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Old 12-30-2012, 11:41 AM   #1 (permalink)
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Air dam and grill block design

So here's what I'm starting with:



You can see the obvious room for improvement with the deep pockets in the front bumper cover and grille. So this begs the question: what is the main goal in creating a grille or air dam cover? By this I mean is it better to prevent air from flowing under the vehicle as the air dam pockets will allow, or to attempt to force air around the front of the vehicle, or both?

This will have some design consequences for the bumper cover. It would be simple enough to just remove the louvered grilles and replace with a flat piece of material, leaving the "pockets" but blocking air from traveling through and under the truck. Would it be more beneficial to smooth the entire leading surface? Or would the difference be negligible?

For the grille block, the same holds true... if I simply fit a piece of flat material inside the grille opening, less air enters the radiator but the "pocket" remains. Ideally, should I attempt to fasten the material to the leading edge of the grille so that the deep pocket is removed?

Thanks for your input!

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Old 12-30-2012, 01:03 PM   #2 (permalink)
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The air dam is meant to keep air from flowing through low hanging items. ie. suspension, axle, oil pan, etc. so it can flow under as long as the airflow doesn't attach to pieces underneath.

as far as a grill block from best to worst as far as aero. I could see all 3 being equals as far as warm up
1) on leading edge. This creates a smooth surface to flow over or around the vehicle.
2) In front of grill. It creates a bubble of air that allows the air to flow over but air would still escape around edges.
3) behind grill. This is the most stealthy but air can now cycle around the grill creating a small amount of drag.
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Old 12-30-2012, 02:13 PM   #3 (permalink)
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I originally thought there was a direct mpg benefit, but after I did some ABA testing on my truck I found I lost about 1% (0.2mpg). However, a grill block does help shorten the warm up cycle, and for my truck, it is letting me get a few more degrees on my warm air intake. And in some (cold) locations, using a grill block is the best way to keep engine temps up...
My grill block (for testing) was basically a flat piece of cardboard covering 80% of the front grill. If you were to change the shape of the front of the vehicle, I am guessing your results could be different, based upon this info at NASA: Shape Effects on Drag
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Old 12-30-2012, 02:54 PM   #4 (permalink)
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It looks like you've got the same front end design as my 2000 Jimmy. (see attached thumbnail - click on it to enlarge) If you don't care anything about "stealth", you could do what I intend to.... attach the bottom of the grille block at the little "lip" in the bumper (a "natural shelf"), and the top to the very top of the grille chrome. This makes a very nice slope back that really "rounds off" the front end nicely. Then I continued the block over the front of the headlights with clear plexiglass. I had a couple of pieces 12" by 24" and they fit well enough I didn't even need to trim them. (although some trimming would help). Then I used 2" wide clear packing tape to go around the sides to the front of the fender. With the exception of the middle piece of cardboard & duct tape, it looked a lot better than I expected it to.

Similarly, for the 2 holes in the lower bumper (where my tow hooks stuck through), there is also a "lip" of sorts around the edges that I found perfect to fit the blocks to. Now, cardboard and grey duct tape isn't exactly the most beautiful way to do it, but either clear pieces of plexiglass so the hole still "shows", or a painted piece of something (fiberglass? - coroplast?) to either match the bumper color or maybe flat black to add a contrast would probably look better. Then I'm thinking about "glueing" it on with silicone sealant under the edges instead of the ugly duct tape.

Now for the air dam question, my understanding is keeping as much air as possible from going under the car is most critical for high speed stability (as in a race car). But for lower speeds (say, 70 and under), streamlining the front by rounding it off without adding more frontal area would be better. So for my car, instead of bringing an air dam straight down to some point the same height as the lowest hanging stuff underneath, I plan to make a "belly pan transistion" from the bottom of the front bumper to some place under the engine/skid plate. This will actually slope down as it goes back, but at the speeds I drive, I'm not worried about "lift". - LOL

If some other aero guru would like to chime in and either verify my theory or correct me, I'd welcome it.

