Prepared Rear Spoiler - How Badly Am I Compromising

As some may know, I have a 1981 Camaro that I have built for the SCCA's C-Prepared autocross class.  In the class, we are allowed to run a rear spoiler:

P. A spoiler may be added to the rear of the car provided it complies with either of the following: 1. It is a production rear spoiler which is standard or optional equipment of a US model of the vehicle or an exact replica in an alternate material. 2. It is a non-production rear spoiler which is mounted to the rear portion of the rear hatch, deck, or trunk lid. The spoiler may extend no more than 10.0” (25.4 cm) from the original bodywork in any direction. Alternatively in a hatchback, the spoiler may be mounted to the rear hatch lid at or near the top of the hatch in such a configuration the spoiler may extend not more than 7½ inches (7.50”, 19.1 cm) from the original bodywork in any direction. The spoiler may be no wider that the bodywork. The use of endplates is prohibited. Spoiler endplates are defined as any vertical (or semi-vertical) surfaces attached in front of the spoiler which have the result of capturing and redistributing air (downforce) along all or any portion of the spoiler. The angle of attack is free. The spoiler may not function as a wing.

I will admit, I am a fan of the car with the factor spoiler, so when I started building my prepared spoiler, I based it off of the factory spoiler.  The factory spoiler has a very shallow angle of attack, so I put a break in my spoiler to angle it up in an attempt to generate more downforce.

So the my question is, am I actually accomplishing what I wanted with the bend and change in angle of attack?  How much do you think I'm giving up vs a flat spoiler at a consistent angle?  Am I creating an aero disadvantage with the stock spoiler making a smooth transition where it meets the deck lid vs no stock spoiler and a sharper transition?

For reference, this is the more traditional spoiler:

So realistically I have three options:

1. What I have now (more on this in a sec)

2. A stock spoiler "extension" like I have now, but with no bend

3. A more traditional prepared class spoiler

There is an option 1B, which is making the upper half of what I have now adjustable, just in the event we want to muddy the water.

Hmmm. What kind of testing can you do?

Seems to me that this is one of those situations that you could bench race to death but you could probably test pretty quickly with the right stuff.

What is your goal? Maximum rear down force (or reduction in lift)? Adjustability to balance a handling trait? Reduce drag? 

Setting a goal will help answer how best to test.

His goal is to be the fastest around an autocross course he can be within the CP ruleset, which I think means max downforce.

Had a good spitball session with him regarding yarn tuft testing (or mixing up some flo-viz), but the problem is that he would like to do it this winter and the car is a bit far from road legal, even though desired optimal effective speeds are only 45-50mph (majority of autox corners are about there).  Anyone know some abandoned roads around baltimore md?

I'm an enginerd, but dont have the aero background to really suss out if its generating downforce or acting like a parachute. The Best I can figure is if he doesnt have flow over the front of the thing its generating more drag than downforce, but I am not even sure I am in the right neighborhood conceptually and its not just all about making a high pressure area backed up over the trunklid instead of directing flow.  I mean, there likely isnt flow on the back side of a spoiler like this. 

What he said...

Apexcarver said:

His goal is to be the fastest around an autocross course he can be within the CP ruleset, which I think means max downforce.

 

Had a good spitball session with him regarding yarn tuft testing (or mixing up some flo-viz), but the problem is that he would like to do it this winter and the car is a bit far from road legal,

 

I'm an enginerd, but dont have the aero background to really suss out if its generating downforce or acting like a parachute. The Best I can figure is if he doesnt have flow over the front of the thing its generating more drag than downforce, but I am not even sure I am in the right neighborhood conceptually and its not just all about making a high pressure area backed up over the trunklid instead of directing flow.  I mean, there likely isnt flow on the back side of a spoiler like this.

There's a joke my aero-engineering group of friends used to make whenever someone posed a question... based on our experience with tests:
The answer is "D) All of the Above"

People tend to get a bit hung up about pressure.  Yes, it is what's going on... but, in a sense.. we use it because it's easy to measure (compared to circulation), and similarly... easy to ballpark with numerical methods.

