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Thread: Reinforcing an area of the fiberglass body to install a hinge?

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    Senior Member beeman's Avatar
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    Reinforcing an area of the fiberglass body to install a hinge?

    What's the best way to reinforce an area of the fiberglass body to mount a hinge? Bond in an aluminum plate? Fiberglass in an aluminum plate? Something else?

    Then, what's the best way to mount a hinge without seeing the bolt heads on the painted side of the body?

    Thanks!
    Dave
    MK3.1 2004 Mach 1 donor. ABS, PS, TC.
    GTM #304 LPE 525hp LS3
    2000 C5 Lingenfelter LS1@489hp
    1999 Corvette FRC/Z06 track car

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    When I was working with carbon fiber cars in college, we used the sandwich method of bonding in a reinforcement plate. One way to conceal fasteners is to glass in a stud, allowing the fastener head to rest between the body and the backing plate. Think countersunk. The fastener never penetrates the body or shows up on the exterior face of the panel you want to grab. Pay close attention to your edges on both the fastener and the backing plate. On a load bearing connection, sharp edges can cause stress marks or cracking over time if they're allowed to contact the body directly. This is especially true for a hinge, which may generate a torque on the panel depending on its alignment. This causes the edge of the backing plate to bite into the body. You'll want to grind off any sharp edges prior to installation to ensure force is distributed gently across the entire backing plate. You can also place a piece of rubber or felt between the fastener head and the body to accomplish the same thing there.

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    Senior Member beeman's Avatar
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    Thanks for the input, I'm envisioning that. How big of a plate? Would you use steel instead of aluminum? I could weld the studs to the plate and not worry about bolt heads being prominent. Is there any benefit to bonding before fiberglassing the 'sandwich'?
    MK3.1 2004 Mach 1 donor. ABS, PS, TC.
    GTM #304 LPE 525hp LS3
    2000 C5 Lingenfelter LS1@489hp
    1999 Corvette FRC/Z06 track car

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    Quote Originally Posted by beeman View Post
    Thanks for the input, I'm envisioning that. How big of a plate? Would you use steel instead of aluminum? I could weld the studs to the plate and not worry about bolt heads being prominent. Is there any benefit to bonding before fiberglassing the 'sandwich'?
    The size of the plate is dependent upon the forces applied to it. We rarely went bigger than a postcard, with most being about the size of a credit card or two. Be cautious with steel - it's more prone to corrosion and the weight makes it more likely to compromise the bond. Something to remember is that the strength of the composite is largely from the reinforcement (glass, carbon fiber, etc). When you're patching something in like this, you don't have any glass connecting the patch to the original surface. In an assembly fully glassed at the same time, the resin cures as a single entity and the reinforcement (glass) can keep a part attached even if the resin fails. If the resin lets go on a part without continuous fiber reinforcement, the connection is toast. You want to prevent vibration as much as possible to control this risk. Bonding the backer to the body before glassing it in is ideal. If you don't, all the retaining force will be concentrated on the cured resin at the perimeter of the backing plate. If you bond the plate first, you get a nice even distribution of force. It will take a much more violent vibration to work the resin loose from the body panel you're working with.

    I'm sure someone else may have ideas as well. There's more than one way to skin this cat.
    Last edited by Jake From State Farm; 01-08-2019 at 09:08 PM.

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    Senior Member beeman's Avatar
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    Thanks!
    What have you found to be the best way to prep the existing fiberglass surface for maximum adhesion by the bonding agent and new fiberglass/resin?
    MK3.1 2004 Mach 1 donor. ABS, PS, TC.
    GTM #304 LPE 525hp LS3
    2000 C5 Lingenfelter LS1@489hp
    1999 Corvette FRC/Z06 track car

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    Quote Originally Posted by beeman View Post
    Thanks!
    What have you found to be the best way to prep the existing fiberglass surface for maximum adhesion by the bonding agent and new fiberglass/resin?
    I would first use a good prep solvent to remove the wax and any oils, then grind with 36 to the glass, not the gelcoat

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    My preference has been to use an aluminum plate with a stainless steel counter sunk screw, (threaded into the plate and use stud lock), if you need a stud protruding from the plate otherwise drill and tap and use a stainless steel heli-coil insert once the glassing has been completed.
    If you anticipate the fastener to be used a lot then I suggest using a stainless steel plate and weld the fasteners to the plate.
    I bond the plate to the existing fiberglass panel using 3m 8115. I also drill some additional holes in the plate to increase surface area for both bonding and reaction loads.
    Once bonded cover with fiberglass for both appearance and additional bonding/reinforcing.
    Cheers,
    Dave

    GTM # 294 Build Start Date 10/12/2009,
    2000 C5 Donor, LS-1 Twin Turbo, AC/Heat, G50-20.02 6 speed, Brandwood Cable shifter
    Build site. http://s679.photobucket.com/albums/vv153/dfraser/

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    Senior Member beeman's Avatar
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    Thanks for all the replies. Dave, how thick of a plate and how large? I'm guessing sufficient thickness to drill/tap?
    MK3.1 2004 Mach 1 donor. ABS, PS, TC.
    GTM #304 LPE 525hp LS3
    2000 C5 Lingenfelter LS1@489hp
    1999 Corvette FRC/Z06 track car