EDIT: The grille/headlight/tow hook covers as shown in the thumbnail yielded 1.45 mpg increase in careful A-B-A testing with a ScanGauge. However, it sealed off the air to the radiator so well, I had to cut a 6" diameter hole in it to prevent overheating.
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Old 12-30-2012, 03:14 PM   #5 (permalink)
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[QUOTE=wmjinman;347792]With the exception of the middle piece of cardboard & duct tape, it looked a lot better than I expected it to.[QUOTE]

So ugly, yet so beautiful. Nice work, now start cleaning it up!
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Old 12-30-2012, 03:41 PM   #6 (permalink)
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[QUOTE=Smurf;347796][QUOTE=wmjinman;347792]With the exception of the middle piece of cardboard & duct tape, it looked a lot better than I expected it to.
Quote:

So ugly, yet so beautiful. Nice work, now start cleaning it up!
Yup! LOL!!! Thanx.

I'm thinking about trying to find a big enough clear piece of plexiglass or lexan to make the whole thing out of it. That might be the "cleanest" look of all - allowing the stock front end to show, and maybe only noticable when reflections hit it and/or looking at it from a side angle. And instead of the single 6" diameter hole, making a row of smaller (2" or 2.5") holes along the bottom of the actual grille area. If I calculate the actual sq. inches, I could make it more than the single 6" hole, but cover some (starting on the outside) with that clear packing tape as needed. If summer comes & I find it's running hot, just start ripping tape off the holes to adjust....
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Old 12-30-2012, 04:06 PM   #7 (permalink)
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Hmm.. Perhaps use the buttons like Jeep vinyl tops do, and form a ring around the top of the bumper and the front of the hood, and the very front of the fenders behind the corner lenses. Then, snap on a clear durable plastic sheeting to cover the area. Getting Lexan to cooperate with curves isn't easy, at least in my extremely limited attempts.

Or, bend some aluminum flat strips, and screw it in under the hood and on the top front of the bumper. One on each side, about 18" apart. Then you have solid mounting points for each corner, maybe seal from the strips back to the grille to direct air (and keep it out of the headlight area), and can figure out a changeable center section. That center section could be made to be filled with a flat piece of Lexan, and you can carry a couple different "grilles" with you in the truck. A 50% block, 80%, 100%, etc.

Brainstorming out loud, seeing what can stick.
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Old 12-30-2012, 04:58 PM   #8 (permalink)
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Yeah, good ideas, Smurf.

I learned the first time, when I went to check my oil, that you don't want it actually attached to the hood! LOL!!! But, no sweat - attaching to the top of the grille trim and the areas over the headlights works fine (with duct tape, at least)

Using 3 sheets could have an added benefit of reinforcing it at the joints by having double thickness there. This would help resist the oncoming air from pushing it into a concave shape at speed (although, I seldom drive "at speed" anymore once I found out how much better my mpg is going SLOW). I'd thought of putting dividers/baffles on each side of the grille to direct air, but then thought it would be better to leave it open so the pressures could "equalize" better. (maybe not an issue, though - dunno). On the other hand, those dividers/baffles could also act as reinforcement to prevent the potential concave effect. Maybe a win-win.

I've never tried it, but supposedly lexan and/or plexiglass can be bent with heat - around 200 deg. F, I understand. So maybe I could "customize" the "headlight covers" so they wrap around to the fronts of the fenders. I had been thinking the way to do it would be to put the whole thing in an oven (the one in the kitchen, of course), but the fiancee isn't too keen on that idea. However, a better option might be to heat up a big piece of pipe, lay the lexan/plexiglass on it where I want the curve, then use a hair dryer or heat gun while applying pressure & see what happens.

...Continuing the brainstorming out-loud...
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Old 12-30-2012, 05:18 PM   #9 (permalink)
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Good work on the front sloping cover. I'm mulling a similar idea for my truck.
For LEXAN, you might try rid-out plastics online. Plexiglass sheets, Acrylic plastic, Lexan sheet, Plexiglass Fabrication, Fiberglass Panels, PTFE I ordered some thinner sheets from them (0.020") for my wheel covers. You could also search for a local business that sells corrugated plastics and such... Polycarbonate Sheets Dealer | Lexan Sheet | Teflon Tube
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Old 12-30-2012, 05:24 PM   #10 (permalink)
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EDIT: Econoram, I just went to your garage & looked at the pictures of your belly pan & front wheel deflectors. I'd like to try to do something along those lines, too. I'd really love to get my Jimmy to where it can do 30 mpg on a tank. Probably won't ever be able to do that in mixed driving though. I have done it on the highway before, though - even before eco-modding.


Yeah, I'm thinking about looking for thinner stuff, too. There's a Lowe's home improvement store about 4 blocks from my house that has plexiglass & lexan. I'm expecting to be able to find something there. If not - - - the search will be expanded.


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