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Thread: x frame for fiero undercarriage

  1. #1
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    x frame for fiero undercarriage

    does anybody know the dimensions for the x frame for fiero undercarriage. and also the type of steel tubing i will need. I appreciate it.

  2. #2

    Re: x frame for fiero undercarriage

    Don't waste your time installing one, they don't do much good.

  3. #3
    Senior Member LZeitgeist's Avatar
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    Re: x frame for fiero undercarriage

    Mine was done (quite well, I might add) thusly:

    1"x1-1/2" rectangular steel tubing - what wall thickness I'm unsure of, but obviously it's better to go 'too much' than 'not enough' -

    Two straight pieces welded to front crossmember and going back directly towards the cradle mounting points on the frame. Attach end of each piece with a short section up firewall to join to frame at cradle mounting points. Spot weld to underbody about every 6-8".

    Create a narrow 'X' with more of the same tubing starting about 6" back from the front ends of the first two pieces, and going back to about a foot short of the ends at the rear. This will cause the crosspoint of the 'X" to intersect about a foot rearward of the forward tip of the fuel tank.

    Create a crosspiece across the rear ends with 1"x1" square tubing. Bolt in place to short pieces added to original two pieces that joined them the the cradle mounting points. (This piece must be bolted, not welded, since it will cross under the rear end of the fuel tank.)


    - under car on lift, standing under rear of car, looking forward


    Patrick W. Heinske -- LZeitgeist@aol.com

  4. #4
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    Re: x frame for fiero undercarriage

    I personally believe that the under body frame does little especially with the relatively small 1" thick material, it simply does not triangulate out any of the forces on the chassis. Interestingly when BMW converted some of their cars into convertables they welded a 3/8" plate along each side of the car from front to back. That would be strong. Most of the weekness, if not all, is in the back anyway at the top of the firewall not under the tub. As an interesting side note take a stick of 1x2" x 12' and lift one end of it up - it will deflect a couple of inches under it's own weight so what use does it have as a structural material?

  5. #5
    Senior Member LZeitgeist's Avatar
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    Re: x frame for fiero undercarriage

    Considering my car is a completed convertible, it obviously does quite a bit...
    Patrick W. Heinske -- LZeitgeist@aol.com

  6. #6
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    Re: x frame for fiero undercarriage

    Well, I'm not saying there is no value in the x frame but here is my specific issues with the design. First, it reduces the ride height of the car. second it adds a fair bit of weight for it's value. Third, it bolsters the strongest point in the chassis (the underside of the seating tub) rather than the weak points. The mistake in the design is that it so closely follows the firewall rather than reaching out towards the back of the chassis in a triangulated form. The basic rule of chassis design is being broken - Forces should be along the axis - Ideally pushing rather than pulling. The idea is that the force required to compress or stretch a tube along it's axis is very high. The force to bend a tube regardless of wall thickness is quite low relatively. The entire rear section of a fiero transfers force rotationally basically from the engine cradle pivot to the top of the firewall. Once the roof is cut off the firewall support is massively reduced.
    Simply because a car is completed does not mean that something obviously works. The Fiero was never know for it's lack of chassis flex. Try out most convertibles that originated as coupes and you find plenty of weakness in the chassis. Does it affect much? Not really, virtually no one cares or notices because the way the cars are driven. Can the Xframe be made better? Easy - stretch out the part that comes from the bottom of the tub to meet the chassis further back on the upper frame rail. Probably $8 in 1"x1" steel and an hour of cutting and welding and the frame will be drastically more rigid.

  7. #7
    Senior Member LZeitgeist's Avatar
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    Re: x frame for fiero undercarriage

    There's many ways of doing the reinforcements - the way mine is done doesn't have the extra areas that you're speaking of, but it handles the same as my friend's all-original Formula. Perhaps the 88's have a different/stronger arrangement for the rear frame than previous years?

    Patrick W. Heinske -- LZeitgeist@aol.com

  8. #8
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    Re: x frame for fiero undercarriage

    thanks for the advice guys,i just want to make sure when i cut the roof off and then add the strech it doesnt twist and bend, like some posts talk about in tne lambo lounge

  9. #9

    Re: x frame for fiero undercarriage

    Quote Originally Posted by LZeitgeist
    Considering my car is a completed convertible, it obviously does quite a bit...
    I have stetched a few cars and there must be more to this X frame than just the floor area. The Fiero has no strength when the roof is removed. My spider is a perfect example of what janice_ho is saying. I used 1"X3'X1/4" rec tuding and I only get about 2 inches of flex at the top of the doors. I have adding about 100lbs more steel into the rear so this does not happen. The floor X really doesn't do much.
    LZeitgeist you must be one of the lucky ones if this all that is in your car.

  10. #10
    Senior Member LZeitgeist's Avatar
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    Re: x frame for fiero undercarriage

    I'm realizing now that this is just one of those topics like religion and politics, that even when presented with hard, real-world facts, those that are set in their ways will continue to believe what they wish.

    The facts are that my Formula is braced this way, and my car is not only drivable, but feels just as solid as a good local friend's Formula. They were driven back-to-back by both of us to test this exact theory.

    I've offered my facts to try and help those who asked for them - I will now leave the thread to those who appear to believe that my car is (apparently unbeknownst to me) actually sitting in my garage in two broken halves, put the top down and go for a ride. Have fun, gang...
    Patrick W. Heinske -- LZeitgeist@aol.com

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