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Old 08-16-2011, 12:26 PM   #11 (permalink)
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think of the foam as a form or shape. It has a certain rigidity, but the glass and epoxy are what is strong. Lay up the outside of your foam shape with a single layer of cloth and epoxy, add filler coats so you can sand safely, and smooth it out. You need those filler coats so you don't sand into the glass. It takes out all the strength if you do that.
Now, turn it over and glass the inside. At this point you can remove the wood framing, if your design allows for that. The foam and glass inside and out are plenty strong.
The inside has no "smooth" requirements. You can imbed short steel straps under several layers of glass and epoxy, and use those to drill up for mounting screws.
If you are moving your tail lights or otherwise covering them up with this boat tail, you can use that pocket in the car for attaching those mounting straps.
That takes care of the midpoint attachment.
Some people have installed inexpensive bumper hitches to take most of the mounting duty. Put the hitch on the car and make a bracket to fit between it and your tail.


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Old 08-16-2011, 01:08 PM   #12 (permalink)
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Here is a simple way to do foam/fiberglass. If you don't care about what the finish looks like, you can skip the bondo, and just paint.

If you are going to do sheet metal over a steel substrate, you may want to look into how ambulances are built. The big box that goes on the back is aluminum welded into a frame, and the skin is aluminum sheet that is adheared with double sided tape (industrial grade no doubt). They spray foam in between the aluminum structure, cut off the excess and double sided tape the interior into place and use vynal as a floor. There are lots of reasons for using tape over rivits/screws. It is less expensive, seals to the outside better (particularly important in an ambulance), holds better, and offers an additional layer of insulation for both heating/cooling and sound deadening.
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Old 08-17-2011, 05:21 PM   #13 (permalink)
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Thanks guys. I looked through the whole "insight tail extension" thread and wyatt's link so I think I have a pretty good understanding of to do it. I'll be starting a thread whenever I finally get a car.
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Old 08-17-2011, 06:31 PM   #14 (permalink)
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have you looked at what I did with my boat tail? I made it out of fiberglass and I had very little skills on how to do it.
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Old 08-17-2011, 07:50 PM   #15 (permalink)
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Originally Posted by HydroJim View Post
I'm currently in the planning stages of all my aeromods and was thinking of the best way to seal a boat tail made out of sheet metal. To understand what I'm talking about look at the blue areas of my attached picture.
My point is a question about design not fabrication: what are the planned angles for the top, bottom defuser, and how are you accounting for the stock tumblehome of the sides? Top and bottom have ideal parameters according to the "template" and other posts available here. Accounting for the tumblehome can be vexing and is one reason I postponed my trunk-lid project indefinitely.

I love the feedback you are getting re: fiberglass and steel. Good stuff, that advice.

See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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