Aero/wing question...with a twist...and my name is not Hyper
 

So, next year I plan on running a wing for autoX. I've been reading the rules for mounting such a rice device, and they are as follows...

-May be no more than 8 sq/ft
-May not hang further back thatn the rear most part of the rear bumper
-May not be wider than the body of the car
-May be NO higher than 6" above the highest point of the car (windshield frame)
-Must mount no further than the center point of the rear axle.
-No part of the wing can exceed 6" forward of the rear axle line.

So with that layed out, I got to wondering, I know everyone tends to mount wings to the most rearward position. Also, we know the Miatas profile isn't very friendly to aerodynamics...and in my case, I don't have a hard top. So I'm wondering about mounting a wing to the forward most possible position (within the rules allowed). My reasoning to seek insight is based partly on the old Chaparral race cars that had the wings mounted in middle of the car and on the idea of catching air right as is expands/sinks off the rear roof line to restrict airflow under the wing but not sit it right up against the top.

Attached is my quick rendering of the restrictions.

What do you guys think would be the effects of mounting a wing like this?

placing it there should expose it to clean air and generate downforce at a relatively low AOA and reduce drag. Going to build one yourself or use an of the shelf solution?

The further back you mount it, the more leverage you get on the rear tires. Put the element wherever you want, but make sure it mounts as far back as possible.

Placing it forward will also apply the downforce right on the rear axle, as opposed to creating a moment about the rear axle which (slightly) unloads the front tires.

It would be a very small difference, and is surely offset balance-wise by the weight of the turbo kit, and placing it further back applies the force behind the rear wheels without adding pendulum-like weight back there.

This is what you want for auto-x.

http://gotcone.com/pgallery/images/2...p/img_6124.jpg

^^^holy shit! lol

Funny thing about CSP...

They are allowed to add an element to the rear of the car, but it cannot act as a wing....so they're running those spoilers. I've often wondered how much difference in benefit there is between a large spoiler and a large wing like Chris Swearingen's car...

http://www.wrroadrace.com/images/chris_s.jpg

You might notice in that picture there is a spoiler and a wing. I believe that to be a legal configuration, but have never run it at a "big" event, so I don't know how it would hold up to protest.

I did a quick cut and past to show the comparison of mounting locations for you. I don't know that the forward location will get as clean an airflow as the rear section will. If you add a diffuser as part of the rear valance, the further back will help improve the performance of the diffuser.

Edit: I added my proposed splitter/air dam and canard changes as well

This is quoted for the truth... The further the point of applied pressure (wing) is from the fulcrum point (rear axle) the greater the torque (downforce) will be applied to the system. It's Physics baby!!!

Yep, you want a wing, not a spoiler like shown on the black car above. This is what we run on the car I am driving.
It is an APR piece. If you tell them you are an SCCA member, they give you a nice discount too.

http://farm3.static.flickr.com/2758/...4c7427fd_o.jpg

When you talk to them (APR) you want the 55" GT-II wing, but you want both elements to be the 7.5" high camber wings. You will wind up building your own end plates (14x14 inches to take full advantage of the allowance) and adding a Gurney flap to the upper element. Talk to them about custom mount spacing as well if you want to mount at the edges of the trunk lid.

sav's description seems slightly incorrect, but thagr81's clears it up to make me happy.

center of pressure * distance from pivot (wheels and/or CG) = rearward downforce moment. further back = more rear downforce.

I think you are going down the wrong direction. For big aerodynamics it would best to take the body off your car and weld a boxfish shaped sheetmetal to your frame.

http://www.carbodydesign.com/concept...Bionic-Car.jpg

Boxfish.

http://www.divegallery.com/boxfish_.jpg

See what I'm getting at?

You should probably worry about your own welding endeavors for now.

