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Old 10-27-2010, 08:15 AM   #31 (permalink)
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If you have enough RAM (4GB minimum) you can run Fusion or Parallels Desktop to run Windows programs on an Intel-based Mac. These do cost ~$80.

Check out CADintosh, DoodleCAD, and other inexpensive programs:

Pure Mac: CAD - Software for Macintosh

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Old 10-27-2010, 10:24 AM   #32 (permalink)
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I think that I will look up the cost for a junk hatch. I was thinking that the odd shape of the hatch this would simplify things a lot. Then I can weld right to the hatch No need to use the hitch at all.

Neil: thanks for the CAD links I might just have to draw up my ideas now.

Any ideas for the bottom angel?
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Old 10-27-2010, 04:21 PM   #33 (permalink)
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Since you are probably going to be making the tail shorter than the optimum design drawing that you posted, I would suggest putting your taillights at the proper height and sloping the bottom up to meet the aft panel with the taillights.

How high should the taillights be? Either put them at the same height they are at now, or if your hatchback has higher than normal lights, check out a sedan and put them at the same light level as the sedan. You will want to look at it to see what looks good and what looks outrageously odd. Believe me, you will get looks.

I recommend making the aft panel that holds the taillights and license plate as small as possible, both in vertical and horizontal dimensions. I would suggest making the aft panel first, and fitting the rest of the panels using the aft panel, the hatch mounting system and the longest distance back that you think you can get away with, which is probably going to be about 5 feet.

The angles of the top and bottom panels should not have any hard bends in them. I'm sure there is an ideal angle for minimum air resistance (15d?), but your overall length may prevent you from using that angle. A tail is going to provide you with a huge (20% to 30%) improvement in FE, so optimizing it is good but not crucial.

I think you could make this out of coroplast and zip ties as long as it has a good mounting point. The problem I had with my coroplast tail structure was the mounting point that failed. Coroplast would be much lighter than any other construction method, and it goes together quickly. Maybe zip tie and tape a plexiglass section into the top panel.

I am probably using plywood if I ever get around to building the Tail Phase II. But I would rather use coroplast, it is the material of the future.
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Old 10-27-2010, 04:37 PM   #34 (permalink)
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I will see about the cloroplast after Nov 2

otherwise I was going to try foam like 3wheeler did.

I have a call into a junk yard to see how much a spare hatch would cost, if it is reasonable I will pick it up and have fun That one I will weld up a frame for the shape of the tail, then cover with what ever material I want.

You are right about making the back as small as possible. I wish I could draw it up but I am having issues importing a pic to base the design off of. Oh well.

I think for length I can get away with 6" on the top (the Kammback is 22" and is even with the back bumper).

What evidence of 20-30% gains do you have? that seems high to me, I was guessing about 10-15%.
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Old 10-27-2010, 05:15 PM   #35 (permalink)
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Want to consider an idea from far left field - how about an inflatable tail?

Make it out of clear plastic and the taillights can stay where they are and you will be able to see back through it, at least until it gets dirty. The weight would be negligible. Place a small reversible pump on the hatch, and run the control wire into the cabin and you can pump it up or down as needed. If you make an inner structure out of sealed tubes it could support an unpressurized outer sheath, and then the volume of air needed to open and close it would not be very large. Tubes would probably provide more control over its shape. Of course, your car is going to look like it has a large used condom on the back when the thing is deflated, and it might look, um, suggestive, as it pumps up. Unlike a fixed tail, this would let you still park in a relatively short space, one only a little longer than the car currently fits into.
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Old 10-27-2010, 05:36 PM   #36 (permalink)
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It is based on observations from the tail I made. I got about 25% improvement in FE over the EPA rating numbers even with a tail on a sedan, you may have better aero with the tail on the hatchback. It is somewhat hard to tell what was the tail and what was driving technique but I think 25% is accurate for the tail only. I could probably do an additional 20 to 30% over EPA by doing EOC and slow driving.

Foam and fiberglass will make a nice looking part but the time to build will be very high. I have worked in the boatbuilding industry and build boats as a hobby. The only time I would build with foam and glass is to make a plug for a mold. In order to get a good finish and lightweight fiberglass part, it is best to lay up fiberglass in a mold, vacuum bag it and pull the part out.

I generally prefer to use chine construction with plywood in stitch and glue construction method for boats. It is faster and easier to get to a finished hull. But that does not give you the nice curves as you can get in foam/fiberglass.

I am still up in the air about whether to use plywood or coroplast for my next tail. coroplast is really an amazing material for the weight and cost, I probably should stick with coroplast.

The program Hull Designer can be used to get cut out grids for panels from a 3D design that you lay out. Then attach the panels at their edges and the 3D design appears.

Carlson Design Plotter/Cutters

This software could be used to make a tail with multiple panels that is more rounded than the somewhat ugly four sided tail that I built.
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Old 10-27-2010, 05:42 PM   #37 (permalink)
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Nice ideas. The inflatable one is out, I like building things that hold their shape.

The cutter could be cool but the cost (24,000$$$) is just a wee bit out of my diy budget.
Where in MI are you? We might be able to meet up and brain storm ideas
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Old 10-27-2010, 05:58 PM   #38 (permalink)
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I use an exacto knife to cut coroplast, or a router if I have a wood pattern to follow.

I am in South Haven. My car is not modded very much right now, I am debating on selling it and getting something else. I am tired of the high rpms at highway speeds. The Versa is a good car in the city, but it would not be good for someone that has to commute on the highway every day.
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Old 10-27-2010, 06:13 PM   #39 (permalink)
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That is a good 3hr drive to your place

I use a knife too.
I did not hear back about the hatch I have a job fair to go to tomorrow but I might swing by the junk yard on Friday and see out it. If I get it then off to the steel store to buy materials I just might have some welding in my weekend plans. What to cover it with I have not decided but I can weld up the rough shape and cover with plastic for now

Metal ideas for the frame?
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Old 10-28-2010, 06:30 AM   #40 (permalink)
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Mine's built out of aluminum, using 1"x1/4" bar for the structure covered with .060" aluminum skin. The bars are held together with drilled and tapped screws and the skin is attached with either pop riviets or drilled and tapped screws. The Lexan window is attached by drilled and tapped screws.

The main problem with coroplast is that it gets brittle after a year or two in the sun unless you keep it protected under a layer of paint and paint doesn't stick to coroplast very well. The only places I use coroplast on my car are the belly pan where the sun doesn't reach it and for the side skirts which I keep painted.

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