How about a change of pace?

Discussion in 'Fiberglass and Composite Boat Building' started by Handtool, Sep 13, 2006.

  1. Handtool
    Joined: Sep 2006
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    Handtool New Member

    Howdy All,
    I know its not a boat but this is the only online fiberglass community that seems to build stuff other than subwoofer boxes... I hope you can help me out and share a little of your collective wisdom as I have no experience with fiberglass.

    I am working on a fiberglass motocycle seat that I had to cut to fit my particular bike. Now I am wondering what is the correct process and products to use to finish the cut end. My idea was to use resin to glue the cutoff end flush with the seat and then use a filler product to fill the gaps and finish it off for paint. Does this sound like the right way to go about it? Would using an epoxy to glue the seat together be stronger? Anybody have any suggestions on products that work well with fiberglass and have the strength to stand up to my 200 lb butt and the vibrations of the bike and road? Thanks...


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  2. snal
    Joined: Jan 2005
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    snal Junior Member

    what about...

    aluminum?...I made this one for my bike.
     

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  3. Toot
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    Toot Senior Member

    What does the seat attach to at the cut-off end? The fuel tank? Forget bonding the fiberglass to metal. It can be tricky, finicky, and vibrations can quickly destroy the bond. Also, the motorcycle chassis flexes when it's leaned over (which is why carbon fiber GP bikes were never really successful- too stiff), so I think you'd have a crack there in no time. Besides, what kind of bike is it? Don't you need to lift the tank to perform some maintenance? I only really know about sportbikes.

    First choice- metal fasteners.

    Second choice- C-clips that you can bond to the seat and clip onto the frame, maybe capable of being screwed down tight. More complicated, but it'll look nicer.

    If you don't like these options, you'll have to give me more details about your application, preferably with pics or a description of the bike.
     
  4. Handtool
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    Handtool New Member

    Thanks for the replies fellas, sorry I wasn't more specific. I plan on mounting the seat with large machine screws into some tapped standoffs welded to the frame. At the front, the screws would act as a mechanical bond, sandwiching the cut piece between the seat and standoffs. I want to finish off the front edge for cosmetic purposes, to close the open end that will be seen through a gap between the tank and seat. I am wondering what kind of chemical bond I should use between the two pieces of fiberglass and what kind of filler product to use to fill in the seams. Will some two part epoxy and bondo do it or should I be looking for something else?

    Here is a pic of someone elses bike using the same seat. I will be doing something very similar.

    [​IMG]
     
  5. fiberglass jack
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    fiberglass jack Senior Member

    sounds good to me ,sand the to parts where u are going to bond lots of epoxy and clamp it or another trick is to throw a couple of dry wall screws to clamp it, then take them out when cured bondo is good to fill the holes and dress the edge, your friend has left a rough look i can see the join so dress it out nice and no one will know what was done
     
  6. Toot
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    Toot Senior Member

    Bondo won't really do much for adhesion. It just sticks to itself pretty well. I wouldn't put more weight than that on it.

    How do you plan on removing the gas tank in the future? Or don't you care? Will it come off as a single piece?

    As for bonding them together, I would use a Hysol product. That's the best for bonding metal to fiberglass, however it still is very prone to delamination by vibration and, especially, heat. Google Hysol and you can get a feel for what their products can and can't do. 2 part epoxy will work too, but its heat resistance may be an issue and cause cracking over time.

    I would use that for the mechanical connection to the tank. I would use at least 3 square inches of bonding surface. And I would be sure I can lift the tank and seat off as a single assembly.

    Once that's done, you can fill it with just about anything. I would probably use the premium line of Bondo products just because it's easy to get ahold of. I'd start with their short hair filler. It has fiberglass strands embedded into it and it's designed for filling larger areas. Then I would finish it off with body filler to smooth it out, since it's exceptionally easy to sand.

    I'd nix the screw idea because screwing into a gas tank and the subsequent resealing opens a whole other messy can of worms. Just apply pressure, maybe a sandbag with tin foil underneath it. You shouldn't need much pressure as epoxy (including Hysol) is fairly tolerant of excessive glue joint bond lines- just don't over do it. You've got to make a joint, not just fill in a gap with glue. So long as you do that, you should be fine.
     
  7. Toot
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    Toot Senior Member

    Bondo won't really do much for adhesion. It just sticks to itself pretty well. I would put more weight than that on it.

    How do you plan on removing the gas tank? Or don't you care? I would use a Hysol product. That's the best for bonding metal to fiberglass, however it still is very prone to delamination by vibration and, especially, heat. Google Hysol and you can get a feel for what their products can and can't do. 2 part epoxy will work too, but its heat resistance may be an issue and cause cracking over time.

    I would use that for the mechanical connection to the tank. I would use at least 3 square inches of bonding surface. And I would be sure I can lift the tank and seat off as a single assembly.

    Once that's done, you can fill it with just about anything. I would probably use the premium line of Bondo products just because it's easy to get ahold of. I'd start with their short hair filler. It has fiberglass strands embedded into it and it's designed for filling larger areas. Then I would finish it off with body filler to smooth it out, since it's exceptionally easy to sand.

    I'd nix the screw idea because screwing into a gas tank and the subsequent resealing opens a whole other messy can of worms. Just apply pressure, maybe a sandbag with tin foil underneath it. You shouldn't need much as epoxy (including Hysol) is fairly tolerant of excessive glue joint bond lines.
     
  8. fiberglass jack
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    fiberglass jack Senior Member

    u can sand the epoxy if any is showing also the bondo is only to shape the rough edges so it can be painted ,the seat is going to be mounted to the frame
     
  9. Toot
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    Toot Senior Member


    The problem I see, though, is that the gas tank has to occasionally be removed for cleaning- especially in climates with a lot of freeze-thaws which tend to make corrosion and such a bit of an issue. So although, structurally, it's mounted to the frame, it's still got to carry it's own weight if you are going to permanently attach it to the gas tank.

    To make the problem even a little more challenging, you've got a throbbing V-twin under there which can vibrate your molars loose and the tank and seat won't always be vibrating at the same frequency. It may not debond the epoxy, but it can certainly cause some small cracks to appear over time.


    FWIW, I do about 12K miles/yr on a V-twin crotch rocket.
     
  10. fiberglass jack
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    fiberglass jack Senior Member

    hes wondering how to finsh the front of the seat , it can be seen through a gap between the gas tank and seat he is not going to bond the seat to the tank look at his friends bike that is what he wants to do
     
  11. Handtool
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    Handtool New Member

    Right, I've got no plans to attach the seat to the tank in any way. Both the seat and the tank will be removable seperatly. I think I'm just going to trim the cut piece down to a couple inches and epoxy the hell out of it and throw some pop rivets in through the top. Hopefully if I use enough epoxy I won't need much filler. Thanks guys.
     

  12. Toot
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    Toot Senior Member

    Got it! I couldn't see the picture he posted yesterday. It wasn't coming up.


    Don't use pop rivets if you can avoid it. The holes you drill in the fiberglass (FRP) will be cracks waiting to happen. If you do use pop rivets, add some epoxy to the rivet and to the hole. This will help reseal some of the glass fibers that are freed and prone to delamination after you've drilled. The problem with rivets is that you have no control over how much compression force is being applied. Since this is a weakpoint with composites, you're better off going with screws or bolts, so long as you can stand the appearance.
     
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