Glad I could bring some science to the conversation... I am a chemist afterall. Haha

Oh shut up. I am being a little silly with the boxfish thing but it's true:

See:

The bionic car - MSN Autos

My thoughts are that you should get a magazine feature before messing up the profile of your car. You have one of the best looking Miatas ever. Seriously..

faeflora-Don't worry, I have a spare trunk lid that i'll be using to affix any aerodynamic aids to...and will be keepinf the one that is currently on the car nice and cherry :)
I wish I knew someone who could help me get the car into a mag. SCC was going to do it, but they folded right in the middle of our discussions.

Chris- Don't worry, I have no forgotten a single thing you have discussed with me about aerodynamics :) I was just contemplating and take all information in. I wish we could run diffusers in SSM :(

thagr- I knows, about wing placement and how the rear axel acts as a fulcrum point. I am curious as to what might be the effects of putting the weight straight down on the wheels.

Why not try for a feature in a good import/car blog?

[threadjack]

I don't know where to start with something like that

[/threadjack]

**threadjack threadjack**

It should be fairly easy to get your car onto SpeedHunters at least - talk to Linhbergh :3

my question is how important is it to have the wing not sit on the trunk but on the chassis itself so that the trunk doesn't absorb the downforce?

Quite important, if your wing is generating any useful downforce. The trunk isn't designed to handle 200, 300, 400, 500 lbs of force pushing down on it.

but how can i tell how effective my wing is besides just feel. or the limit of the trunk.

I wouldn't discount feel. Setting up a car is both what makes the fastest car and how the driver feels about the car.

Decent wings will have downforce spec sheets, such as the GTC-300 that Emillio sells (link here APR Performance - GTC-300 - CFD Data & Analysis ) and that is the best way to get information. I'd generally say, if the manufacurer of the wing doesn't put out drag/downforce numbers, don't count on it being more than a styling choice.

One minor detail I seem to have forgotten. I would be using a large wing/spoiler like Chris's car and am looking to get as much as I can in downforce in the 20-55mph range and I figure catching the air right off the slope of the top (top would be up obviously), I would be able to get as much air as possible over the wing.

As for mounting the wing, I would mount it right on top of the ridege that holds the trunk seal which would keep trunk flex to a minimum and transfer of downforce to the body efficently.

Now, I do plan on eventually getting a different wing for track days...one that will sit as far back as possible and as high as possible....because there are no rules on track days.


See, on a nice sloped back car (like if I somehow got my hands on one of those new autokenexion fastbacks), a wing mounted far back works well with minimal AOA because the wing is forsing a lot of air over its top and the air going under isn't really turbulant.

*warning awesome picture*

http://www.seriouswheels.com/pics-20...l-1024x768.jpg

Attached is a pic of a NC in a wind tunnel and a description of the area in question.

http://www.imagelinkers.com/out.php/...vortex1111.JPG

Does that build more downforce at autox speeds than their GTC-300?

Yes.

The GTC wings are 3 dimensional airfoils. That means they are shaped to accomodate the airstream as it comes over the hardtop. In autocross, I don't have a top on my car.

I also think the dual element wings are capable of creating more down force than a single element wing given the same square inch surface area. I think 5 is the point of diminishing returns with regard to the number of elements. SSM limits you to two unfortunately, which I have never understood given the area limit.

Btw, these folks are popular among the SCCA autocrossers:

Wing Shop home page

Some of their airfoil data:
Wing Shop aerofoil facts page

Looks like at 50mph their dual element one at 20 degrees angle of attack is making 84 lbs downforce for the penalty of 2.5 hp

It would really depend on what type of air you wanted to catch with the wing. If you wanted to catch air that has already been introduced to turbulence moving the wing forward would help. However, if you wanted to catch clean air which is more linear in flow, the rearward mounting would be better. The flow that is seen in the NC can be helped with the use of vortex generators at the roof-line drop off to direct the air more linearly up and over the rear window and trunk lid but flow under the wing. This would be ideal... I will try to find the article where I saw the CFD's of before vortex generators and after...

