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Aerodynamic Mods...


Rocan
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Who's done what? There is a ton to improve on with our 02s. I've spent a few years now studying fluid dynamics (yet I haven't taken the class yet... darn prerequisites keeping me from having fun) and I can't wait to have some fun with our 02s. I know the hill climb guys go all out (diffuser, flat belly, wings, etc), but us street guys should have some fun too. The following is my thoughts on a stock 02 without adding a front spoiler or doing too much lowering (which would both improve things). 

 

We all know how much lift our cars generate stock at speed. A lower front spoiler helps a ton, But I think we can do better. Flat bottoming the car wouldn't be too hard and should increase downforce a noticeable amount while decreasing wind resistance. The radiator also isn't ducted and a lot of flow is lost around the radiator. Some simple ducting will increase cooling and decrease drag significantly, but to do it correctly it will need to be ducted front and rear. Not a problem, but with a proper belly pan some kind of venting will be necessary (otherwise the air entering the engine bay will be stagnant and things will overheat, though I may be over-thinking this since our hoods dont seal the engine bay THAT well). 

 

The front wheels have a lot of exposed area especially on stock ride height. removing US spec spacers helps a good bit, but some simple deflectors as are common on a lot of new vehicles should clean things up.  

 

Some things may come as a surprise, but I actually feel that the 5mph bumpers improve the general flow characteristics over a no bumper or small bumper setup. Really we are fighting an uphill battle here since the front of our cars is shaped in a way that it forces air under the car, hence why we get so much lift. 

 

As we all know, exhaust vents into our car pretty heavily if the rear end isn't sealed up well. This tells me that there is a ton of turbulent air right behind the trunk area. There isn't much we can do up top thanks to the relatively boxy shape of the 02 (though it really is surprisingly good for a car designed in the 60s!) but if you don't mind losing trunk space, the tank and spare wheel well can easily be removed and a diffuser fitted to keep the rear end planted and improve flow both under and above the car. I actually feel that a diffuser would make such a big difference on our cars (considering how big we could make it without a spare wheel or tank) that if I ever ditch the rear seats, I wont mind losing the additional trunk space in order to get the better aerodynamics. 

 

 

 

Why am I obsessed with this? Because aerodynamics is the easiest and cheapest way to get greater fuel economy, a higher top speed, and faster acceleration above 60mph. I suspect 40+mpg is attainable at 55mph by just having a light right foot and a few hours spent on smoothing out the underside of the car. Also aerodynamics are fun. I once got bored in a parking lot while waiting for my parents at a market, so I bought some painters tape and found some cardboard boxes. A little thinking, a lot of taping, and a half hour later I made the front of my fathers ford explorer look pretty ghetto. His "what the F***" soon turned into a "wow!" when I was able to average an additional 8mpg just by making the front end smoother with a few dollars worth of tape. Prototyping with tape and cardboard is a great way to plan out what you need later when you make the final version out of plastic or composite material (or aluminum, if you like). 

 

Here are some pictures to help some of you visualize what the goals in streamlining are. 

 

aero_1.jpg

 

air%20flow.jpg

 

 

To those who think aerodynamics don't make a big difference, go talk to the 50cc Bonneville guys who get 120+mph out of 20hp. 

 

http://50ccs.blogspot.com/2007/09/50cc-land-speed-records.html

 

 

 

Disclaimer: This information is very very basic, and there is much more than meets the eye when it comes to aerodynamics. Adequate cooling, air ventilation, and many other practicality aspects must be considered when changing the airflow around a car. Although large improvements can be made with simple modifications, there comes a point when further improvements may become too difficult to achieve without major modifications, similarly to how you can get a lot of power out of a stock engine, but it is hard and expensive to get a bit more power out of an already modified engine. 

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Interesting thoughts. I've thought of smoothing the underside of the car, and moving my fuel tank to accommodate a diffuser. But I don't really know how to calculate the optimal design, so will be following this thread with interest.

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ditto Ian - next m2, whenever time and money permit that to happen, will get a flat floor and some kind of diffuser, but current thoughts are not based on any indepth knowledge, but that's never stopped me doing anything before!

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This is a very interesting topic. I also want to fool around with the underside of my 02, and see if I can design a decent diffuser for it. What intrigues me also is improving flow over the rear window.

Look at this photo, it probably help the rear wing a lot.

0.jpgbmw2002hansreith2gm3.jpg

Sorry for the crap size, but you can still see the plexi/lexan extension over the rear window.

If you are willing to cut your hood, you might want to vent the hot air out as far forward as the flow would allow, it would decrease drag at the base of the windshiels as well.

Regards

Edited by Cyclone101
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I like the flat bottom idea, and am currently debating what should be tucked up inside.  If you troll the LoCost boards, they have a flat bottom, which covers driveshaft and exhaust.  That's a bit extreme for our cars, but I have wondered about tucking the exhaust up into a tunnel in the floor to get it out of the airstream.  The problem comes at the diff, where you'd have to route it over the rear trailing arms.

 

I've also thought of making a splitter that extends back under the engine to seal the engine compartment from the road.  This would require an exit from the engine bay to get rid of the incoming air flow.  This would also potentially increase downforce on the top of the car.  I was thinking of using louvers; basically making an insert with louvers that could be welded or riveted into the hood.  Kind of like in this pic.  I've learned how to make the louvers by hand, but might have a die made up so I could use my hydraulic press to make them.  I could then make extras, if there was enough interest.  