I'm not sure that tufts will show you a whole lot of information in this scenario.  Most likely the flow is going to be relatively attached to the back hatch, even at 60mph.  And the tufts won't really show a difference between the 196's spoiler, and Gimp's.

Now, offhand... it looks like Gimp's spoiler is wider than the 196 car's... and (based on my reading of the rules), neither of them are near the maximum allowed in the rules.  That implies to me that the cars are front-end aero-limited?  Is the car loose through sweepers... or is it pushing?  Also, because your current implementation has more area... you might be closer to parity than you think with the other car.

Although, I take issue with any rule that states a spoiler (which is really more like a "plain flap")  " may not function as a wing. "  The car is the wing, and the spoiler (or any device) is actually a flap... but that's perhaps a philosophical difference.  And, I realize the sisyphean effort that getting into a semantics argument with SCCA rulemakers is. 

I'll see about pulling in a crude "camaro" profile shape with spoiler into JavaFoil... and running a test or two.  But, it's gonna be a couple days, at a minimum.

edit:
By the way, what's the 'angle' of the 'oem' spoiler?

Get some sensors that measure the position of the rear suspension. c5 corvettes, or any car with air ride will have something you can use. I'm not much help finding empty space, but as you stated you don't need to be driving fast. 

Then just test for which spoiler compresses the rear suspension the most at steady state 50 mph or so. 

I think in theory the spoiler works by effectively increasing the size of the rear of the car (draw a line from top of roof to top of spoiler). When a car angles down on the rear window a trunk, this acts like a wing and creates lift. The spoiler - ahem - spoils the wing effect by reducing the effective angle the air goes back down. That would seem to indicate that the vertical height is the critical dimension to maximize. BUT!!! I think it is also possible that with the right angles underneath the rear of the car and the right spoiler angles, the spoiler could function like an extension of the diffuser, helping to pull air out from under the rear. Could you put some holes in the top of your bumper? Then your dual angle or even a curved spoiler might help to keep air from below the car attached further up the spoiler. 

Again, hard to say without running some software or real testing. 

This might give you an idea of the OEM air dam angle of the center section. It's a Firebird but they used the same center section, deck lid, etc.   The Camaro end caps that mount to the quarters are said to reduce lift more than the rounded ones used on the birds. The light color in pic below is the original shape. I made a tall version and (eventually) will test difference between the tall one and the original regular size.

[URL=https://app.photobucket.com/u/NOTATA/a/6b6f76c0-5830-42f9-94b2-3399d5fddd0f/p/d48be983-483d-4120-9fd8-93e349888b5d][/URL]

Thank you all for the comments and the visual aids.

I will get a measurement of the angle of both my stock spoiler and the extended one tonight when I put the car back on the ground.

As far as a diffuser - no dice.  Not only are we not allowed to run one, but I don't even have a trunk floor.  The aero under this car is band for sure.

I know my spoiler is a bit undersized, but I was trying to stay well inside the rules.  I have a lot of front splitter on the car, so I can probably use more aero at the rear.   Hard to say what the car is really doing right now, as my amount of test time has been limit, so I'm just trying to get to a "theoretical" better starting point.

OH MAN! if you don't even have a trunk floor then I think the holes in the rear bumper could work really well. Right now your trunk is a giant parachute to the air under the car (which is hopefully minimized by your splitter, but will still be there). Letting some out in an upward direction could both reduce the parachute effect of the trunk, and reduce the pressure differential of the wake created just behind your car. Double drag reduction!

They might still be illegal due to other SCCA rules, but if not I think they would definitely be worth considering. 

So some numbers. 
 

The section of spoiler that follows the factory spoiler is at 54 degrees. 

That's likely too shallow to be effective without the bend in it  

The extended section, after the bend, is at about 83 degrees.

Which is probably a little too much. 
 

Joe (apexcarver) made the suggestion to hinge at the bend and make things adjustable. Thoughts?

Alright... I haven't forgotten about this.  It's just... 'been a time' of late.

