Prepared Rear Spoiler - How Badly Am I Compromising

Gimp (Forum Supporter) said:

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. 

 

I read this ^^^ a while back and was reminded today that I'd intended to ask you about it. Not familiar with SCCA rules.

The stock gas tanks were contoured pretty nicely for air flow. Rounded forward bottom, into flat bottom, then rounded and angled nicely up to the rear bumper. Having one in place might smooth out (speed up) the airflow under the car thereby moving the COP forward and possibly increasing front grip (and/or reduce drag). Now that leads to the question "Does it have to be a fuel tank, or can it just look like the stock tank from the back or under the car?" Ya know, use a stock tank to make a mold to make a lightweight fiberglass panel that just appears to be a stock tank.

In reply to NOT A TA :

Unfortunately, I think the only way I can do that is to put a factory trunk floor and stock tank back in. Anything else would likely run afoul of the rules. 
 

Eventually, I want to move the radiator to the rear. The way I'd like to mount it may have the side effect of cleaning stuff up. 

On the topic of angle, I'm seeing 60 to 80 degrees as the optimum, all things considered. 
 

https://grassrootsmotorsports.com/articles/simple-spoiler-tips-aero-expert/

More updates!  So I have been talking with competitors and working in the garage to refine the spoiler and make sure I'm within the understood letter of the rules.

The biggest question to date has been these two parts of the rules: "The spoiler may extend no more than 10.0” (25.4 cm) from the original bodywork in any direction" and "The spoiler may be no wider that the bodywork."

The first bit, 10" from the body, would make you think you can go 10" wide than the bodywork, if you were so inclined.  The second bit, calls into question which part of the car is the "bodywork."  Is it the widest part of the stock body, where the spoiler attaches, the width of the flare, etc?

After speaking with much smarter and accomplished people in the class who have been down this road, the clarifications given over the years basically say that the spoiler attachment point is your initial width reference, and for the overall width, follow the same sort of logic for the front splitter - "It shall not protrude beyond the overall outline of the car as viewed from above or aft..."

So, time to make the donuts... or, I guess, spoiler.

I began shaping the "extensions" on the end of the spoiler.

The intent here is to close up the gap previously pointed out, and to get things to fit nice and tight.  You can see the first draft of the upper shape in these images too.

Looking from the back, I could probably make some extra money renting the space out to an advertising company.

And from the front:

So, now to this part:

"The spoiler may be no wider that the bodywork."

The back edge is easy.  Once trimmed, I'm well within the bumper.

Unfortunately, the extensions will need a little trimming to be legal.  That said, I don't think I'm going to hate the look (red line).

The plan is to bend a flange into the extensions, temporarily attach them with clecos, and trim to the final shape.  After that, I can start looking into adding the hinge to make things adjustable.

Gimp (Forum Supporter) said:

On the topic of angle, I'm seeing 60 to 80 degrees as the optimum, all things considered. 
 

https://grassrootsmotorsports.com/articles/simple-spoiler-tips-aero-expert/

JG and I talked before this article came out and those are basically my words...

The way to get close to the optimal angle is to draw an imaginary line between the bottom of the spoiler and the whatever part of the rear glass/roof/hatch intersects first. The spoiler should be pretty close to 90 degrees to that line. A little either side of 90 degrees is close enough.

On the end panels you are building, you may want to curl the outboard/upper tips forward a little AND then redraw your 10" body extension line. It will make a small difference to the amount of material you use, but the forward curl will benefit the rear downforce by helping to resist the outward/upward flow of the air spilling of the trunk/deck. The effect is small, but you are trying to get every last ounce of downforce for your application.

I like how Sleepyhead described the car system as a wing with flaps. He and I agree that you have to consider the car as the primary aero shape and all the stuff attached are modifiers.

In reply to stafford1500 :

Well in that case, thank you very much!  This sort of info, as I'm learning, is hard to come by.  I really apprecaite all of your hard work and willingness to share.

"The way to get close to the optimal angle is to draw an imaginary line between the bottom of the spoiler and the whatever part of the rear glass/roof/hatch intersects first. The spoiler should be pretty close to 90 degrees to that line. A little either side of 90 degrees is close enough."

If I understand that correctly, you are talking something like this:

I set the angle finder at 90 degrees and compared the angles.  To get my spoiler to match the angle angle finder, I only need to increase the angle by 14 degrees, which is totally within reason.  Am I understanding that correctly?

"On the end panels you are building, you may want to curl the outboard/upper tips forward a little AND then redraw your 10" body extension line."

It's hard to tell from the other photos, but there is a slight curve to them.  I promise, I'm listening!!!

Do the rules allow a fence where the proposed curve on the ends is? If so, do the folks who know more than I do think one might be useful?

NOT A TA said:

Do the rules allow a fence where the proposed curve on the ends is? If so, do the folks who know more than I do think one might be useful?

