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Old 12-05-2012, 02:58 AM   #1 (permalink)
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overlaying template on softtop?

Hi everyone,
Planning to do some aero testing with some cardboard this winter with my MR2. I don't have time to tuft test just now but I figure doing a template overlay is a good first step...except I don't have a clue as to how to do this with my softtop!
Here's a good side shot of a Spyder:
http://www.modifiedplanet.com/wp-con...eed-wheels.jpg

The 2nd last hoop and last hoop form a nice angle that you'd expect from the end of a roof, but I don't know if I should treat it differently. At highway speed the fabric tends to lift up a little so it doesn't look as bumpy.

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Old 12-05-2012, 11:51 AM   #2 (permalink)
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If you're building a Kamm with the top up, use that. If you're building one to replace the soft top (Miata hardtop style) use an image with the top down. If the top is down you'll have to use the windshield and roll bar as a starting point and plot your own roof apex.

Here is a more realistic side view.



With a (soft?) top, for head room reference.


Be wary of frontal area when designing the roof.
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Old 12-05-2012, 02:00 PM   #3 (permalink)
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Here's this. I drew in a "Stuff the Wake" shape that would optimize the Cd of this car in the lower image there.

Here it is bigger. (Ecomodders forum format doesn't let you put very big pics in here, so here it is sort of by itself)

Bigger MR2 Pic
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Old 12-05-2012, 10:30 PM   #4 (permalink)
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Yikes.

I should probably say that I am not trying to build a Kammback or roof, I feel like that would take way too much time especially if I want the softtop to work (I don't actually like top down driving that much and I'd rather have a hardtop or targa anyday, but since the top is there I do put it down if it seems like it's not going to rain and I'm not going on the highway). The plan is to first investigate the flow on the back with tufts in 2 weeks, and then start with a spoiler/rear deck extension. I don't want to do anything too drastic because the car looks really freaking cool from the back stock and I don't want to mess that up. Aside from that I'm going to try removing the rear "diaper" under the muffler because it is almost certainly adding drag, and not only that it's quite big. I might consider putting a smooth "diffuser" in its place, if it's not too difficult to do.

If I can figure out how to stylistically do some buttresses along the side I will consider that.

How about vortex generators to try to get the air to attach earlier?

Last edited by serialk11r; 12-05-2012 at 10:54 PM..
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Old 12-06-2012, 06:25 PM   #5 (permalink)
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Quote:
The plan is to first investigate the flow on the back with tufts in 2 weeks, and then start with a spoiler/rear deck extension. Aside from that I'm going to try removing the rear "diaper" under the muffler because it is almost certainly adding drag, and not only that it's quite big. I might consider putting a smooth "diffuser" in its place, if it's not too difficult to do.
A spoiler or [horizontal] Bonneville plate would occupy the black area under the curve.


My suggestion, wheel spats:


Quote:
I don't want to do anything too drastic because the car looks really freaking cool from the back stock...
Quote:
How about vortex generators to try to get the air to attach earlier?
OK, now you're just trolling.
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Old 12-06-2012, 09:19 PM   #6 (permalink)
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Quote:
Originally Posted by freebeard View Post
A spoiler or [horizontal] Bonneville plate would occupy the black area under the curve.


My suggestion, wheel spats:


OK, now you're just trolling.
lol, I'm not, I consider vortex generators to be pretty low key as far as aesthetics go. The concern is that the rear end is so far inside the template, maybe it would be useful to get the air to attach earlier but I don't really understand how it works, so it's just a guess.

Thanks for the wheel spats suggestion, that really wasn't on my mind. I think they'd have to be more subtle than what you drew in because the stock springs are actually somewhat soft so I wouldn't want to risk my spats getting destroyed.

However I don't have the stock mudguards shown in that image, so it'll be a little harder to attach to the body. I did study the rear bumper behind the wheel and it seem like a good aerodynamic shape, but it just sits a bit high as of now. If the stock struts need replacing I will probably just lower the car a little while I'm at it, right now the car is able to go through most of the smoother speed bumps at a full 20mph (there are those annoying speed bumps in some areas that are a rectangular block, and it's impossible to not scrape on them no matter how slowly I go because the car just falls off the end and impacts), I wouldn't mind having to slow down to 10 and be more careful about curbs. If it's lowered that would reduce the wheels' drag by some amount. The fenders poke out past the wheels a decent amount so I'm not too concerned with the wheel spokes themselves.

The trickiest modification I'd like to do is some buttresses along the side a la Ferrari 360 Spider, but the aesthetics are once again a bit tricky.

