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Old 07-12-2019, 03:02 PM   #1 (permalink)
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Rear decklid spoiler: design discussion

I am considering a "quick" build of a decklid spoiler to hav some fun testingat El Mirage july 21. I am thinking of a small one that extends only as far as the rear bumper (legal limit), is flat/flush with decklid, and curves around side tumblehome as far as taillights. Make it of expanding foam, molding it on the car using wax paper for a "mold release". I might incorporate easily removed side panels for a "box cavity." Maybe finish with fiberglass. Maybe paint it OEM black.

I am thinking of neodynium magnets for attachment but too little time maybe for an order online, so... thoughts about attachment? Other shaping methods and tools?

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Old 07-12-2019, 08:37 PM   #2 (permalink)
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Duct tape over painter's tape held mine on....at <40 MPH speeds....until the rain destroyed it...that was back when I was still using duct tape covered cardboard on the Civic.

Maybe attach aluminum mounting brackets through the holes for the license plate?

Wild idea: extend your bumper further back so you can go longer on the tail extension.
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Old 07-13-2019, 01:47 PM   #3 (permalink)
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Quote:
Originally Posted by mpg_numbers_guy View Post
Duct tape over painter's tape held mine on....at <40 MPH speeds....until the rain destroyed it...that was back when I was still using duct tape covered cardboard on the Civic.
I'll consider that, but I also hope to remove and reinstall it for testing.

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Originally Posted by mpg_numbers_guy View Post
Maybe attach aluminum mounting cor brackets through the holes for the license plate?
Maybe some combination of that and a small amount of tape for the purposes of testing, then something more permanent if I like the results.

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Originally Posted by mpg_numbers_guy View Post
Wild idea: extend your bumper further back so you can go longer on the tail extension.
Hahaha! Can't do that (yet), but I am a little concerned that it might be too short if Idon't push the legal length a little. What do you (or anyone else) think of a small box cavity shape to augment the shorter spoiler?
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Old 07-14-2019, 12:25 AM   #4 (permalink)
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I'll consider that, but I also hope to remove and reinstall it for testing.
Painter's tape doesn't damage the paint like duct tape, which is why I used it. It's durability and stickiness leaves a lot to be desired for permanent use though.

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Hahaha! Can't do that (yet), but I am a little concerned that it might be too short if Idon't push the legal length a little. What do you (or anyone else) think of a small box cavity shape to augment the shorter spoiler?
I saw a small benefit to mine, but mine extended out about a foot past the rear lid. How far can yours extend legally?

A box cavity would probably help some, the tricky thing though is tail light visibility, and if the cavity is too far inward (i.e., starting inward of the tail lights) then it's benefit would likely be nil.
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Old 07-14-2019, 03:54 AM   #5 (permalink)
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I used coroplast for my decklid extension, matching the curve of the trunk lid and adding 7Ē to the length, adding big side plates for a sort of box cavity formed at the back of the trunk lid, and Iíve seen some benefit from it, though my construction left a little to be desired and Iím considering getting brand new coroplast and making a MK IV design... Iíve bexome fond of wrapping visible portions of my coroplast in CF vinyl to hide the texture, makes the mod appear more pleasing to the normies...
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Old 07-14-2019, 11:16 AM   #6 (permalink)
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Quote:
Originally Posted by mpg_numbers_guy View Post
... I saw a small benefit to mine, but mine extended out about a foot past the rear lid. How far can yours extend legally? ....
I remember measuring about 7 inches once a long time ago when I considered this, but I'm going to have to measure again. I have a feeling I'm going to want to push the legal limit a little. Did you have any method for measuring the benefit of the spoiler? Did you do any tuft testing or coast-down testing or anything like that? If you have any pictures handy I'd love to see a picture of what you installed.