 

The other thoughts I had was to make fender flairs with a vent at the rear to exhaust the air from the front wheel well, and to exhaust the air from the rear wheel well out the rear, possibly into the diffuser flow.

louver-1.jpg

Edited by Ian
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Here's a link to a pdf from 1980, where the author used his 2002tii in a wind tunnel at Duke University to get some numbers for aero reduction.  The bottom line is that an air dam with a splitter and rear trunk lip spoiler can give the most bang-for-the-buck.  

https://dl.dropboxusercontent.com/u/2328398/2002/2002.aero.pdf

 

Additionally, a hoop spoiler like on the old CSL's could help aim air down towards the trunk lid.

Spa-1976-07-25-003.JPG

 

Hey Lee, I didn't know you were advertising in the '70's ;)

Edited by Ian
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Thanks for the links guys. That article confirmed my thinking, which is nice. I was considering how to improve flow over the rear window, an although the csl hoop spoiler is moving in the right direction, it is not enough IMHO, and the design may increase lift without a spoiler. A better solution would be to have a smooth Continuation of the roof line down halfway past the windshield. If you draw a line from the roof line to to edge of the trunk, you get the general idea. That accompanied by a tall spoiler should make the flow a lot nicer. I'm thinking along the lines of a Merkur XR4ti wing after extending the roof line back and down to cheat and make a sort of boat tail, just with gaps. The gaps between the new roof line and spoiler should be small enough to allow flow OVER the gap rather than under, and the low pressure zone created above the trunk would increase downforce significantly while decreasing drag. I'll try and make a sketch later.

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Here are two nice links for aero mods for the DIY racecar builder:

 

http://grassrootsmotorsports.com/forum/grm/flat-bottom-aero/37013/page1/

 

http://www.gmecca.com/byorc/dtipsaerodynamics.html

 

I'm interested in the discussion of dimpling the floor of the car.  I'm thinking of making a flat floor for my '02, as I have to repair a bunch of rust in the floor anyway.  I've thought of running beads or raised panels to stiffen the floor, but this just adds drag to the panel.  Dimples might work, but would take a lot of work to make a die and then get all the dimples arranged properly. However, I wonder if reversed beads might work?  I have a bead roller, and cold just flip the dies so that the bead forms upward in the floor (towards the roof) instead of downwards towards the road.  Might this not work similar to dimples?

 

We do have to be concerned about cooling.  I've wondered about making removable panels to cover the driveshaft opening.  These could be attached with rivet nuts, but you would have to make sure you got adequate air into the tunnel.  Perhaps NACA ducts in the panel to route air into the driveshaft bay, similar to what is seen on the plastic undertrays or the e46? Then dump the transmission tunnel air out into the rear diffuser.  

 

Fun stuff!

Edited by Ian
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Awesome links Ian! 

 

Heres another good link on basic aerodynamic principles, ala NASA.

 

http://www.nasa.gov/centers/dryden/pdf/88628main_H-2283.pdf

 

 

I honestly do not think dimpling is worth the effort. The point of dimpling is to help the boundary layer disconnect from the surface... If you pay attention to some prototype vehicles that use dimpling properly, you will find that they only put dimples on the back side of the curves, where they want separation to occur. It's a similar concept to vortex generators. 

 

I was thinking about the continuation of the roof line and adding a spoiler some more and I think the best we can do would be a kamm tail, since you don't really want to exceed 22 degrees when angling the roofline (boat tailing). Simply adding a lip spoiler and the csl roof spoiler would actually be worse for drag, but probably help downforce a bit. Not what I am after. I have yet to do a sketch and calculate the angles... I'm a bit busy but will try to do a sketch by tonight. 

 

 

 

Another good link on boat tailing/truncated boat tails (kamm back) 

 

http://ecomodder.com/forum/showthread.php/maximum-angle-boat-tail-8927-2.html

 

 

Ecomodder is awesome for those who don't know about it. Those guys make hypermiling cool. 

Edited by Rocan
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Someone is making a rear window spoiler for the 02 it was in the old site, he makes all spoilers & fender flares .I am going to put one on my Pig Cheek Hybrid to complement the wing , I also intend to make a lip spoiler between the legs of the wing ala the Bat mobile e9

 

2012-09-05195330.jpg

 

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A better solution would be to have a smooth Continuation of the roof line down halfway past the windshield. If you draw a line from the roof line to to edge of the trunk, you get the general idea. That accompanied by a tall spoiler should make the flow a lot nicer.

 

 

Look at the rear window of this car, it has what I think you mean.

You can see it clearly at 1:57

 

Edited by Cyclone101
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cyclone, that is precisely what I mean. That car as a whole probably has most of what we are discussing... diffuser, air dam, and I would bet money it's flat bottomed. 

 

...Jesus though, that is one hell of a car! 

Edited by Rocan
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Someone is making a rear window spoiler for the 02 it was in the old site, he makes all spoilers & fender flares .I am going to put one on my Pig Cheek Hybrid to complement the wing , I also intend to make a lip spoiler between the legs of the wing ala the Bat mobile e9

 

That would be Rey Rivera, REY949 is his user name, he is in SoCal.

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Using the very common 4:1 ratio for boat tailing, I was surprised to find that the rear end of the 02 is almost spot on for a kamm back of the correct proportions. With a window louver (more or less exactly whats in the video) and a small trunk lip (more or less the 2002 turbo lip) a 20* kammback can be made without much work at all and without interfering with rear visibility too greatly (so long as the louver is clear plexi). Aside from the improvement in aerodynamics, I think this would look pretty badass. I would estimate about 5% aero reduction from this alone. 

 

With a diffuser, flat underbody, and front spoiler I think around 15-20% reduction in drag is attainable with about 10% increase in downforce (maybe even more considering that the front lifts in stock form). 

post-42517-0-57701900-1367805749_thumb.j

Edited by Rocan
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