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?

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. :D

I'll consider that, but I also hope to remove and reinstall it for testing.

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.

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?

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.

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.

I used coroplast for my decklid extension, matching the curve of the trunk lid and adding 7” to the length, adding big side plates for a sort of box cavity formed at the back of the trunk lid, and I’ve seen some benefit from it, though my construction left a little to be desired and I’m considering getting brand new coroplast and making a MK IV design... I’ve 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...

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 think this looks really good, and it's really close to design I have been thinking about:

https://ecomodder.com/forum/attachme...p;d=1563090839

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.

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.

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.

https://ecomodder.com/forum/attachme...1&d=1529866887

https://ecomodder.com/forum/attachme...1&d=1529866739

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.

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.

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 they’re wide enough to park your Honda in, let alone a layer of coroplast lol...

I’m 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...

Thanks for remembering my old box cavity study.

http://ecomodder.com/forum/attachmen...p;d=1373843754

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:

https://ecomodder.com/forum/member-c...coming-off.jpg

https://ecomodder.com/forum/member-c...er-remains.jpg

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

Besides length, my guess would be that the sides weren't helping due to not being right at the edge of the car for obvious legal reasons, and the bottom should probably continue straight back from the end of an underbody panel (it may already have been doing that, it's just hard to see from the photo if such was the case).

Vortex generators are quite debatable as to whether they improve or hurt fuel economy, but this article talks about someone fixing a similar detached airflow spot on the rear window and trunk lid of his Mercedes. Might go for an interesting ABA test.

What do you use to cut coroplast? Unless it's straight lines, I find it pretty hard to get a "perfect" cut on coroplast, no matter what I use. I haven't tried a jigsaw yet though lol..

Had a fail with the spoiler prject Thursday/Friday. Expanding foam in a wax paper and cardboard mold had expected difficulty curing but had unexpected difficulty filling the entire void and therefore picking up the full civic trunklid body contour. Bummer. Gonna make an easily removable cardboard version for coast-down testing and dust trails at El Mirage tomorrow.

My thought is that generally a box cavity is a way to simulate the boattail. In effect, the angle of departure for the flow to the trailing edge of the cavity members should not exceed what would remain attached with an actual boattail design, and research has shown that to be an effective design parameter.

I understand your claim, but don't really follow that linked article's argument. I am not saying it is wrong. I am saying it is beyond my training. Lemme ask you to comment on the test I did, based on this thread: https://ecomodder.com/forum/showthre...tml#post602571

Just working off my own intuition myself, but I think the reason boxing it had minimal effect over the simple spoiler could be viewed in terms of area. I.e., there is a lot more potentially turbulent air coming off the top of the car vs. the sides. I think that changes as proportions shift with more vertical design areas such as with station wagons and trucks.

I just took the whole thing off. It got wet in unexpected rain and even with black paint it warped a bit and it became hard to imagine it was working great. Nonetheless, my average to work was quite a bit lower.

I am going to rebuild and I think I will go with a fiberglass over cardboard build.

I suggest pink house insulation in 1/2" thicknesses. Or Styrofoam since both shape easily.

Dollar tree has 1/4 thick paper wraped foamboard for a buck a sheet that I use for electric model airplanes.

Alumalite signboard material is also a more permanent option that could look nice. I haven't worked with it myself though. Kinda wondering if it wouldn't work out to make some single-sheeted panels as well with coroplast. Don't ask me what it takes to bond polystyrene (polypropylene?) to aluminum though . . :rolleyes:

Prime the aluminum with self etching primer, and use foam safe sikaflex (221?). The stuff is fairly easy to work with but once cured there's no getting it off. It cleans up when wet with plain water too, so that's nice, but it dries quickly.

polymetal & magnets
 

Considering a 6mm high 4'×8' sheet of black polymetal and 3mm high or 4mm high neodymium magnets with countersunk holes in them. Washers, counter sink screws, nuts, and some sort of plastic or rubber insulator for the paint/magnet contact area would complete the kit. Would make underside supports from scrap because turnbuckles seem too heavy and conspicuously like a racecar. People already mistake Black & Green for a drift car (lol).

I could use a hole saw to set the magnets into the polymetal so the sheet is flush with the decklid. Thoughts?

another prototype
 

Wanting to have the flat spoiler back on the car because I'm trying to get 70 MPG and it seems to be a working mod, I decided to prototype again last night. I got the paper wrapped foam board for a dollar/sheet at Dollar Tree as was recommended earlier in this thread. It cut really easily of course. I put it in place. I gave it a layer of black remnant paint in order to give it water resistance. The paint, possibly because it's only on one side, warped the form as it dried overnight. So I found the spoiler kind of sticking up a little when I came out this morning, and it was pulling apart the duct tape connecting it to the supports under the spoiler. Here it is after I devised methods for holding it down more firmly and a little more flat... But I'm disappointed. I guess I should have painted on both sides or not painted it at all. Nonetheless, I'm going to run it. Polymetal will be the next, final version, I think.

https://ecomodder.com/forum/attachme...1&d=1564678063

https://ecomodder.com/forum/attachme...1&d=1564678192

That doesn’t look too bad? Not gonna wrap it down over the sides?

