Del Sol Aerodynamics

[krousdb]

So in another thread there was a discussion about the Del Sol and the relatively high drag (CD 0.378) that is caused in part by the targa design...

Originally Posted by MetroMPG

about the del sol... i wouldn't be surprised if that Cd is right.

the car's rear window design is a killer. airflow separates at the trailing edge of the roof, and likely doesn't re-attach by the end of the rear deck because it's fairly short. so you've got a relatively large trailing wake as a result.

to the wind it looks like a mini short-bed pickup truck with a tonneau cover. a "del camino"?

the "buttresses" on each side of the back window probably help to some extent (helping contain a bubble of air that the over-roof airflow can ride over without going instantly turbulent).

but if i were the betting type, i'd say the del sol is a particularly good candidate for aero mods in that area that would have a measurable impact on hwy mpg.

So I am stuck with a design that isn't optimal. But I love the look and its nice to have a removeable top and a rear window that opens.... which got me to thinking....

If I cruise with the side windors cracked open and the rear window open fully, might that decrease drag by providing an air flow through the rear, effectively bursting the bubble? Maybe the added drag caused by opening the windows would be mitigated by reduction of drag in the rear? A nice flow of air on a hot summer day?

And what about when the top is off? Leave the side windows up and the rear window down? That would provide more distance for the air flow to reattach at the rear? Both times I have had the roof off I was impressed on how quiet it was. With the side windows up and the rear window down, there was definately a flow of air coming over the front windshield and through the rear window. No turbulence inside of the cabin.

Am I on to something here?

[MetroMPG]

Originally Posted by krousdb

If I cruise with the side windors cracked open and the rear window open fully, might that decrease drag by providing an air flow through the rear, effectively bursting the bubble?

easy enough to test. do some before/after wool tuft runs.

i suspect you'll see turbulence all the way down the rear deck under closed-car driving. if you can achieve attached flow at the rear of the deck under some combination of window/roof opening, then you know you're on to something.

maybe all you need to do is put your heat/ac blower on MAX and blast the air out the back window cracked open a hair. like the renault concept car

[Matt Timion]

now that you have the supermid you can test this on the fly. If you manage to find a flat piece of land just give it a shot.

[krousdb]

Originally Posted by Matt Timion

now that you have the supermid you can test this on the fly. If you manage to find a flat piece of land just give it a shot.

Yes, thats the problem. Nowhere flat. I guess I could try a long stretch of highway, but without cruise control, I can't be sure that I applied the same pedal pressure up and down the hills. I need to think about this awhile. First I need to setal some of my wife's red yarn for the trunk lid. :-)

[krousdb]

Well I havent gotten around to taping yarn on my trunk lid and watching for turbulence with the windows closed and then with the top off and the rear windows down... But I did have some interesting results today....

Every weekend I drive to the inlaws house to check on it while they are in Florida. It is about a 25 mile round trip. The elevation is about 100ft lower at thier house than at mine.

So the trip out was 60.11 MPG, starting with the engine fairly warm, about 130F.

Before leaving for the trip back I removed the top and rolled down the rear window. Temp was 54F and the winds light and variable. While driving I put my hand out the rear window, over the trunk lid and could feel the air flow, so there is a possibility that the air flow could actually help the areodynamics by smoothing out the turbulence on the trunk lid due to the targa roof. Hopefully it offsets the drag created by having the roof off.

The trip back home was 62.7 MPG at a 100ft+ elevation change.

While this is not a scientifically valid test (really need to do two round trips, one with and one without the roof) it seems evident that driving with the roof off at 25-40MPH speeds does not significantly hurt FE.

Current tank, which also includes the trip back from the gas station...

32 miles, 59.5 MPG. :-)

W00t!

[Matt Timion]

That's amazing. I'm wondering if you have dimishing margin returns depending on the speed used. This would be an interesting thing to test at three different speeds

[krousdb]

So I finally got around to taping yarn on my trunk lid. One row at the end of the lid, one across the middle and one across the front, near the rear window. It didn't take much speed to see what is going on back there. The rear ones flail about toward the rear of the car. But the front and middle rows immediatly flip over and flail about toward the front of the car. No matter what speed, same results. Rear window open with side windows open, no change. Rear window open with roof off, no change. One hell of a lot of turbulance.

