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Old 10-28-2006, 09:14 AM   #1
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Question Rather confused...

I'm still learning about aerodynamics, but I've been repeatly stumped when it comes to the rear of a car.
Which is actually better, the rounded corners like most cars, or the chopped off strait down rear that creates a kammback?
Or does it really depend on the cross-section of the car?

That being said, what the heck do I need to do to these rear corners to make less drag? Working with one panel at a time to make the design less aerodynamicly challenged. These are first.
Anything at all is possible, so hit me with the best ideas.

As it stand right now:



Pardon the mess. Er, I mean creative chaos.
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Old 10-28-2006, 09:20 AM   #2
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Have you seen the civic aerodynamics thread? The end model that basjoos is using is about optimal from my understanding.
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Old 10-28-2006, 09:30 AM   #3
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Which is actually better, the rounded corners like most cars, or the chopped off strait down rear that creates a kammback?
I've never been able to find a conclusive answer to that question myself.

EDIT: I'm assuming you're asking specifically about the specific side/rear corner/transition, as opposed to the general shape of the back of the car.


I've found contradictory answers as well. (Which might also just be me misunderstanding the particulars of the answers than them actually being contradictory.)

Your work looks interesting! Are you going to glass over the foam? What kind of foam is it? (I read somewhere that some kinds dissolve under the resin - I'll probably be doing something similar to make the permanent version of my partial Kammback .
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Old 10-28-2006, 09:35 AM   #4
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If its the "to radius or not to radius" question - aka the side/rear transition, here's an earlier thread that may or may not help... Help me understand aero: rear corner radii, yes or no?
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Old 10-28-2006, 09:51 AM   #5
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Quote:
Originally Posted by MetroMPG
I've never been able to find a conclusive answer to that question myself.

I've found contradictory answers as well. (Which might also just be me misunderstanding the particulars of the answers than them actually being contradictory.)

Your work looks interesting! Are you going to glass over the foam? What kind of foam is it? (I read somewhere that some kinds dissolve under the resin - I'll probably be doing something similar to make the permanent version of my partial Kammback .

I did see the Civic topic, but the Civic is more hatch than this car. Imagine it being more like a DelSol or MR2.
So I'm unsure if the same ideas would apply to it.



One dollar sheets of insulation foam from Lowes! Resin would gobble it up, so I seal my stuff in fiber roof sealer first then glass over it. Gives me the same foam composite for 1/4 the cost.

Unless I hear otherwise I'll just leave the corners alone and add a partial Kammback wing like you did, Metro(which looks great, by the way). Might also drop the center section down and bring the underside up to meet it like they did in the Honda HSC. Only the center section would be deeper.
That would create three different streams though, wouldn't it?

Oi oi! Aero be damned, style is so much easier.


Honda HSC for referance(or direct coping if I feel lazy):

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Old 10-28-2006, 09:57 AM   #6
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Quote:
Originally Posted by MetroMPG
Are you going to glass over the foam? What kind of foam is it? I read somewhere that some kinds dissolve under the resin
Normal white foam (eg -new tv box packing) will disolve or deform when a solvent or an epoxy is put on it., whether that be from glues or f/glass.

It is also not the best to use as shaping with a sander or file can rip out individual balls of foam leaving a hole.

Unfortuantely the best stuff that you can glass over directly is what they make surfboards out of.
Its great to use , but expensive.

If you want to go the cheap way (white foam) that too is possible.
Just shape it up the best you can and then paint the whoel thing with a few good coats of water based house paint (not gloss).

After that is realy - realy dry (days) coat over the whole thing with body filler.
Use a good filler that is easily sanded.
Cheap fillers go like rock and are less flexible and more prone to pinholing.

After the shape is finalised you can glass over the top or make a mold from this piece.
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Old 10-28-2006, 10:00 AM   #7
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Quote:
Originally Posted by MetroMPG
If its the "to radius or not to radius" question - aka the side/rear transition, here's an earlier thread that may or may not help... Help me understand aero: rear corner radii, yes or no?
Ah! That's exactly the same question I was asking!
So yes, it really helped and the information in that thread seems to point to keeping the edges sharp like the bottom part of the bumper.

Thanks agian, that link ubber helped. I'll be sure to hit you up agian on the next part.
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Old 10-28-2006, 10:05 AM   #8
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Hey I think if you used epoxy on the white styrene foam it will not attack it but what you want is the urethane foam - just don't inhale the sanding dust - it's really bad stuff cancer causer.
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Old 10-28-2006, 10:08 AM   #9
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Quote:
Originally Posted by onegammyleg
Normal white foam (eg -new tv box packing) will disolve or deform when a solvent or an epoxy is put on it., whether that be from glues or f/glass.

It is also not the best to use as shaping with a sander or file can rip out individual balls of foam leaving a hole.

Unfortuantely the best stuff that you can glass over directly is what they make surfboards out of.
Its great to use , but expensive.

If you want to go the cheap way (white foam) that too is possible.
Just shape it up the best you can and then paint the whoel thing with a few good coats of water based house paint (not gloss).

After that is realy - realy dry (days) coat over the whole thing with body filler.
Use a good filler that is easily sanded.
Cheap fillers go like rock and are less flexible and more prone to pinholing.

After the shape is finalised you can glass over the top or make a mold from this piece.
I use lightweight spackling instead of body filler. MUCH lighter(since I won't be removing the foam weight is key), and easy to sand. The roof liner seals everything from the resin and when glassed everything turns out peachy and really light.

I never could get housepaint to work for me, as the heat of the resin always melts the foam anyhow. If I just stop being cheap and bought Epoxy resin I wouldn't have to worry about any of that.
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Old 10-28-2006, 10:50 AM   #10
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Quote:
Originally Posted by nonnef
I never could get housepaint to work for me, as the heat of the resin always melts the foam anyhow.
Thats why I recomended going over it with body filler , the heat is a lot less.
I have made quite intricate bonnet scoops this way and its been fine.
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