Fiero notchback rear aero (NASCAR COT wing)

I'm going to a track day on the Roval course at Auto Club Speedway in Fontana next weekend, so I am rushing to implement a rear aero solution that adds downforce and aerodynamic stability (by moving the center of pressure toward the rear). Buttonwillow was fine with a loose car, but the walls at Fontana are less forgiving.

I purchased a Nascar Car of Tomorrow (COT) wing from Ebay (see earlier discussion). This is a carbon fiber wing made by Crawford Composites. It has a 54" span without the endplates. The COT wings were $3000 new, but can be found secondhand for $500 to $1000. They use an LNV109A airfoil. NASCAR stopped using these and switched back to spoilers because these wings generated a large amount of lift when traveling backwards (i.e. during a spin) which increased the probability of cars going airborne (1).

This type of airfoil performs very well in motorsports use (high L/D ratio) but is sensitive to imperfections in the surface (2). Luckily mine doesn't have any rock chips that I could find. We'll see how long that lasts.

There are two wings available -- one was used on show cars only, and is not really a structural part. Don't buy one of these. You can tell if it's a real race wing because it has CNC machined aluminum ends (you can see this with the endplates off), and 6 fasteners hold each of the mounts on. There are only 4 on the show wings. I think the show wings also don't have a slot for the gurney flap. There is some further discussion about the show wings in the earlier thread.

Carbon fiber endplates are available in two different configurations: curved and straight. The curves ones are aerofoils set up to pull toward the left. Duh, NASCAR! They also include a 1" aerofoil extension on each side, giving a total wing span of 56". The straight endlplates are not aerofoils themselves, but they do still have the 1" extensions. I don't have these right now, so I'm stuck with some aluminum endplates that the previous owner made, and have a 54" span.

The mounting plates are about 38" apart. This coincidentally lines them up perfectly with the end of the bumper crush structure on the 1987-1988 Fiero Coupe/Formula rear bumper.

I finished the steel bumper adapter brackets last week:

I mocked up the wing position on some perforated angle steel, and played around with some cardboard mockups for the upright shape.

This is one of the early mockups:

The wing is as far forward as possible without interfering with the opening of the decklid. The decklid misses it by 1/8". I could lower the wing but then it would have to go even further back. It's currently 11" above the decklid if I remember correctly. It's just slightly higher than the roof.

I settled on a design for the uprights but I'm not super happy with how they will look. I'll finish them off and see how it looks; I can always make new ones later. I need to get this project done in the next couple days so I don't have time to be picky about the design. I cut the uprights from 3/8" 6061-T6 aluminum. I still need to drill the holes and trim them a bit -- hopefully I'll have that done in the next couple days, but I want to finish my harness bar first. I bent the tubing today and cut 4 of the 6 brackets, so I should be able to knock that out tomorrow.

The reason I mounted it so far back and high is to improve the chance of it ending up in relatively clean airflow.

Once I have finished my smorgasbord of projects, I'll get tape some yarn to the rear clip and see what it's actually doing.

Should have this finished in a couple days...

i got two of them for 150 bucks. i think you paid a lil to much

BUT that being said. best bang for the buck and looks great

v8exocet wrote: i got two of them for 150 bucks. i think you paid a lil to much BUT that being said. best bang for the buck and looks great

That's not a normal price or even close to it.

ive gotten half a dozen total under 250 each. just gotta keep an eye on ebay and CL

v8exocet wrote: ive gotten half a dozen total under 250 each. just gotta keep an eye on ebay and CL

That's a pretty awesome deal. However, was it recent? There isn't a completed listed on ebay with a non-show wing for less than $500 in the past 3 months. The prices on these wings have been on the rise since the beginning of this year.

yeah, like within a month got them via craigslist

I guess you live in a region of the country where NASCAR exists. We don't get that stuff on CL here in SoCal... just IKEA furniture and wheels for the hellaflush crowd.

myrtle beach south carolina lol

when will you be showing the install of the undercar neon light kit that you are now required by law to put on the car?

I would suggest that there's a business opportunity here. If you can really get them on a regular basis for under $250, put 'em on eBay and double your money.