So I headed toward I-79 to take a highway test run to see what sort of mileage I can get now that I am SuperMID equipped. Temp was 60F with 8 MPH tailwind to start. Of course on the way back ot was a headwind. Terget speed 55 but I bled off speed going up hill down to 50, and let speed increase to 60 on the downhills.

Results:
- 22 miles with an 8MPH tailwind, 58.4 MPG.
- 22 miles with an 8MPH headwind, 56.5 MPG.
- 44 mile RT, 57.4 MPG.

What a pleasant surprise. I was only expecting 50MPG. The Prius is good for 65 MPG under the same conditions.

Lots of errands today also.

Current tank, 144 miles, 58.8 MPG.

[krousdb]

So I am wondering what the turbulence looks like on by trunk lid. Is it a swirling tornado with a horizontal axis is it more chaotic than that. I was thinking about a few options for smoothing out the turbulence, both which would leave my trunk unuseable.

First I could install a clear flat plastic plate that would close in the area between the side butresses, the roof and the trunk lid. This would give me some transition from the roof to the trunk lid instead of the cliff that I have now.

Or I could build a wedge shape out of clear plastic that would sit on the trunk lid, between the side butresses and extend from the roofline to the end of the trunk.

Can anyone draw me a picture of what the airflow would look like in each of the three setups, stock, flat plate and wedge?

Also, I am considering cutting vents into the rear bumper cover. There is an area just above the black lip that would make the vents very difficult to see. Any thoughts as to the benefit of this?

[SVOboy]

Also, I am considering cutting vents into the rear bumper cover. There is an area just above the black lip that would make the vents very difficult to see. Any thoughts as to the benefit of this?

I have heard this doesn't do too much, and I thinking that the better route would be to model yourself a rear diffuser, I will try to get some pictures and what not of DIY rear pans/diffusers for your car later today, or mehbe just general if there are none for del sols.

[SVOboy]

http://www.geocities.com/frontsidesk8er/delsolaero.xls

There ya go, should be hosted well and stuff, I will test download it again to make sure the link works in here.

[MetroMPG]

yay! krousdb is going to mod up his aero stuff! (we'll have rear wheel skirts on that puppy before you know it!)

Originally Posted by krousdb

I was thinking about a few options for smoothing out the turbulence, both which would leave my trunk unuseable.

you need a third option that preserves trunk functionality like this one i've been thinking about: an inflatable plastic shape that you can blow up like a beach ball, or with a 12v compressor, for use on highway trips. when not needed or wanted, deflate and store in the trunk.

in my wacky aero daydreams, i envision making an inflatable "boat tail" for the back of my car, to taper the flow a little further to reduce the cross section area where the flow separates. it would be lightweight, using straps to attatch to the hatchback edges (top & sides) and the bottom of the bumper.

the challenge is: (a) finding some material like the kind that air mattresses are made from that you could somehow heat bond the edges together; (b) for larger structures, you'd need to build internal baffle-like structures that would serve to tension the shape and keep it from "ballooning" in the pressure differences it's exposed to. (think of the shape of a convertible roof at highway speeds - not at all what it looks like when parked).

of course, a smaller inflatable shape on the deck of the del sol wouldn't have as much difficulty retaining its form compared to the big boat-tail needed to "fix" my upright hatchback design.

First I could install a clear flat plastic plate that would close in the area between the side butresses, the roof and the trunk lid. This would give me some transition from the roof to the trunk lid instead of the cliff that I have now.

i have read that you don't want to transition more than 11 degrees from the top or sides, or you end up with flow separation again. which means you may not be able to taper down from the roof edge right to the very tip of the trunk deck.

i suspect the optimal shape you end up with, observing the 11 degree limit, will have your taper ending some height above the trunk deck. surprise: it looks something like the back of a crx, prius or insight.