I tried popping the profile into JavaFoil, and messed around with things.  But, I kept getting higher downforce without the spoiler... which I know is wrong.  It's probably a 'settings' thing, and I had a similar issue when messing around with a miata profile earlier this year.  So, clearly I'm doing something wrong.  One of these days I'll get around to learning OpenFOAM.  That would probably benefit this kind of study more anyhow, what with the low speeds and high voritcities.

Anyhow... stop meandering Buffalo!

Conceptually speaking, I wouldn't worry too much about the the break "messing things up".  Yes, the "more normal" spoiler probably traps a larger positive pressure at it's base.  But, the lower initial angle keeps flow attached, and allows the spoiler to move 'back' more off the car, increasing it's effective 'moment' w.r.t. the rear axle (which it's effectively levering against).

Offhand, I'm not sure how much I'd worry about "parachuting" the spoiler.  Any spoiler angle above ~15deg, and you're already making the trade that downforce is more important than drag... and around 45deg you're effectively admitting that you don't care about drag at all.

I like the idea of 1B... "make the extension adjustable".  I dunno about how flat/straight the top of the OEM spoiler is, but experimental a/c types tend to use piano hinge for this kind of thing.  And you'd probably want to make the adjustable part happen on the back side of the elements.  But, generally speaking, adjustability has lots of benefits... since you can adjust the balance to driver preference... or to suit the conditions (rain!).  Plus, front-end downforce tends to be "get as much as you can", and so you're left with the back end to adjust and balance out.

I wouldn't be afraid of taking the extension up to 90deg... and I would consider thinking about the ability to add a 'gurney' to the center part as well.  At 80mph, I doubt the drag increment is going to be that bit a deal.

edit:
Also, hopefully this goes without saying, but the spoiler would be considerably aided by having some kind of stiff, lightweight, panel in place of the t-tops

sleepyhead the buffalo said:

 

edit:
Also, hopefully this goes without saying, but the spoiler would be considerably aided by having some kind of stiff, lightweight, panel in place of the t-tops

Tinted Lexan FTW. 

We had a car come threw with a spoiler and we tested the aerodynamic properties of said spoiler set at a 45*-ish AOA. The Guys at Showtime were kind enough to allow us to release the data, and asked us to mention https://www.franklinroad.com/mens/bottoms/denim/page13.html?mode=list 

the results of the spoiler were quite profound.  this was a good aero spoiler. aka it was very highly engineered and will produce less drag than most home-built units. 
First. Air packed up on top of the spoiler. This air increased the theoretical frontal area and acted like a pileup on the highway, sounds bad, because it is. This increased the overall drag of the vehicle by over 42%. that's the same as having a Miata with the drag of a full-size truck. any time you mention drag and truck you know it's not good. 
this did increase downforce substantially. almost 400 lbs. we are talking a 4:1 Lift to drag ratio. for reference, our wing makes 15:1 Lift to drag.    
The moral of the story is to be careful with your AOA and height on splitters. going bigger might be acting like a parachute and slowing you down.    

 

sleepyhead the buffalo said:

I like the idea of 1B... "make the extension adjustable".  I dunno about how flat/straight the top of the OEM spoiler is, but experimental a/c types tend to use piano hinge for this kind of thing. 

edit:
Also, hopefully this goes without saying, but the spoiler would be considerably aided by having some kind of stiff, lightweight, panel in place of the t-tops

Thank you again for the continued discussing and feedback, both here and over DM.  I think I've got a few ideas on how I can improve things, and I'm hoping to grab some cardstock soon and mock up some ideas for you all to critique.

As far as drag, that's why I have all the bald eagles, right? ;)

As to the two bits quoted above - 1. the top of the stock spoiler has a gentle curve, but I can totally get away with a piano hinge without it being too strange and 2. I have something in the works for that roof.

Well, I lost the last post I was working up, so let me see if I can recapture the magic.

Got some work and some research done over the weekend, and I've started version 2.0 of the spoiler.  I started with a slightly taller piece of material so I can get to the max height of 10" from the body (more on that later).

