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Thread: Rear Frame Rails

  1. #1
    Senior Member DYNO TECH
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    Rear Frame Rails

    For a back half door car you guys prefer the one peice mandrel bent rectangular frame rails, .134 round tubing, or rectangular fabricated?

    I lean towards the mandrel bent rectangular rail, however I think sometimes the tubing strength might be compromised because of the distortion caused by the bender dies. I have been considering just fabricating the entire rail for that reason.
    'A government big enough to give you everything you want, is big enough to take away everything you have.'

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  2. #2
    Senior Member DYNO OPERATOR
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    Trick question.

    I know of no one building frame rails or rear back halfs with round tubing that are only using one bar of tubing. On the other hand I have seen plenty of rectangular tube rear back halfs that are using one bar of tubing.

    So my answer is round tubing with plenty of diagonal reinforcements, gussets. You will end up pound for pound with more ridgidity.

    Ed
    " Let all things that hath breath, praise the Lord. Praise Ye the Lord" Psalms 150 vs 6.

  3. #3
    Senior Member MASTER BUILDER
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    Rear Frame Rails

    I have used each of the methods to back half a car that you have mentioned and have also used 1 5/8 chrome moly and they have all worked equally well, because of extra bracing. The main thing that you need to do with any of them is to triangulate to make sure that the upper shock mount is solid and that the ladder bar/4-link mounts don't move forward or backward. I take the two bars from the top of the cage as far back as I can put them on the frame rails. Then I add the X-brace to the upper shock mount bar and back to the cage. I have tied in extra bars to the top of the 4-link brackets back to the floor at the main hoop and from the crossmember near the bottom brackets forward to the subframe connectors or the main frame. If you don't add these bars, I have seen the crossmember bend, the upper shock brackets flex, and these can cause safety and chassis tuning issues.

    Good luck,
    Bill M
    '85 Monte SS with a 539
    Best ET 8.90 @ 152

  4. #4
    Senior Member DYNO TECH
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    Thanks for the response guys.

    I have done plenty of back halves with MS rectangular tube with both mandrel bent and fabricated rails. Was just looking for some different perspectives and preferences on this.

    Thanks, Bill

    Ed: A.R.T., Alston and a few others offer kits simular to the one below, this is just an example of what I was refering to. Obviously rear strut tubes, X-brace and much more diagnal bracing is required to make it rigid enough.

    'A government big enough to give you everything you want, is big enough to take away everything you have.'

    Thomas Jefferson


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  5. #5
    Senior Member EXPERT BUILDER
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    I like the round tube stuff as shown in the pic, not hard to fabricate at all.
    Agreed , lots of bracing and tieing into the roll cage is good.
    A lot of guys with street type cars seem to prefer the rectangular tubing rails for some reason.
    The mandral bent rec tubing is easier for the manufactures to put out, and sell to guys that want to do the job themselves.
    It is not difficult to make up a sectioned frame rail and weld together.

    The trouble with these kits and frame rails as supplied is they are all a universal fit, which means, it fits everything, but fits nothing well....
    Ed

  6. #6
    Senior Member DYNO TECH
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    I agree completely, I will custom fabricate and fit everything for my application. Again this was just used as an example.

    So from the sake of strength and weight you would all go with the round tube instead of box?
    'A government big enough to give you everything you want, is big enough to take away everything you have.'

    Thomas Jefferson


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  7. #7
    Senior Member EXPERT BUILDER
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    blittle, I prefer the round tube frames, as I feel it looks cleaner and more professional. It will allow for a little bit lower riding car and more room for exhaust and mufflers. If you have access to a tube bender, they are no problem to make up and you can fit them were you want them.

    Some guys still like or prefer the 2x3 box for some reason for street cars, they say it is stronger for the street, but I think it is because it looks more like the stock frame rails.

    Last year I upgraded the frame on a vega for a guy, the frame was built in the 80s, square tube, 2x3 and had been added to with 2x2 square to the front with some weird suspension set up from a chevette, (you remember those??) It had worked well for the original owner for many years.
    The guy took it to another chassis guy .. :shock: .. to replace the cage which would not pass cert to to wall thickness.
    After the new cage was installed, he brought it to me to install struts on the weird 2x2 setup.
    To make a long story short, we ended up replacing the complete old frame with a new round tube frame under the new cage. Kind of a backa**wards way of doing it , but it turned out ok.
    The vega is now back in my shop for 4 link install, as it was originally a ladder bar car.

    you can see pics of it on my website, its the red vega, I dont know how to post pics here.
    good luck with the project....
    Ed

  8. #8
    Iv'e done both kinds as well, but I definitley prefer round tube. It's cleaner andjust as strong or even stronger when done properly. In my opinion it is much easier to add the braces to round tube.
    Go with the round tube you will not regret it.


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