[krousdb]

Originally Posted by MetroMPG

i have read that you don't want to transition more than 11 degrees from the top or sides, or you end up with flow separation again. which means you may not be able to taper down from the roof edge right to the very tip of the trunk deck.

i suspect the optimal shape you end up with, observing the 11 degree limit, will have your taper ending some height above the trunk deck. surprise: it looks something like the back of a crx, prius or insight.

Right you are! The roofline is 11" above the trunk lid and the end of the trunk is 42" from the end of the roof, giving me a 15 degree angle. For an 11 degree angle, the tail would need to end 3" above the trunk lid.

[krousdb]

Hmmm. This might not be that bad looking after all.

[kickflipjr]

Not you too...

Wouldn't it be awesome if someone sold aftermarket wheel skirts that looked good. They could start with making them for civics (because they make every aftermarket part for civics).

Oh that cardboard dosen't look too bad. It would look better if it was red and made of steel

[krousdb]

Originally Posted by kickflipjr

Not you too...

Wouldn't it be awesome if someone sold aftermarket wheel skirts that looked good. They could start with making them for civics (because they make every aftermarket part for civics).

Oh that cardboard dosen't look too bad. It would look better if it was red and made of steel

I have a can of color matched spray paint so they will be red. Im thinking corrugated platic if I can get my hands on some.

[MetroMPG]

woo-hoo! another convert!

(i figure if i get enough of you guys adding wheel skirts, mine will become a little less conspicuous. no? didn't think so.)

[krousdb]

Kinda looks like an Insight doesnt it?

[Compaq888]

your design looks okay but the cartboard should go further down, and should flow with the rest of the body instead of just being straight. How come you aren't doing any under car airdonamic stuff? Lowering your car will just make the air more violent under the car. Get everything sealed under the car and you should see another couple of mpg.

[Matt Timion]

Originally Posted by MetroMPG

woo-hoo! another convert!

(i figure if i get enough of you guys adding wheel skirts, mine will become a little less conspicuous. no? didn't think so.)

This summer I plan on learning how to fiberglass. I'll then produce myself a set.

So is it generally accepted here that the mudflaps HURT aerodynamics? I'll remove them in a heartbeat if they do.

[krousdb]

Originally Posted by Compaq888

your design looks okay but the cartboard should go further down, and should flow with the rest of the body instead of just being straight. How come you aren't doing any under car airdonamic stuff? Lowering your car will just make the air more violent under the car. Get everything sealed under the car and you should see another couple of mpg.

I cant go any lower because the body tapers in and the cover will hit the tire. Yes, I will be doing the underbelly pan too. Just need to get my hans on the coroplast.

[Compaq888]

Originally Posted by krousdb

I cant go any lower because the body tapers in and the cover will hit the tire. Yes, I will be doing the underbelly pan too. Just need to get my hans on the coroplast.

then use metal since it bends. Make it bend around the tire.

[SVOboy]

I think he means the wheel well will hit the tire.

[n0rt0npr0]

if you have home depot or lowes in your neck of the woods you can get that Coroplast there

AND FYI those of you wanting to use this stuff for the underbody belly pan...

When force drying in conventional screen jet-driers, temperatures should not exceed 100 degrees F (43 degrees C). Higher temperatures will tend to warp or distort the Coroplast sheet.

[MetroMPG]

Originally Posted by krousdb

I cant go any lower because the body tapers in and the cover will hit the tire.

i had the same issue with mine. i took them lower by forming a curve in the cardboard (which you can do with whatever material you end up using). the curved shape in my "experimental" cardboard units was retained with a piece of coat hanger wire duct taped to the inside (back) of the skirt.

photos: they make the skirt look flatter than it really is, but see...

http://www.metrompg.com/posts/grille-blocking-part-2.htm

also, i used the rear mud guards as part of the "structure" that supports the rear/bottom part of the skirt. without them i would have had to form a complex curve to get the skirt around the rear of the tire and back to the bumper.

if you remove the guards, you lose that option. and i suspect the aero penalty of leaving the guard on is trumped by the larger benefit of a full skirt.

[Flatland2D]

Here's a few of my experiences driving my Del Sol, but nothing real significant that hasn't already been mentioned.