That puts the "factory" angle of attack at roughly 56 degrees.

I say "factory" angle of attack because I will be hinging the new spoiler right above the factory spoiler to make things adjustable.

So now to the question of width.  The rules state "The spoiler may be no wider that the bodywork."  Well, what does that mean?  After speaking with my competitors, the general consensus is that the spoiler width is determined by where it mounts to the bodywork.  Yes, the rule could be more clear, but I'm inclined to agree with my competitors and that's the width we will be going with (say that five times fast).

The outer edge is even with the rear flare, the dashed line is in line with the factory body at the door, and the red line is the width where the spoiler meets the body.

So now we have to talk about another rule.  "The spoiler may extend no more than 10.0” (25.4 cm) from the original bodywork in any direction."  To me, that means the spoiler should taper and follow the shape of the body as it falls off to the side of the car.

To that end, a few folks (when digging through image archives of other second gens) agree with me:

But there are also some folks that seem to have set the 10" mark at the center, and carried that to the outside, making the ends much taller than the 10" rule:

I will likely err on the side of caution, but I'm curious as to what you all think.

Oh, and as far as dirty air over the roof, I'm cooking up something for that.

If they look dirty, it could be because they are "molds..."

Gimp (Forum Supporter) said:

So now we have to talk about another rule.  "The spoiler may extend no more than 10.0” (25.4 cm) from the original bodywork in any direction."  To me, that means the spoiler should taper and follow the shape of the body as it falls off to the side of the car.

 

To that end, a few folks (when digging through image archives of other second gens) agree with me:

So, to me, the definition of "extend no more than 10.0” (25.4 cm) from the original bodywork" reads a little different.  It creates a situation where you don't have to 'pull in' following the bodyline, because you're still 10" away from the body crease at the top of the fender...

and is mainly precluded (occluded?  permuted?) by the limitation that the spoiler "can't be wider than the bodywork"... which is understood to be known as "the bodywork the spoiler is attached to"

edit:  I should add, that I don't intend to claim the 'as previously used' interpretation is wrong; and I don't want you to buck the rules unnecessarily... and/or get protested.  But, I read that rule as a 'Volume" that extends 10" in every direction from the point of contact with the body... limited only by the requirement that you not exceed the width of the body where the spoiler is attached.... and you could easily have a ruler with you to prove that you're still '10" from the body'.  But, that might be something you run by everyone else first?

One other thing, that I've been meaning to point out... is that you might take some time to work on closing up this gap:

and, yes... closing up that gap is related to why I'd encourage you think about pushing the 10" from the body rule.  And, it something that Not-A-TA brought up, and you can also see it in the CFD image Johnny posted... it's a feature of all spoilers... but the OEM Camaro spoiler in particluar.  Which is, that the design is effectively a "dive plane" (if it was in the front, people would call it a canard).  But, basically, the angle, and sharp edge of the OEM spoiler is intended to create a vortex to roll up on the backside of the spoiler, creating a low pressure zone on the backside... which increases downforce.

Like I said, fortunately thanks to 9LR, we've got evidence that all spoilers do this... so, we're not causing the spoiler to "act like a wing"... we're just trying to maximize our use of the phenomenon within the limits of the rules.  And, it's most likely a thing that we'd be able to do if the section behind that part of the OEM spoiler was "snugged up good and tight" against the OEM part, and has the same "angle" as the OEM section.  Although, it being best that both section be at the same angle is a guess.

And that raises a bit of a conundrum... since it'd mean you'd either have to split the wing into adjustable sections; decide to try the thing at the "OEM angle"; or... something else that would probably lead to a spoiler shape that is so contorted, everyone will be scratching what's left of the hair on their wondering if it actually works.

In reply to sleepyhead the buffalo :

Thank you for pointing out the gap.  I fully intend of having my side extensions meet right up against the contour of the factory spoiler and close up that gap.

At the moment the plan is to split straight across, including the upper half of the side extensions.  Hopefully I'll have something better to show soon.  I can tell you I'm definitely scratching my head.

