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Thread: Retrofitting trans tunnel bracing?

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    Retrofitting trans tunnel bracing?

    This is my 12th season autocrossing FFR 1630, 7 regional class championships and a couple of National Tour wins.

    But I've got a Mark I with no trans tunnel bracing. My FEA of the FFR frame (as David Borden mentions in his Mistress tribute) confirms the trans tunnel bracing is highly effective for increasing both torsional and bending rigidity.

    I believe a properly designed brace could be bolted in (removable because I need to get the transmission out the top).

    Are there any factory options for retrofitting Mark II+ bracing into my car?

    Thanks,

    James

  2. #2
    Administrator David's Avatar
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    James, I just pieced it together and intially left the top cross tubes off and created a shear plate instead. Thats how I would do it if I were you...

    Welcome to the site!!

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  3. #3
    President, Factory Five Racing Dave Smith's Avatar
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    Thanks Dave for the tips (teh guy was a top finisher in SCCA Nationals autocross = cred). Jim here at FFR can help as well if you call him.
    Dave Smith, FFR 001
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    Senior Member Gordon Levy's Avatar
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    It can be done and has been done in the past. The first thing you need to look at is if you have the wide or narrow tunnel. I know it was around this time when FFR changed from the wider tunnel of the early cars to a narrower tunnel. The narrow tunnel is too narrow for the backbrace. The tunnel was widened again when the Mark 2 came out that had the backbrace. It you have the narrow tunnel it would probably be less work to add structural door bars instead.
    99/2000 NASA PSO Champion-2005 west coast FFR challenge series Champions
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    Senior Member Mike N's Avatar
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    James.

    I know that Wade Chamberlain has retrofitted a back brace as well but welded his in. I did a fair bit on analysis on my MkI chassis and added the door bars just because it was easier, they are somewhat effective and I think you would notice the difference but they are not as effective as the tunnel brace.
    Mike............

    FFR2100 - 331 with KB supercharger - T5 - 5 link rear 3.08's and T2 Torsen.

  6. #6
    Administrator David's Avatar
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    Honestly guys, I can check in FEA but I dont think the door bars are going to help much without the backbone and taking it all the way like I did in my design around the perimeter of the car. Even leaving just a couple tubes out will negate all your work. The time I spent in SW was very enlightening.

    James, we can try to work through some senarios with you but the backbone is going to be one of the single best mods you can do to the chassis. You will basically double the chassis stiffness with it.

    David
    Mk4 Build Thread: http://thefactoryfiveforum.com/showthread.php?141-David-s-Mk4-Build-Thread

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  7. #7
    Senior Member Mike N's Avatar
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    Here's what I did. David I agree that this was not a huge improvement in torsional stiffness only about 9% from what I remember but it does improve the bending a lot more and I did find that it was quite noticeable in how less nervous the rear of the car was after the install. I really need to do the tunnel work but that is a lot of work to get access to the attachment areas. This was all 1.5" x .12" wall DOM 1020 steel.



    Mike............

    FFR2100 - 331 with KB supercharger - T5 - 5 link rear 3.08's and T2 Torsen.

  8. #8
    Senior Member Gordon Levy's Avatar
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    Dave, I would agree. I was just thinking about the relative work to do the mod if he has the narrow tunnel vs. the wider. If he had the narrow tunnel he ideally would remove and replace both floor panels, the U panel, modify the rear bulkhead and the 4 tunnel aluminum pieces.
    The wide tunnel would only require modifiing the rear bulkhead and replace the 4 tunnel top pieces.
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    Thanks for the tips. I did extensive analysis on all sorts of door bracing- there is some increase in bending rigidity, but little torsional. The design pictured would be better, but I don't think will work with my frame. It can be fixed if the driver is willing to have a bar through the kidneys.

    What I found (an amateur analysis- FFR has the real engineers of course) is that the trans tunnel is far more effective. However, the whole tunnel structure exists solely to position the two diagonals (forming the "K" on the top). Remove those two diagonals (as many did to fit different transmissions) and you lose 90% of the stiffness of the tunnel.

    This is because a steel square tube is not at all stiff in bending, slightly stiff in torsion and extremely stiff in compression and extension. The design of any bracing is to transfer a twisting or bending force into a compression or extension force. Then you put a tube on this line and there's your stiffness.

    It's easy to add lots of steel and weight but only tiny amounts of stiffness- FEA is a must.

    I never tested a shear plate- interesting idea.

    The end result of my research simply confirmed that FFR really knows what they are doing with the frames. Thanks for the offers Dave and Dave; will be in touch.

    (Don't know if my tunnel is narrow or wide...)

    Thanks!

    James

  10. #10
    Senior Member Mike N's Avatar
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    Quote Originally Posted by Ophitoxaemia View Post
    The design pictured would be better, but I don't think will work with my frame. It can be fixed if the driver is willing to have a bar through the kidneys.
    James my chassis is FFR2100 and is a Mk1.5 my Kirkey seat is right against the rear bulkhead and the side bar does not come anywhere near my kidneys. I did split the aluminum panel right there to allow the side bar more room. The bar is right up against the bracket for the door latch and allows for the door to remain operational with no interference.

    Mike............

    FFR2100 - 331 with KB supercharger - T5 - 5 link rear 3.08's and T2 Torsen.

  11. #11
    Senior Member BigLeo69's Avatar
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    i cut out the cross-member to get the tko mid-shifter in.
    i then decided to not replace it. at the front of the tunnel on top 1st piece, i installed 1/8" plate steel (glued screwed and tattooed) not to be removed, about 4 " from the mid-shifter. then the rear panel i installed the aluminum as is supplied and cut the tranny tunnel aluminum at the rear just to cover about 8".
    now the tunnel still has 2 pieces left to install the middle one about 10" long to be screwed down at about 1.5" on center on all the 3/4 tube, as a removable piece and as a gusset the rear piece will be secured like the front piece but over the top of the cut Al. tunnel pc. now the top of the tunnel has 1/8 steel plate, where i can tap or fasten any covering. it also is insulated under and on the sides.
    sorry no pics i still having issues posting pics, they say my pics are too big, never had that issue before.>>>>>
    how did ffcars do it?
    Mk 3.1 #6945

  12. #12
    Senior Member Mike N's Avatar
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    There's no size limit on the other forum. I have the photos I posted hosted on FLICKR.
    Mike............

    FFR2100 - 331 with KB supercharger - T5 - 5 link rear 3.08's and T2 Torsen.

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