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    Senior Member beeman's Avatar
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    Another question, what would the pros/cons be of adding 1/8" or 3/16" rivets in addition to the adhesive and fiberglass? It would help me line things up prior to permanently bonding too. Could slightly countersink on the paint side.
    MK3.1 2004 Mach 1 donor. ABS, PS, TC.
    GTM #304 LPE 525hp LS3
    2000 C5 Lingenfelter LS1@489hp
    1999 Corvette FRC/Z06 track car

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    If you do it right the rivets are not needed. I would also recommend the 8115. Make sure the laminate that you put over the plate is appropriately thick and then even if the plate works loose over time it will remain captured. Many times wood is used as a load distribution plate because it bonds better to the glass/resin than metals. The fasteners are then not as robust, but you give something to gain something.
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    VRaptor SpeedWorks, LLC's Avatar
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    Quote Originally Posted by beeman View Post
    Another question, what would the pros/cons be of adding 1/8" or 3/16" rivets in addition to the adhesive and fiberglass? It would help me line things up prior to permanently bonding too. Could slightly countersink on the paint side.
    No matter what you do, eventually they will show up on the paint side....might not show up right away, but eventually any hole or rivet head will print thru the paint as the filler/primer shrinks over time.
    Shane Vacek
    VRaptor SpeedWorks, LLC
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    Turn-key GTM, SL-C & Ultima GTR Built to Your Specs!
    Offering a full line of GTM Upgrades and Custom Parts

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    Quote Originally Posted by beeman View Post
    Thanks for all the replies. Dave, how thick of a plate and how large? I'm guessing sufficient thickness to drill/tap?
    I "try" to use the rule for both nuts and studs, that thread depth is not less than the thread diameter. e.g. 1/4" thread = 1/4" plate.
    This can be reduced if you use a counter sunk screw as the head takes the load.
    Using a fine pitched thread can get you a little leeway on reducing thread length, but not much.
    Cheers,
    Dave

    GTM # 294 Build Start Date 10/12/2009,
    2000 C5 Donor, LS-1 Twin Turbo, AC/Heat, G50-20.02 6 speed, Brandwood Cable shifter
    Build site. http://s679.photobucket.com/albums/vv153/dfraser/

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    Senior Member dallas_'s Avatar
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    Also check out "bonding fasteners" at places like Veck Fasteners.
    FFR 7123 tilt front, Levy 5link/wilwoods/LCA's, webers.
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    Quote Originally Posted by beeman View Post
    Another question, what would the pros/cons be of adding 1/8" or 3/16" rivets in addition to the adhesive and fiberglass? It would help me line things up prior to permanently bonding too. Could slightly countersink on the paint side.
    Try not to if the rivets will be seen.
    I have had good luck placing a couple of layers of aluminum foil or wax paper from your pantry between the clamp and the piece to be glued to prevent the clamp getting glued.
    The 8115 can ruin your day if you don't pay attention.
    Cheers,
    Dave

    GTM # 294 Build Start Date 10/12/2009,
    2000 C5 Donor, LS-1 Twin Turbo, AC/Heat, G50-20.02 6 speed, Brandwood Cable shifter
    Build site. http://s679.photobucket.com/albums/vv153/dfraser/

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    VRaptor SpeedWorks, LLC's Avatar
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    To expand on that last post, we use clecos and 8115 together all the time......instead of using rivets. Drill 1/8" holes and use clecos and that works to both align the 2 parts as well as clamp them. Spray your clecos down with some silicone spray and let them dry prior to use to prevent the 8115 from sticking so hard to the cleco that it can't be removed. Once the epoxy is cured, you can pop your clecos out and fill the holes with more 8115 after cleaning any silicone residue out of the hole with some degreaser.
    Shane Vacek
    VRaptor SpeedWorks, LLC
    www.vraptorspeedworks.com
    Turn-key GTM, SL-C & Ultima GTR Built to Your Specs!
    Offering a full line of GTM Upgrades and Custom Parts

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    Senior Member beeman's Avatar
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    What is the light brown adhesive that FFR uses to bond the hood rigiding frame and the inner/outer door together? It's impressive. Is that the 8115?

    Thanks again for all the tips!
    Last edited by beeman; 01-09-2019 at 06:03 PM.
    MK3.1 2004 Mach 1 donor. ABS, PS, TC.
    GTM #304 LPE 525hp LS3
    2000 C5 Lingenfelter LS1@489hp
    1999 Corvette FRC/Z06 track car

  20. #17
    VRaptor SpeedWorks, LLC's Avatar
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    I don't know what the light brown stuff is that FFR uses.....my guess is that it's just fiberglass resin mixed with a thickener?.....like talcum powder. The 8115 is black.
    Shane Vacek
    VRaptor SpeedWorks, LLC
    www.vraptorspeedworks.com
    Turn-key GTM, SL-C & Ultima GTR Built to Your Specs!
    Offering a full line of GTM Upgrades and Custom Parts

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