No fences or endplates. 

Gimp, your images and understanding of my comments is spot on.

Since I spend my time working at closer to 175mph, the effects are more significant, but the physics works at all speeds (mostly).

In reply to stafford1500 :

Thank you very much!  That means a lot!

I owe you big time for sure.

I have some other plans for 175mph+, but that won't be for quite a while.

so, here's another thought, about something that hasn't been fiddle with (but seems legal)... and I'll let stafford chime in a bit

the rules state you can't have an "endplate" which is defined as a vertical face ahead of the spoiler... and it's implied that applies across the "width" (or span) of the spoiler.

one thought I've have, is that there's no restriction on adding a 'gurney flap' (i.e. a small forward extension at the top of the spoiler, placed at 90deg to the spoiler face).  I had been thinking about that maybe in the middle section of the spoiler.  But, stafford's suggestion about curling in the outboard portion of the spoiler would indicate to me that they might be of the most benefit out there.

edit:
If you can get a bit of an elevated picture over the back of the car that fits at least from the centerline out to the edge, I'll try to give a "paint.net" explanation of what stafford is getting at.  It's described in words in the article you linked to.  But, some of the flow particulars might... uh... 'whoosh' over your spoiler, so to speak

In reply to sleepyhead the buffalo :

I'm 99% certain gurney flaps aren't legal, but I don't see where they are strictly forbidden.  For example, having a bend in my spoiler (more than one plain) has been determined to be okay.

I'll do some research there, and I'll try to grab a photo like you're asking for!

Big no on the gurney flaps. 

Gimp (Forum Supporter) said:

Big no on the gurney flaps. 

That's fine.  I understand the limitation against them... even if I don't particularly see where the limitation against them.  It's possible that that limitation is buried in a section of the rules I didn't go digging through.  I try to limit how much I dig through the Solo rules.  I only have so much hair left.

In reply to sleepyhead the buffalo :

I agree that it's not clear in the rules, but I spoke with long-time CP racers and they all agreed and no one runs one. 
 

Sure, I could add one and bait a protest, but that's messy. Maybe I'll write and get clarification. 

Making a little bit of progress.  I got the flanges made on the end pieces (thanks Larry) and cut them so the deck lid will still be removable.

Here is the main body of the spoiler, free of the end pieces:

This is the bottom half of the end piece.  This will stay permanently attached to the car:

It's attached to the back, eventually, by a bunch of rivets.  You can see the flange on the back that helps locate and support the main spoiler:

The upper part of the extension is riveted to the main spoiler, so it will come off when you take the deck lid off:

And here is the whole thing, with both ends fitted:

Now, on the topic of angle.  Using the method that stafford1500 pointed out, the spoiler is currently 5 degrees short of the "optimum" angle.  I'm in a debate with myself, and with Larry, as to whether or not to cut the main spoiler body and add the hinge (along the cut line in the end plates) to get to the optimum angle.  He doesn't like the look, and I'm less concerned about how it looks.

How bad is being 5 degrees short?

Can you guys explain this angle relative to the rear window stuff a little better?  I'm missing exactly what I should be looking at.  I'm working with a bit more level trunklid vs the sloped camaro one gimpy is dealing with here, and am using the same CP ruleset

In reply to Patrick (Forum Supporter) :

I'll send you some pictures this afternoon.

So after a lot of debate, back and forth, measuring, etc, I finally cut the damn spoiler to size.

So we are ready to body work and paint it right?  Wrong!

So, it works.  It mounts how I want, it looks how I want, etc.  But I painted myself into a bit of a corner when I first started this build, and I can now see a way out, so I'm going to take it.  The problem is with the endplates.

When I first started this, I imagined that there might be a hinge that I'll use to adjust the angle, so I cut things in certain places to allow for that.  What I wound up doing was creating a "keystone" that locks the rear deck lid, when assembled, in place a little too hard.

So, my plan is to cut the lower end plate, horizontally, along the bottom edge of the main spoiler, and then remake the upper endplate as a taller piece that will get welded to the main spoiler.  Seems like the work is never done.

So, there it is.  I'm obviously getting one more chance at shaping things, so now is a great chance to get last minute thoughts and opinions in, or wait until it's finished and painted ;)

Problem solved!

I had enough material where I could remake, and have Larry at 20sqfab weld on new endplates for the rear spoiler.

They actually wound up fitting better than the last versions, in addition to making it possible to remove the trunk lid.  Win win.

We added a retention tab to help keep things located and supported.

So now I just need to bodywork and paint, and we'll be in business.

There was another aero update, but that's back in the main thread... here is a look.

looks good to me

Got the spoiler all painted up.  Thank you to everyone in this thread for all of their help.  I love how it turned out.

That should work!

Bitchin'