneven in areas along the length. Depending on an OD or ID measurement of the pinch weld it seems to consistently be 6.5" on both sides. There might a 1/16" differential from drivers side to passenger side. The drivers side may be a little more. I do also have to factor in the thrust alignment because the differential / spring assy is not centered and I understand this is normal for some of these cars, sorry now I'm opening another can of worms. At one point during this debacle I actually changed upper and lower control arms on the drivers side thinking that the lower arm might have been out of tolerance but there was no change in the caster problem so it is not a bent lower control arm which I suspected. I have concluded that it has to be an out of square frame. One other point on the locating holes in the frame and body, the on center spread dimension from left to right is greater for the frame than the body probably 1/8 - 1/4" so the holes may line up on both sides on the length axis but will never line up on the width axis. Right now the passenger side locating hole (body to frame is dead centered) the drivers side is not centered on both axes and what is even further disappointing is that the hole alignment is biased in the wrong direction meaning that if I moved the frame to line up the holes on the drivers side it would make the situation worse. So in the restoration of twelve different cars in over 40 years of this hobby I've never had one project that didn't have at least one weird problem that I struggled with... usually its my inexperience! Thanks to everyone that chimed in, your input has been valuable.
     

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