Last edited by serialk11r; 12-06-2012 at 09:31 PM..
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Old 12-07-2012, 03:38 AM   #7 (permalink)
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When I had the suspension done on my weekly driver we lowered it until it looked *good* and then raised it about an inch so it only scrapes on some of the speed bumps. Apparently it helps to traverse them an an angle, if possible.

OTOH, my VW panel van can roll across the extruded curbs that they use in parking lots. It would get spats before lowering.

The spats are on the aero template are optimal, the factory mud guards are the other extreme, and anything in between would be beneficial. A reasonable material would be conveyor belting. It's made to take abuse and would just flex and rebound. Here's a sketch done for a pickup truck:


You might also look at the approach on Basjoos' Civic, a double walled air dam between the front and back wheels. It seems to work for him.

To get the buttresses to work with your soft top they would have to be inflatable. And to reach the template line they would have to be like tail fins. And then they would only affect 20% or less of the air moving over the top [maybe].
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Old 12-08-2012, 10:52 PM   #8 (permalink)
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Hmmm so the difficulty with rear wheel fairings is that as you can see the wheel is almost right up against the bumper which is curved, so it'll be a little hard to attach things to.

I noticed that the rear diaper was replaced for 300 dollars by the previous owner looking through some service logs, which makes me feel bad about taking it off. I guess I should pay it some respect and put some coroplast or something where it used to be.

Lowering would be easier and improves appearance at the same time, although it costs money to do and I don't want to cough up money, heh. However if I lower the car, and then install larger rear tires (205/55/15 instead of 205/50/15 for 10mm increase in height off the ground) then that'll offset it a little, and give the car a bit of "rake" towards the front which will help marginally with the wake.

I don't think the buttresses will interfere with top operation because the top is narrower at the top than at the sides. The buttresses can come up until the window's round corner starts, and I think the top should fold just like it does stock. However it's tempting to fabricate a Kammback starting from the buttresses that goes over the end of the top!

I need to just sit down for a few hours and do a really nice sketch of my car during break or something, then I could imagine how buttresses would look. Currently I am thinking painting them metallic gunmetal gray, that should look good against the red of the car. I don't mind them being not high enough for optimal drag reduction if they look really good haha.

I will still tuft test but it seems like a spoiler will have to be raised some amount over the rear deck if it's going to get anywhere near the template and not poke out too far behind. One of the best things about this car is how easy it is to park because it's so short!

Last edited by serialk11r; 12-08-2012 at 11:01 PM..
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Old 12-09-2012, 05:41 PM   #9 (permalink)
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Quote:
Hmmm so the difficulty with rear wheel fairings is that as you can see the wheel is almost right up against the bumper which is curved, so it'll be a little hard to attach things to.
Abaft the rear wheel, it looks like you could have a V-shape with a vertical rear edge. And in front they could be rounded instead of the flat version in the drawing (it was for a truck). we're only talking about the tread width 205mm. Front-to-back about 200mm in front and 300-350mm (~1 foot) in back would be good proportions. Then lower it and let them wear off on the bottom.

Quote:
I noticed that the rear diaper was replaced for 300 dollars by the previous owner looking through some service logs, which makes me feel bad about taking it off. I guess I should pay it some respect and put some coroplast or something where it used to be.
Going strictly by the side view drawing, it looks like it's not hanging lower. A rear partial belly pan might be enough.

Quote:
...However it's tempting to fabricate a Kammback starting from the buttresses that goes over the end of the top! (I will still tuft test but it seems like a spoiler will have to be raised some amount over the rear deck if it's going to get anywhere near the template and not poke out too far behind. One of the best things about this car is how easy it is to park because it's so short!)

I need to just sit down for a few hours and do a really nice sketch of my car during break or something, then I could imagine how buttresses would look...
Do it! Try laying the top view Sven7 posted over a top view of the GM EV-1 electric. It is a very aerodynamic car, but doesn't follow the template as much as it does Jaray:


Do you know what a Carson top is? As much as you say you use the soft top, a Carson top with a nice skylight or T-top might serve well.
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Old 12-12-2012, 01:10 PM   #10 (permalink)
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Study pics. Here's a hardtop conversion of an MR-S, with a flying buttress design. There have been some who attempted to fit a Spoon fastback on an MR2...Good luck on your addition.
http://www.dieselupload.com/files/1/toms_mr2.jpg
http://www.dieselupload.com/files/1/toms_mr2a.jpg

Spoon fastback. http://i73.photobucket.com/albums/i2...pe2003/109.jpg

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