Let's start from one of the the worst aero platforms, why not

The classic Beetle.

I'm married to the body, so that's staying. Nothing sacred about it, though, so I'm game to slice it up in the name of lift/drag reduction.

Brainstorming toward SCCA E Modified while remaining streetable. Splitter, belly pan, spoiler are legal, with some size limitations. 

As I understand it, Beetles make a LOT of lift at the back, and a small spoiler at the top edge of the rear glass can help the flow detach from the body.

I'd like to run some computational fluid dynamics tests to plan for aero rather than just taking a wild guess. Thoughts on how best to go about this? Recommended inexpensive/free CFD software? How would I need to create/manipulate the model in the simulation?

There's a new book called "modifying the aerodynamics of your road car". I grabbed it at a local library. 

Written by a very grm guy. He details how to do some very cheap aerodynamic testing with wool tufts, pressure sensors, pitot tubes and ride height sensors (not cfd, but he does claim that good real life testing is probably more accurate than bad cfd...) 

Absolutely worth the read - and I'm not even all the way through it yet!

He does use the beetle as one of the examples in the book. I'll try to reread closer tonight and report back.

I thought beetles had a very low Cd compared to other cars of the era? Didn't know they created lots of lift at the rear. 

CFD Starting point:  https://www.openfoam.com/

In reply to T.J. :

Found a wiki page that listed the Beetle as a Cd of 0.48, only slightly better than a Mitsubishi Pajero or a Jeep Cherokee. Not sure how well it stacks up against its contemporaries, but its a 70 year old design; maybe its better than, say, a '48 Ford Tudor or something.

As far as lift goes, the flow adheres to the roof and follows it all the way to the rear edge of the engine cover. Free air continues flowing horizontal, so you're asking the attached air to occupy an increasingly large space between free airflow and the body. Low pressure on the rear bodywork. Theres a decent study or two out there with tufts that shows you can detach the flow with a very small "visor" at the top edge of the rear window.

At some point, I suspect more spoiler is not better, therefore the flow analysis.

This might give you an idea of the flow for a stock body:

Roof Spoiler

You're not the first to consider this, there is some solid data out there on what to do. 

In reply to JohnInKansas :

That makes a lot of sense. Now that I've thought about it, it was my high school physics teacher who claimed they were very aerodynamic/low drag. He also took points off my on a paper about friction because he said there was no such thing as weight transfer to the rear wheels when launching. I still wonder how he explains wheelies. He may have very well been correct when compared to other cars from the 40's.

In reply to pinchvalve :

Yeah, I've had that site on my bookmark list for years now. I'd just like to go one step farther and figure out what the optimal placement, size, and attitude is, and what kind of numbers can be expected as a result (drag coefficient, downforce).

T.J., pinch's link goes into more depth about why the Beetle is almost a really good aero design, if you're interested.

In reply to JohnInKansas :

Thanks, I checked it out. Interesting.

Here's the same idea with some wool Tuft testing. You can see the flow separating with the spoiler added. Should reduce lift but I dunno if it makes drag worse or better.

In reply to Robbie :

Haha, those photos were taken as part of the study pinchvalve linked a couple posts ago. The author does give him proper credit though.

In my mind, it should help with drag as it should decrease the size of the column of air the car has to pull along behind it. Should. This is why I want to run some tests.

So you could do something halfway between and a lip and this monstrosity... diminishing returns on track for something this big but just disrupting the flow with the window lip is perhaps not enough.

https://beetledozier.blogspot.com/2012/02/retrofitted-kammback-ability-to-improve.html

I've thought of doing something like to my DD for gains but I'm worried that it'll have to be too heavy to survive weekend RallyX duties:

https://ecomodder.com/forum/showthread.php/project-making-permanent-metro-kammback-extension-3518-26.html

Also, don't underestimate the ease application and cost for significant gains found with Vortex Generators:

http://hikickracing.blogspot.com/2011/01/effects-of-vortex-generators.html

http://www.autospeed.com/cms/article.html?&title=Blowing-the-Vortex-Part-4&A=113219

BTW, Julian of autospeed is THE GRM'r of OZ as best I can tell. One could do worse in this world than reading everything on his site.

Difdi Al-Jabal 68 said:

Also, don't underestimate the ease application and cost for significant gains found with Vortex Generators:

http://hikickracing.blogspot.com/2011/01/effects-of-vortex-generators.html

http://www.autospeed.com/cms/article.html?&title=Blowing-the-Vortex-Part-4&A=113219

 

BTW, Julian of autospeed is THE GRM'r of OZ as best I can tell. One could do worse in this world than reading everything on his site.

He's also the author of the book I pictured. ;) I agree, I think I'm going to reach out to him after I finish reading his book and another of his I have on order from the library. 

Keep in mind that 30 ish degrees is about the worst angle for a rear window and 10-15 is about the best in terms of drag. above about 40 degrees everything is similar up to 90. But that is speaking about drag, not lift. 

Vortex generators do work, but must be tested to make sure they are working in the way you want them to. Easy to add drag without increasing downforce if you do not test them. 

In reply to Robbie :

Ahh... I remember faintly a reference to another book he was working on but obviously I'm not up to date. How much lift does a Beetle have to combat with nearly all the weight in the rear? It does have a short wheelbase though doesn't it. Definitely one of those wicked problems.