Air coming over the targa top wants to loop back around and enter inside the rear window. I've noticed this when leaves have fallen on the trunk. When I drive off with the windows down I get a car full of leaves.

Just cracking the side and rear windows is enough to get a good amount of airflow through the car. Without cracking the rear window, the difference is definitely noticable. I have also wondered if this would acually help MPG by filling the vacuum behind the rear window. Hopefully I can test this someday.

I haven't noticed much difference in MPG with the top on and windows up as opposed to top off and windows down. However, this wasn't the most accurate test. At the time I was only getting 25MPG highway with 87 octance gas. Check engine light came on for fuel system. Later I learned a little more about my engine (B18C1) and found out it required premium (I started it out on this when I first bought it, but didn't know it was required and used regular gas). Anyway, two hours of highway driving in each configuration yeilded the same MPG. If I had a better way to test this there would probably be some measurable difference, but I didn't find anything significant. Went back to premium and fuel economy went back up (~35MPG highway).

I've also tried to think of ways to fill in the void behind the rear window with some kind of cover, but there is the trunk problem and the device would have to be clear. I think we're pretty much stuck with that unless someone really wants to change the look of their car. If it bothers you that much, then a '92-95 Civic would be one's best bet for aerodynamics (one of the lowest CD's of cars in this price range).

[krousdb]

Originally Posted by n0rt0npr0

if you have home depot or lowes in your neck of the woods you can get that Coroplast there.

I have both in my neck of the woods and neither have it nor have they ever heard of it. I ended up getting some today at a sign shop. Guess I will work on it this weekend.

[krousdb]

Originally Posted by Flatland2D

Here's a few of my experiences driving my Del Sol, but nothing real significant that hasn't already been mentioned.

Air coming over the targa top wants to loop back around and enter inside the rear window. I've noticed this when leaves have fallen on the trunk. When I drive off with the windows down I get a car full of leaves.

Just cracking the side and rear windows is enough to get a good amount of airflow through the car. Without cracking the rear window, the difference is definitely noticable. I have also wondered if this would acually help MPG by filling the vacuum behind the rear window. Hopefully I can test this someday

Yes, I have noticed the same thing. I believe I mentioned earlier in this thread that I have already tested the effect of opening the rear and side windows. I placed rows of yarn across the front, center and rear of the trunk lid. As soon at you start moving, the end row of yarn points to the rear while the yarn on the middle and front rows flip backwards toward the from of the car. I treid all combinations of opening windows and removing the roof and in none of the cases did it cause the front and mid row of yarn to flip toward the back. So I have concluded that the added drag from opening the windows will not help reduce drag at the rear.

Ryland suggested a spoiler at the roof angled down. Might have possibilities.

[krousdb]

So I got some coroplast yesterday for the underbelly tray. I have spent lots of time on how to approach it. I ended up buying four 2'x 8' pieces and paid $40. No, Lowes and Home Depot do not carry coroplast in my area. I have been to five stores and people have no idea what I am talking about.

The del sol has a black moulding that runs on each side under the door that runs from the back of the front wheel well to the front of the rear wheel well. This moulding screws into frame with 5 or 6 screws. These screws will make a great attachment point for the coroplast. I will start on the drivers side 2ft wide from the rear bumper toward the front of the car, as far as 8ft will go, attaching with the moulding screws and into the lip of the rear bumper. I just have to figure out how to secure the other two sides. What I will end up with is the left and right side covered with about a 1ft strip down the middle that will not be covered, mostly because the exhaust runs down the middle and I dont want to start a fire. Hopefully I can finish it this weekend for testing next week.

Wish me luck.

[SVOboy]

I ended up buying four 2'x 8' pieces and was raped to the tune of $40.

Would you really consider 10 bucks for a sheet of plastic you chose to buy and accurate comparison to rape?

[krousdb]

Originally Posted by SVOboy

Would you really consider 10 bucks for a sheet of plastic you chose to buy and accurate comparison to rape?

I was venting because paying $40 for something that would cost $24 had it been available in my area at Home Depot seems excessive to me. Yes it was my choice so rape was not technically a correct way to put it. If you were offended then I apologize. I will edit my post.