I just bought a 480 hp LS3 that has 15 miles on it for $1500. That's not typical, so it would be bad form for me to drop in on someone's thread and tell them they paid too much.

novaderrik wrote: when will you be showing the install of the undercar neon light kit that you are now required by law to put on the car?

I won't install one; I'm just going to try to sneak by without getting caught. I only drive this car to and from the track anyway.

EDIT: Oops, just realized the law you're referring to is in the NASA rulebook. What color should I run??

EDIT2: I just read on automotiveforums.com that the green lights are the faster ones... hmmm. Still not sure.

Keith Tanner wrote: I would suggest that there's a business opportunity here. If you can really get them on a regular basis for under $250, put 'em on eBay and double your money. I just bought a 480 hp LS3 that has 15 miles on it for $1500. That's not typical, so it would be bad form for me to drop in on someone's thread and tell them they paid too much.

i merely suggested to wait out for a good deal. there are many to be had. you may have to pay 50.00 in shipping, but there are enough out there on craigslist (hint: searchtempest) to warrant the adjustment in average price IMO.

For those just joining us, please excuse the noise above!

Now back to your regularly scheduled programming...

Got the wing uprights done and mounted! Finally some rear aero to complement the front.

With force applied by hand, the bumper beam is deflecting more than I'd like (but the 3/8" 4130 steel brackets I made are sturdy) so I'm going to add some reinforcements before hitting the track. The top of the brackets are attached to the relatively thin sheet metal of the bumper beam, which is unsupported on the front edge. I think I can just need tie that edge into the frame or box the beam to make it sturdy enough. It isn't unsafe the way it is now, it just deflects rearward by about half an inch up at the wing before it becomes solid.

I'll get back to this after I finish my harness bar tomorrow night. Err, if all goes well... hah.

more pics!

The aero induced deflection may not be a bad thing. The wing is at a higher angle of attack (downforce) at low speeds and as the speed increases the wing settles to a lower angle of attack and lower (but more conservative) downforce level. This higher speed condition keeps the wing in a more stable range with respect to stall. Add in the fact that the vehicle pitch may actually put the whole car in a slightly more nose down attitude and the deflection (aero elasticity) may be a good result. The downforce from the wing increases with the square of the speed, so the wing makes more physical force with more speed, even though it will be at a lower angle relative to the ground.
Most top level series count on the aero elasticity to maximize performance between low speed and high speed sections.

With its vertical rear window, the shape of the Fiero notchback presents some interesting challenges for rear aerodynamics.

I'll go into this more in my post-trackday analytical update, but I think the curved edge at the back of the notchback roof (the Fiero GT's fastback roof has a sharper trailing edge) contributes to filling the rear notch area with turbulent air.

If the edge is sharpened and straightened to allow clean flow separation this could improve the performance of the wing, and possibly also help suck more air from the decklid vents to improve engine bay ventilation and the effectiveness of the front splitter (by drawing more air from under the car and thus increasing underbody airflow velocity).

However, clean flow separation at the rear of the roof may also contribute to rear lift generation because it reduces the amount of air filling the notchback area.

Such a modification would also increase the size of the wake (more drag) which may be why GM didn't make the clean break in production. I wish I had a high resolution 3D model of the notchback Fiero to do some CFD on that, as I don't know how important it really is -- the boundary layer at the trailing edge of the roof may be thick enough that the specifics of the edge don't really matter.

I'll take some more pics this weekend at the track. Gotta finish my harness bar tonight.

stafford1500 wrote: The aero induced deflection may not be a bad thing.

That's an interesting point that I hadn't considered. I agree with your assessment, but in this case the amount of force required for the "initial takeup" is minimal and might make this elasticity ineffective.

If I get a chance tonight (ok, if I remember to), I'll put a pull gauge on it and measure the force. I think it's so little that it will already start to deflect before any significant drag is produced.

More pics as promised.

Uprights turned out alright... They look a little funny from some angles, I think I'll trim the top so its the same thickness all the way up.

It turns out it doesn't really flop around after all. Seems pretty stable up to 130 mph (I didn't go faster, decided to play it safe since I don't have a full cage). I stuck a GoPro to the rear window so I could see if it was shaking or doing anything weird at speed. No problems in the video review.

