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Thread: .100 mild steel chassis update

  1. #1

    .100 mild steel chassis update

    I have a 68 COrvette that is a round tube car at .100 thick tubing. I must tear into this thing (very deep) to update to get certified. I do understand that there are concerned about the driver area. I must replace the main hoop and the halo and the x braces in the doors and the cross bar under the dash. My quiestion is that the place where the main hoop lands is actually the crossmember, and it is .100 mild steel. The down bars on the front land on the same .100 mild steel. The rocker bar is also .100 mild steel. I assume the main frame rails are the same. Is there going to be a problem with the NHRA with joining the chrome moly to the mild steel and will I have to change the crossmember?? How deep does it have to go. The driver position is also behind the main hoop. Any info about the funny cage for that?? Thanks!

  2. #2
    Senior Member MASTER BUILDER
    Join Date
    May 2003
    Owingsville, Ky
    If you are going for a SFI25.4 spec for 7.50 and slower, then all of the tubes mentioned are a problem. The main rails need to be a minimum of 1 1/2" x 0.108" MS round or 2" x 2" x 0.065" and the driver rocker bar needs to be a minimum of 1 5/8" x 0.118" MS. The rear crossmember needs to be one piece of 1 5/8" x 0.118" MS or 2" x 3" x 0.083" if it is a 4-link car, but can be 3 pieces if it is a ladder bar car. The lower rear kidney bar (the bar that goes from side to side on the main hoop at about 10-18" up can be 1 1/2" if it is a 4-link car, but needs to be 1 5/8" if it is a ladder bar car. There are a lot of issues, so I would recommend that you purchase the spec from SFI for yourself for about 35.00 and you can have it in just a few days. The specs show the different size tubing for the FC cage because of the driver being behind the main hoop.

    The sportsman spec of 8.50 or slower doesn't require a FC cage, and some of the other bars mentioned, but it would be much safer for you in case of an accident to have all of the proper sized tubing.

    I redid a tube chassis car a few years ago with the same problems. I just left the main rails and had to redo everything else. You can add the proper size tubes beside the existing frame rails, so you don't have to completely remove them. Here is a link to show some of the pictures of that work

    Good luck,

  3. #3
    Thanks so much Bill. I see you have done the work on yours to get the problem solved. I thought a that a friend of mine told me that the frame rails I have were ok. They are 1-3/4. He told me that the tech guy was mostly concerned about the bars around the driver. Seems kind of a general statement so I am going to be sure before I start welding. This is going to be difficult!!

  4. #4
    Senior Member SENIOR BUILDER
    Join Date
    Nov 2007
    Tukwila, WA
    Sounds like you need to replace the cage. not what you want to hear I know but sounds like tubing wall thickness is to small and not alot of room in that vette.

  5. #5
    Senior Member RACING JUNKIE TheYellaBrick's Avatar
    Join Date
    Mar 2008
    Emmett, Idaho
    Just remember to build in a lot of 'triangulation' in your high force areas. Slight triangulation is waay superior than parallel tubes. Never 't' into a tube without continuing that path of force on the other side of that tube into something solid, ie; main frame rails.

    Example; forward head loop into door bar then down into main frame or lower main hoop. You stop at the door bar and a hard hit on the head loop will collapse that door bar and it all comes down around your head either trapping or crushing you.

    Front to rear Triangulation also gives you a 'bridge' type effect to your chassis for more precise suspension tuning. A flexible chassis is impossible to make consistent.
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