Verifying Frame Horn Heights What is the Datum Plane?

Discussion in 'Race car chassis tech' started by guyver002, Aug 10, 2021.

  1. guyver002

    guyver002 Well-Known Member

    Hello all, so I am wanting to see what kinda measurements I get from my frame horns lately as I am tackling putting the front end back together after some core support patching. As invaluable as the service manual is I couldn't find what the "datum" is supposed to be for me to measure frame horn height. I don't think they would leave something as important as that to just use the ride height as the vehicle rested on it's wheels with the myriad of options and their different weight combinations and then measure from floor to frame. So does anyone know what the Datum is supposed to be so I can used the tolerances given in the Service Manual?

    Attached Files:

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  2. Dano

    Dano Platinum Level Contributor

    I'd think that if you set your height "T" at all 4 locations, that becomes your datum plane. Of course this is assuming a level floor & should all really be done on some sort of a jig/surface plate/frame machine. I have a similar issue & trying to decide how to proceed. I haven't checked the floor in my shop but I know that's really not the right way to do it.
  3. Max Damage

    Max Damage I'm working on it!

    I would think you can just level the frame (between the Ts) using carpentry tools in both directions (front to back and side to side)?
    Last edited: Aug 11, 2021
  4. Dano

    Dano Platinum Level Contributor

    On a level floor that'd be fine. The issue w/doing that is suppose the floor isn't level (very commonly it's not, hence why you get pools of water) and drops off ΒΌ" between the "T" & say the "V" measurement. You might think the frame is bent when it's fine or vice-versa: You think it's fine when it's bent.

    Even when you're establishing your datum at you outlined, say you have to shim your stands to get it level: Is that because the floor isn't or because the frame is bent or even was just welded together funky from new. You just don't know w/o a level datum plane to begin with.

    Google "race car jig" & look @ the images. I'm probably going to have to build something. My dilemma is I'm going to be starting w/a new foundation so to speak - Tailpanel, trunk/pass floors/rockers, saddlebags, etc. & rebuilding the original shell onto that. I can either bolt the foundation to the frame or not - I'm thinking bolting it to the frame that will be used for the car is best & w/the new mounts so as to build the shell to conform to any irregularities in the frame but either way it needs to be on a level jig of some sort & I can't really use the floor for the reasons above. I don't want to weld the whole thing together just to find out it's really twisted.

    Maybe I'm way too anal too &/or overthinking this way too much (both possibilities:)) but... I haven't even taken into account the curvature of the earth over 18' (ok, I'm joking there:)). I guess if my floor happens to be dead flat than I'm OK but the chances of that are slim.
    Last edited: Aug 11, 2021
  5. 1973gs

    1973gs Well-Known Member

    The datum plane is a level plane based off of specific holes in the frame. If you look at a frame chart, it gives specific holes and height settings to hang frame centerline gauges. When the gauges are installed, all of them should be seen as one when looking at them from the front and rear. It's like looking at a fence, only horizontal instead of vertical. If the frame is straight, the gauges should all line up and appear to be one. If several don't line up, that's where the damage is located.
  6. Dano

    Dano Platinum Level Contributor

    Something like this?
  7. 69a-body

    69a-body Well-Known Member

    If you have any question about frame, it may be worth trip to frame shop. I took a rolling chassis and was given a before and after sheet of all dimensions. Frame was out of square and also diamonded in the middle. All the final numbers were 1/16th to 1\8th from nominal. I would consider that better than off assembly line. Cost $400
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  8. Max Damage

    Max Damage I'm working on it!

    I think if you level your stands using a known straight piece of lumber or bar stock, you can then use a clear piece of tubing with water in it to measure the difference between the various areas of the frame. The tube ends can be marked up like a ruler.

    Of course if it's bent and needs straightening, the frame shop would be handy.
  9. 1973gs

    1973gs Well-Known Member

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  10. 1973gs

    1973gs Well-Known Member

    That will work assuming that the frame is supposed to be level and frames aren't always symmetric. That's basically what I did when I back halved my car. I knew the history of my car and knew that it wasn't in any accidents so the frame was at least as straight as it came from the factory. I leveled the frame and used a plumb bob to mark the centerline and all body mount holes. I also used a frame chart and added many diagonal measurements before I cut the frame.
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  11. hugger

    hugger Well-Known Member

    Easy way to do this that takes the floor out of the equation is to use 2in square tube check it for straightness,...lay it on the floor then shim it to level, now have a point of measure then level the frame there at the cowl area, measure your front rails down to your 2in tube,...same on the rear check for twist another 2in tube place corner to corner of the belly with a level on it will tell the story there
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