Plenty of grip at high speed, and the car's balance has changed at lower speeds (50 - 80 mph) too. Definitely a lot less tendency to oversteer. I never spun all weekend.

Close up of the first prototype of the wing mounts:

Some awesome shots by CaliPhotography:

I finally dumped my prototype parts in favor of some better-looking components.

I designed some new wing angle-adjustable endplates and had them laser-cut from 0.090" aluminum, then finished them with some dark gray camo vinyl scraps I had laying around.

I also designed new wing stands and had them water jet cut from 3/8" aluminum. I used my router with a template rounding bit to profile the front and rear edges for better aero, then painted them with Rustoleum semi-gloss black.

CAD models:

Finished parts:

Looking good! I love these wings. There are four of them sitting beside my desk All spoken for.

I think you could improve the efficiency of that setup with some bigger end plates. I believe these were set very close to the trunklids on the COT cars so the end plates were restricted in size due to that. Adding more area to the end plates, particularly on the lower part, should improve the function of the wing considerably.

BTW, I know of at least three or four variants of them. Race, Test (race without tethers) and a couple of show versions. One of the show versions I have here has no visible fasteners for the mounts and is made of a very different CF than the race/test units. It's quite flimsy - but I've been thinking that cutting some ribs for it and running a spar down the center before filling it with expanding foam would make it work well.

How hard would it be to incorporate active aero....

Hmmmmm, whose Miata was it that had the motorized rear wing - http://grassrootsmotorsports.com/forum/grm/active-aero/70768/page1/

Keith Tanner wrote: Race, Test (race without tethers) and a couple of show versions. [...]

Very interesting to know! I thought there was just Race and Show. Mine has the holes and steel end inserts for the tethers, so I guess it's the race wing. It didn't come with the tethers themselves though.

Do they all have the same width (without endplates)?

I recently started developing processes for casting rigid polyurethane parts where I work. I'm thinking about casting some fiberglass reinforced cast airfoil extensions that interlock with the CNC-machined end inserts of the Race wing and maybe even bolt into the original tether location. Then these wings can be made wider (at the expense of some weight). I could cast in an aluminum reinforcing rib and steel nuts for receiving the endplate screws.

I forgot to post my downforce / drag calculations for the NASCAR COT wing. The coefficients were generated using the airfoil coordinates from Airfoiltools.com for the LNV109A plugged into JavaFoil.

2014-05-17 Update: I fixed the drag calculation above. In the previous version of the chart it was about twice as high as it should have been.

Note that this is the force generated by the wing, not the actual force at the tires, which can vary due to fulcrum effects. There is no vehicle-specific data in these calculations. Also this data does NOT include a simulation of the gurney flap which extends ~0.5" up from the trailing edge of the wing.

I had settled on running the wing at 16 deg at Buttonwillow in the 13CW configuration (~75 mph average speed, fast corners at ~90 to 95 mph, with a lap time of 2:09). That track configuration has relatively short straights with low top speeds (a little over 100 mph) compared to Auto Club Speedway where I reach 130+.

I was running it with that configuration at Auto Club Speedway too but that track only has one high speed turn and it's the very high speed banked oval, so I think I can run a lot less angle there (or none) for better lap times.

They're all the same width as far as I know, although the flimsiest Show wings don't have removable end plates.

Did you figure what angle it goes into stall? Looks like it's still reasonable at 15 degrees.

Keith Tanner wrote: They're all the same width as far as I know, although the flimsiest Show wings don't have removable end plates. Did you figure what angle it goes into stall? Looks like it's still reasonable at 15 degrees.

Max downforce occurs at 19 degrees with a Cl of 1.855 and Cd of 0.12305. Compared to the specs at 16 degrees, that's a very small gain in downforce for a huge loss in efficiency. L/D is 19.61 at 16 degrees and 15.08 at 19 degrees. The efficiency drops rapidly above 16 degrees, which is why I didn't plot it beyond that.

I'll see tonight if I can get JavaFoil to simulate the gurney flap.