EDIT- Found it... http://autospeed.com/cms/A_3059/article.html

Thanks for the info :)
But I don't think VGs will work all that well at autoX speeds :( Not to mention I would need a hard top to use them, and to use a hard top I would need to remove my roll bar for one that is HT compatable...too much work.

The question I am wondering is, for lower speeds (autoX), will it be a viable option to catch the air coming off th eroof line just before it's about to become turbulant along with catching clean air? Might there be any benifit?

Ah... Was not aware we were chatting about auto-x (I may have missed it). I would still want to catch clean air hitting the wing as compared to air that has had its velocity decreased by flowing over the roofline...

Is the discount vallid for NASA members as well?

Have a great day,
Jaerd

And the hardtop will add 40+ lbs to your car - not good

Not if you get one from Axis Power Racing... :p

http://lh4.ggpht.com/_ScASycO1DOk/Sy...Sg/6kyp7p0.gif

not sure, but this may help you with deciding on the wing placement.

Another question... Will this be for top up or down setup? Because that will have an effect.

And that is legal in SSM?? I doubt it.

I've said multiple times that it would be for top up action.


SSM doesn't limit running a hard top regardless what it's made from.

Yeah, right. Show me a Miata OEM hardtop which is lighter than 40+ lbs.
Or which rule allows you to run non-OEM hardtop:

http://cms.scca.com/documents/Solo_R...Solo_Rules.pdf

Hint: no such hardtop rule exist that I know of
The only hardtop related allowance in the rules involves Miatas but it is to remove the soft top so you're conveting the car to the NB Club Sport specs.
So, it is either top down, no top or an OEM hardtop and that's it.
If there is no specific rule allowing you lighter aftermarket hardtop, then the use of such is forbidden

TUNA, sharks, dolphins and even *I* swim faster than a boxfish, LOL. How would you like a tuna shaped car?

I figured it was for top up... Just questioned due to the CFD image posted by another user. I never said the APR Hardtop would be legal for your class. I just said you don't have to add weight when adding a hardtop.

Regionally, I doubt they would care if I showed up with a CF hard top...and the ATL region is pretty big. I mean, adding a hardtop regardless of material is added weight...and offers no aerodynamic aid at such low speeds. I have been through the ruls and have not found where it specifically says anything about hardtops and SSM. Care to direct me toward the exact section?

What screen name does he go by on the forums?

/threadjack

Linhbergh posts on cr.net (I know, I know D: ) as sonique128.

It offers as much aerodynamic aid as the spoilers and wings at these speeds. And if it was allowed you would see some of the wild CSP cars (like the Schenkermobile) with something like carbon fiber 1 lb hardtop specials at $$$$ cost.

That's the thing - if it doesn't say anything -> only stock is allowed. If you can't find something in the rules, it is forbidden.

The only allowance is deletion of the soft top, which converts the car to the Club Sport package (the one which had the OEM removable hardtop only and no soft top):

I have to agree a non stock hardtop is illegal in SSM. That's why most top CSP cars and above convert to the club sport no top at all configuration.
Then why do it?


EDIT:

While pursuing the rules for something else, I came across 16.1.n which "may" allow an alternate removable top
Might be worth a letter for clarification if you really want to pursue it.

And once again, another topic wanders off course... Sigh.

To the OP:
I see what you are saying about the wing being mounted over the rear axle directly to get direct downforce on it. However, you will also not receive as much force being placed on the wing due to the turbulant air striking the wing as compared to clean linear flowing air that is undisturbed. If it were me, I would make a sacrifice and better distribute downforce to the true fulcrum point (center of gravity) and apply that greater force throughout the entire car than at a specific point while trying to scavenge air that has already had an influence on the car. This will be make for a better handling and more predictable handling car as a whole. :2cents: Hope this helps better explain my logic...

That is for some potentialy dangerous hatches attached to the roof to get replaced with safer alternatives. The hardtop is not a roof-attached thing though, it is the roof itself. Maybe worth a letter but the answer is predictable ;)