So, in case the scratching your head is about "all the lines"... here's what I'm proposing would look like:

i.e. the rules don't say "the spoiler can only be 10" above the bodywork.  Extend is not a direction, other than away.  Something can extend up, sideways, and longitudinal-ways.  But, that's just my interpretation.

Now, "Point the Third"... which I haven't got to yet.
One benefit of not paying me... is I had the chance to go dig around and read your Solo Nats report from last year.  Which included discussion of "the front end is oversprung, and skipping around a lot" and "there's front push"

The thing with Aero is... and it's part of why I stress the concept that "the body is the wing, and everything attached to it are flaps".  When you add a spoiler (or wing) to the back of the car, it acts down... but it's a distance away from the car's center of mass.  So, the total affect is a moment which measured from the the flap to the CG and interacts with the ground through the springs of the suspension system (i.e., springs, shocks, tires, linkages, etc).  So.... depending on the flap (and some other stuff)... the rear downforce may try to 'lift' the front end.

That's all a really longwinded way to say... it's possible your last wing was making too much downforce.  Which means... if we add more area... you may not have to run as much angle in your extension.

In reply to sleepyhead the buffalo :

First off, I owe you at least a beverage of your choice should we meet.  I'm blown away by your help and I really appreciate you putting time and energy into this.

To the shape/profile you have laid out, I think you're 100% in your interpretation.  I'm going to do the old "marker on a string" trick to get the 10" out to the edge, resulting in what I hope will be exactly what you drew out.

"the body is the wing, and everything attached to it are flaps". 

While somewhere in the back of my head I had been thinking along those lines, I've never heard it so well articulated.

Yes, at Nationals the car really pushed.  It didn't matter what we did with the shocks, it would push and skip, especially at lower speeds, but the effect was really everywhere.  I'm happy to report that softening up the front springs made a world of difference when we ran the car at the Dixie Tour earlier this year.  When I mentioned my old spring rates to other folks with similar setups, they looked at me like I was crazy, further confirming that I went way too stiff out of the gate.

While I don't think the old spoiler was creating too much downforce, I will be keeping it and comparing it to the new one I build at a test and tune in the future.  The nice part is I can make a bunch of different profiles and they swap out with six bolts.

I picked up the material for the end extensions today.  I'm going ot make a few more templates, sketch out the profile, and post it all here for feedback before I cut anything else up.

sleepyhead the buffalo said:

 

 

 

If I was building a tall deck lid air dam and trying to maximize it's usefulness for Auto-X speeds I would do exactly what sleepyhead recommends in this pic ^^^. I've spent waaaaaay too much time studying 2nd gen aerodynamics.The more squared off Camaro end caps are out in cleaner air and supposedly wind tunnel testing done back in the day showed they worked better than the Firebird ones. I do not know if that referred to further reduction in lift or if the vortices created by being out in cleaner air helped reduce drag, perhaps both?. Supposedly the Pontiac engineers wanted the Chevy style end caps but because Chevy had done the testing or some such thing they were told no. I was going to get a set of Chevy end caps and remake them to fit Firebird Quarters but I'm already far to deep in my project to dedicate time to that. I need to get my car back on track.

In reply to Apexcarver :

Abandoned? Not really.

However, at the right time of day, or during the right holiday I remember a fair amount of industrial areas and similar places that didn't have anyone around.  Should be able to find one of those, trailer it there, rip up and down the road a couple times?

Robbie (Forum Supporter) said:

Get some sensors that measure the position of the rear suspension. c5 corvettes, or any car with air ride will have something you can use. I'm not much help finding empty space, but as you stated you don't need to be driving fast. 

You don't even need sensors for that.  Rod attached to the unsprung mass with an o-ring on it, a tab with a hole in it attached to the sprung mass.  Rod goes through the hole, as the suspension compresses it pushes the o-ring down.  There's your measurement.

You'd need to keep an eye out for bumps and the link, and not launch it hard

Made another mock-up based on advice from here  

I think we are getting close.