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Old 02-06-2010, 11:28 PM   #21 (permalink)
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Hi,

I've been do a little work on the drawings for my CarBEN EV concept:

The seats are intended to be stretched mesh over frames, so they will be as thin as possible, and breathable for good cooling. The arrangement of the seats are still not settled (nothing really is!) -- the idea is to have the driver sit in the center and to have everybody enter through the rear hatch (which includes much of the rear fascia). The battery pack I've drawing is 6" thick and is 4 feet square (8 cu. ft).

The angles are from the highest and lowest points to the trailing edge, and then tangent from the trailing edges.

The frontal area is 25.543 sq ft (see the 1 foot grid on the front view in the drawing) -- and IF the Cd is 0.10 (higher than the blue clay model of the Boxfish concept it is based on) -- then the CdA = 2.5543 sq ft! Wow, if I can pull that off...

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Old 02-07-2010, 10:43 AM   #22 (permalink)
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The M-B Bionic concept illustrates that a blunt front is okay, but a low drag car needs to have a very small rear. That explains a large part of the difference between the Cd=0.095 clay model and the Cd=0.20 concept car, which had a relatively large rear. The rest may be due to adding door handles, wipers, ground clearance, and panel gaps. CarBEN's rear is much larger than that of the clay model. As a percentage of frontal area, it's somewhere between that of the Cd=0.25 Insight and the Cd=0.188 EV1.

Traditionally, tall vehicles need a wide track for stability. However, going electric and mounting >1000lb of batteries below the floor gives you the option of being tall but not wide. See the Commuter Cars Tango for an example of this.

If you can find a way to make your car a foot or two narrower while staying within your project's goals, you'll end up with a much lower Cd, and a much much lower CdA. Such a car should be called the Trout, instead of the Boxfish. A conventional streamlined car has very long overall length to taper its roof down to meet its floorpan. Instead, the Trout is very narrow to achieve a similar Cd with a shorter overall length by having its sides taper inward until they meet. The Trout does well in a wind tunnel, but less well in a crosswind.

Thanks for the thought-provoking thread.
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Old 02-07-2010, 01:39 PM   #23 (permalink)
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Neil, how much reference material do you have to go from besides the one picture of the blue mock up model that MB made ?

I've been looking for more images of the original model ( the blue one ) but haven't seen any.
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Old 02-07-2010, 01:44 PM   #24 (permalink)
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I think that this project embodies the spirit that Ecomodders have.
When car manufacturers tease us with vaporware concepts that they never really intend to build .... we just build the damn thing ourselves !
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Old 02-07-2010, 02:47 PM   #25 (permalink)
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Neil,

Great idea, great work.

When you make the car, better make molds of the body work so you can sell
them and recoup your R&D costs. If you built around a readily available, older
chassis, you have a better chance of interesting others in your design.

It looks to me like he fifth seat is causing some serious design problems;
headroom, legroom, etc. Is it really necessary... maybe a fold down jump seat
instead?

Without it you could tighten up the vertical dimensions/angles back there and
would still have a useable trunk/cargo volume.
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Old 02-07-2010, 03:18 PM   #26 (permalink)
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Thanks for your responses, folks! I've been thinking a lot more about this project, and I will be writing things down and doing some more drawings. Here's what I'd like to do to check/refine the aerodynamics:

Build a scale wind tunnel! For the 1/24th scale of this model, it would be pretty easy --and I can even use my belt sander (upside down on the floor of the tunnel) to mimic the moving roadway, and to spin the tires...

Is this something I can find on Instructables? How would I make a scale sized smoke trail device?
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Old 02-07-2010, 03:24 PM   #27 (permalink)
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This will be an Open Source project!

Very important: This will be an Open Source project! I want to have others be able to use anything, or all of the ideas that are used for this. It's the principles of how and why this design can work that is important.

Here's what I mean by Open Source -- essentially the same as open source software, applied to engineering:

3 posts tagged ?open source information? - NeilBlanchard?s Blog on Vox

Quote:
Like how open source software -- or scientific peer review works, I think that we need to have open source information; about anything and everything. All good ideas need to be shared, and built up, and revised, and improved -- for the benefit of all. Patents are not working; not in the way they were intended. And good solutions to the world's challenges are too important to "sit on".
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Old 02-07-2010, 03:57 PM   #28 (permalink)
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The trouble with small models is that to keep the Reynold's # the same, the wind speed is increased in inverse proportion, and you'd be getting into sonic effects, (perhaps including those of the neighbors. )
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Old 02-07-2010, 04:31 PM   #29 (permalink)
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Thanks Bob -- I'm sorry to hear that! Yes 24X the air speed would be a big challenge! I guess that is why the pictures of the Mercedes scale models show them in a fluid tank? :-(

Okay, a few more points: yes that one picture is the only one I've found of the blue model with wheels. The (slightly revised) frontal area of the drawing of the CarBEN concept is 25.814 sq ft. and the area of the rear fascia is 10.166 sq ft. so the taper is ~2.5:1. Also, please note that my drawing is approximately 4" narrower per side on the rear fascia, than the wood model is.
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Old 02-14-2010, 12:08 AM   #30 (permalink)
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I've been working on the drawing of my CarBEN concept EV:

CarBEN 1/24th scale wooden model of EV - Updated 13 Feb 10 - Vox

If anybody would like a copy of the DWG file (or the DataCAD AEC file) I'd be happy to send it to you. I hopefully will be starting on the 3D CAD model soon.

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