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I used coroplast for my decklid extension, matching the curve of the trunk lid and adding 7Ē to the length, adding big side plates for a sort of box cavity formed at the back of the trunk lid and Iíve seen some benefit from it...
I think this looks really good, and it's really close to design I have been thinking about:



I have some clear lexan, and I've been thinking about using it to build supports that would fit into the trunk lid gap like yours do. That would still allow visibility to the part of the Civic's tail lights that are on the trunk lid itself, if that makes sense. But in truth, supports mounted directly to license plate screws is the simplest. I think you're correct that is small box-cavity-style support that is well in from the edge of the body in order to avoid obstructing the tail light is not going to be effective areodynamically at all.
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Old 07-14-2019, 12:37 PM   #7 (permalink)
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Cardboard.

I use cardboard for a lot of quick prototype work.

Once you have it to where you like things, you can coat it with polyester resin and fiberglass cloth. This makes it quite rigid and strong. For looks, just wrap it in Ebay Auto body wrap which is also fast and cheap to apply.

Look up "Cardboard Construction Techniques" on YouTube for a vast array of how to videos.
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Old 07-15-2019, 12:01 AM   #8 (permalink)
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Quote:
Originally Posted by California98Civic View Post
I remember measuring about 7 inches once a long time ago when I considered this, but I'm going to have to measure again. I have a feeling I'm going to want to push the legal limit a little. Did you have any method for measuring the benefit of the spoiler? Did you do any tuft testing or coast-down testing or anything like that? If you have any pictures handy I'd love to see a picture of what you installed.
I posted some pictures and details here on the Civic's build thread. I really can't recommend doing what I did due to it not holding on well as well as just not looking the best aesthetically. 19bonestock88's is a much better design.





Didn't you try something similar here with a box cavity once?

I'll have to see if I can find it again, but there was someone on here with a 7th gen Civic coupe who ABA tested a decent improvement with a full box cavity - almost a boattail - at the rear end of the car starting at the trunk lid.

Quote:
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I have some clear lexan, and I've been thinking about using it to build supports that would fit into the trunk lid gap like yours do. That would still allow visibility to the part of the Civic's tail lights that are on the trunk lid itself, if that makes sense...
I thought about doing that, but then I used my small amount of available clear plastic to do those mirror boattails I tried out. Another user on here (might've been thedonkeycrx) got pulled over for having clear lexan "obscuring" the taillights on a VX a long time ago, something to keep in mind depending on how strict the police are in your area.
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Old 07-15-2019, 10:58 AM   #9 (permalink)
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I really can't recommend doing what I did due to it not holding on well as well as just not looking the best aesthetically. 19bonestock88's is a much better design.
.
The success of my design comes primarily from my driving a Saturn Ion. My trunk lid is designed so that the end plate is plastic and the end plate makes up the last couple inches of the top, my spoiler overlaps the trunk lid top, by the same amount as the end plate, so I could screw the spoiler directly into the end plate for a secure attachment. The panel gaps are also very typical of mid 2000s GM, in that theyíre wide enough to park your Honda in, let alone a layer of coroplast lol...

Iím getting to where I wanna redo it though,(though I am thoroughly in love with the look) I shoulda chose some better coroplast, taken more time with my cuts, made my underside supports more carefully...
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Old 07-15-2019, 12:09 PM   #10 (permalink)
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Thanks for remembering my old box cavity study.



The results were totally equivocal, and the experience put me off on the idea that there was anything much to be gained. However, I have wondered a little if it wasn't because the box cavity was a bit on the small side. Since then I have added a rear diffuser, substantial wheel well covers, & smoother, closed wheel rim surfaces. Studies of these kinds of mods that I have seen tend to show that there's a lot of interaction between the different mods. So, I have wondered whether a differently shaped and longer trunk lid spoiler would have a better effect. I don't know, but I think there's reason to think it might. And I'm going up to El Mirage this weekend anyway...

On the basis of photos of dust and water trails on the car, aerohead taught me to see the vortices coming off the C Pillars and spinning off behind the car:





The tufts and the dry zones in the mist water suggest both an area of recirculation near the middle of the rear window/trunk and vortices spinning I guess clockwise off the c-pillars as air from the top collides with air from the side. So I am thinking of a shape that might reduce the energy in that vortex. I think I might try a design that wraps around the curves at the corners over the tail lights. I will make it a little longer than legal, too, probably, given what you guys have been running. If I understand correctly, there might be potential there. I am probably wrong again. lol

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