Rear decklid spoiler: design discussion

[California98Civic]

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.

[19bonestock88]

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

[California98Civic]

Originally Posted by 19bonestock88

... 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.

[mpg_numbers_guy]

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

[California98Civic]

Originally Posted by mpg_numbers_guy

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.

[mpg_numbers_guy]

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.

[California98Civic]

Originally Posted by mpg_numbers_guy

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.

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?

Originally Posted by mpg_numbers_guy

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

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.

[Piotrsko]

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

[California98Civic]

Originally Posted by Piotrsko

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

[California98Civic]

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.