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Old 11-18-2007, 01:47 AM   #1
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Just putting a utopian thought out there for what would be a pretty cool way to improve your Cd. Inspired by this concept car, the bugE three wheeler and Basjoos' second windscreen. How hard do you think it would be to get an "optically blown cast acrylic clear canopy" made for a full sized car? Either having one part from the headlights to the roof and another detachable boat-tail or have it made in two parts with a split right down the middle which opens up like the doors of a De Lorean to let you in (using the roof rack mounts?). You can by a whole BugE kit for $3300 I wonder how much of that is the body, And I wonder if acrylic casting scales up very well. And i wonder how heavy it would be. As you can see I know absolutely nothing about this topic but I just think it would be pretty easy to optimise the aerodynamics shape in one big hit rather that constantly trying to make little changes. It would also look pretty amazing. Plus once you made one and got it to work you could sell it as a kit say for a Honda civic or whatever. Not expecting this is possible/feasible but it would be good to know the specific reasons why it wouldn't work (assuming there are reasons). Cheers
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Old 11-19-2007, 04:30 AM   #2
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I have some experience with clear acrylic canopies for boats. Tried this 10 years ago and failed. It was very hard to make them optically useful. The technology may have improved since then but probably not a low-tech or inexpensive solution.
Another problem is that cars are certified according to factory specification and you may run into legal problem doing extensive modifications. At least where I live...
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Old 11-19-2007, 06:43 AM   #3
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Like HAHA said, this isn't going to be cheap or easy. I assume, based on what you've said, that you intend to make this a DIY garage-tech project. Forget about casting. Molds would be much too expensive for a hobbyist, or even for a low-volume production run at the size of the parts you're talking about. You may want to look into vacuum forming or blow molding of sheet stock. Here is a link to a project using a similar type of process to make a recumbent bicycle fairing. If you're looking for curves this would probably be your best bet. Also, rather than trying to make a cocoon to enclose the entire car, it would probably be more feasible to work on the front and rear sections separately.

You'll likely run into some trouble getting the right shape. Expect to go through a lot of material$ in trial-and-error. You may need to blow it into a mold, which sounds to me like a finishing nightmare. Also expect differences in material thickness to make it a bit like driving while wearing someone else's prescription glasses. last, I'd consider something a little less brittle and fragile than acrylic. If you do this you will be very upset if the first rock from a truck tire destroys all of your hard work. Maybe consider Lexan (polycarbonate).

Not an easy project, but if you're up for a challenge check out what the streamliner (HPV) guys are doing.
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Old 11-19-2007, 11:05 AM   #4
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Glider aeroplanes sometimes have nice 360 degree canopies that look good. Too small for your application but probably less expensive than the F-16 canopy variety that is used so often on powerboats. Their manufacturing process seem to be working so this would be my first stop if trying to do something similar.

Btw. The experiments I referred to above were made with molds.
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Old 11-19-2007, 12:00 PM   #5
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Quote:
Not an easy project, but if you're up for a challenge check out what the streamliner (HPV) guys are doing.
How about this canopy

2007 UCF HPV (human powered vehicle)



It's not optical "grade" though... As theclencher said, it's freaking hard to keep wrinkles out. We used an industrial vacuum former thanks to Lockheed Martin's facilities and a sheet of PETG plastic (soda bottle plastic ).

As far as rating for optical gradeness... 100% being perfectly optical and 0 not being able to see anything... It's around a 60-70%. The distortions are a result of the heating elements. If there was a heat diffuser (rather than exposed element), it would be much better... The "wrinkles" are about 1-1.5" spaced apart and they aren't so much wrinkles as they are localized thin spots where the plastic got warmer than the surrounding areas. We also have some optical "issues" as a result of tight curvature (like eyeglasses ). For us, it's not a big deal and not really noticeable

Oh, and the mold...


It came from that^^. The surface was damn smooth too That was vacuum bagged off a female tool in two halfs - then combined with fiberglass tape.


------
So that's a rather expensive solution, if you're not doing other items like we were....

But that being said, PETG is amazingly flexible. You can pretty much fold a 4x8' sheet in half and it won't break (until you really make the curvature tight). If you build a suitable frame to support the edges and geometry, you might be able to do some simple shapes.... But for complex and tight curves (like our canopy windscreen), it's not a good option

------
As for how HPV streamliner canopies are made without using a vacuformed mold


Indirect and even heat using a template rather than a mold works great for closer to optical quality. BUT, the drawback is - you're limited to making bubbles with a cross section that matches your template. We couldn't do that for our canopy due to the sharp leading edge and overall wide shape :/

http://www.adventuresofgreg.com/HPVlog/04-02-06.html
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Old 11-19-2007, 01:59 PM   #6
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This is the canopy I referred to. It was vacuum formed over a mold. The problem was that it had to be very thick in order to protect the crew in case of an accident at 100 knots. None of the samples came out perfect even though some customers found them acceptable and actually used them (as can be seen in the picture). Later, the design was altered into this, which was much stronger, safer, better optically and more production friendly...
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