Not on this version. This is one of the two shapes that I tested at El Mirage. It tested as being almost as effective. And since its design is simpler, it doesn't obstruct any part with a tail light fixtures, and it doesn't present complications for getting into the trunk, thought I would test it out and see how it performs on the road.

On my commute this morning it seemed to help. I know that can just be confirmation bias, but remember this is the same design that I tested at El Mirage and their a much better test showed that it does actually benefit. Anyway, my tank average is creeping closer to 70 again after two days a falling off towards 68.

Nice! Makes me wish I had tried harder to get a better on on my Civic. It's looking good!

I took it off today because it was warping enough to burst all the duct tape. It just kept warping over time. I think the paper wrapped foam board is probably not a great candidate. But I NEED another model because without the spoiler again I struggle (again) to get over the 70mpg threshold. Today's morning run yielded 68.xx again. I was well over 70 in that same trip average when I took the spoiler off this morning. These are not scientific data points, obviously, but given that this is the design that tested approx 6.5% better in coast down testing at El Mirage, I am confident I am probably seeing the costs of not running the spoiler, at least in part. I may fiberglass my original cardboard template today.

That's one of the issues I had with mine. I didn't see a way to securely attach it without drilling an aluminum plate onto the hatch or doing something with fiberglass.

Maybe a cheap hatch from someone parting out a car you can drill into? but that would be quite the undertaking.

I chose not to fiberglass today. The template is badly warped and I have no other materials. Think I could fiberglass over paper-wrapped foam board? $2 would get me enough for the experiment tomorrow. Think it might warp as the glass & resin dries?

No drilling. I still think I will want to use maybe 5 neodymium magnets, 3 or 4mm high, with countersink holes to take screws that will point upward from the decklid. A hole cutting drill bit will make it possible to set the magnet into the spoiler so it lays flush on the decklid. Maybe 4 good layers of plastidip at the magnet/trunklid contact point could reduce unnecessary paint damage. A few self locking nuts could secure the spoiler to the protruding screws on the trunk. The spoiler material I am considering making out of 6mm black polymetal. Polymetal, I am assured, will not as easily warp, like these cheaper prototyping materials have.

foamboard is really neat stuff.
worst case, 2layers. walmart FB is thicker/denser than DTFB. SO FAR seems mostly VOC proof in light applications. the stuff I make my toy planes from is gas and fuel resistant.. minwax polyurethane makes it everything resistant

This makes me want to experiment further with foam board before committing to polymetal. So I got a little more paper wrapped foam board ($2.xx for two sections at dollar tree). I already have polyester fiberglass and enough remnant interior latex paint to try what this guy did... Five coats of interior latex paint worked for this guy: https://www.youtube.com/watch?v=dru51u-vLCs

Last night I made a fiberglass decklid spoiler using my cardboard template as the model. Polyester resin. Dried flat over night. At 7am it was flat. At 2pm when I got home it was warped in the sun. I am really frustrated with this constant warping of every material I attempt to use. I am reading that a heat gun can render it somewhat pliable to flatten again, but I am pessimistic.

Maybe fiberglass it in a frame and cut it out after? Else slump-formed sheet ABS?

I gently forced the fiberglass flat yesterday and weighed it down with bricks. By morning it was holding flat without the bricks but I kept them in place and let direct sunlight heat it again all day. Just finished working it witha heat gun. Trying to learn how to manipulate this matterial for future. The thing is ugly and amature, but it is only meant to be temporary. I will make another soon. This goes onto the car before I leave for work in the morning, maybe.

Change your epoxy? Polyester resin is famous to warping when dry and taking forever to really cure.

I already have a supply of polyester, so I am committed for now. And supposedly the polyester is the new style. A thought about making it more rigid... maybe some wire mesh, thicker than a screen, between two layers of glass.

Here is what it looks like today, after it was reattached yesterday. Ugly! I am out of black duct tape and I have some old pink duct tape from my daughter that has been laying around for years... using it up. Not gonna waste paint on this temporary build.

https://ecomodder.com/forum/attachme...1&d=1565711753

A later iteration might be straight-sided to that crease that comes off the C-pillar, with downturned winglets to the corner of the tail light.

Any issues with warping today?

Yup. But I am not entirely surprised. Polyester takes forever to cure it seems. So far it is not really serious warping, but...

Clearly there needs to be extended curing time (like a month) or some sort of structure or reinforcement within the piece. Such issues make the material less appealing. Basically, it wants to expand in the heat of the sun, and so it warps. The cardboard underside might not be helping, but I remember a previous piece I made for the interior and as soon as I took the cardboard off (soaking in water) the piece warped without heat.

Maybe multiple layers would help?

Heat affects polyester cure. Needs to be really warm and held flat for a week or so to expedite cure.

Yes, I have been reading about that online. Some people say even as much as 30 days. I want to take back a little bit what I said about warping yesterday. Before driving this morning I went out to take a good a closer look at it, and although it has deformed it's quite slide this time, unlike that first morning after I had made it tonight before. I believe that's probably because of the heat curing I had done in the last few days. I left it waited flat out in the bright California Sun for several days, and I don't want evening I went out there with a heat gun to reinforce that heat to get it up pass the temperature is it would have been experiencing from the Sun, and then left it waited overnight and another day flat. There's no doubt about it... it has not warped as much this time as it did last time.

Glue cardboard sheets into a block and cut the part so the corrugations run normal to the long dimension.