Coming home from vacation with the fiance, and our flight just got delayed thirteen hours, so naturally I'm going down a rabbit hole.

I'm playing around with various ideas to maximize my car's speed for next year (just like everyone else, right?) and am toying with different ideas for aero. Currently, the car is in NASA TT5 class, which is somewhat limited as far as aero goes. I'll be moving the car up to TT3 class after the final race in October this year, which opens up a lot of possibilities for aero.


Current aero setup as it sits.

Current general outline of the car is as follows: ~185whp 2.5L Duratec swap, 235/40/17 Maxxis RC1's on 17x10 wheels, 9LR V2 Big **** (67" chord to match body width and roof height), DIY airdam with a 2" splitter (although TT3 rules allow up to 5" of splitter protrusion). Car's got an OEM hardtop as well currently.
Next year I'll be adding power to the car, likely via a forged high compression 2.4L Duratec engine build (shooting for 250-260whp) or an EFR turbo kit. I'll also be bumping up to 255/40/17 RC1's, or maybe even larger.

I was daydreaming about additional aero accoutrements last week when a post @Fireindc reminded me about Occam's Racer's DIY aero site. Specifically this post about the substantial amount of drag reduction provided by a fastback. Miata Fastbacks and Aftermarket Tops – Occam's Racer

At the same time, I had a post pop up on FaceBook that got me thinking about additional rear end aero, in the form of drag wings. While I wouldn't just toss a super long tail on my car, it got me thinking about how a fastback and longtail could interact with each other and potentially create a serious reduction in drag. Renderos racing did something similar with their old longtail fastback for the NA/NB already, and I'm sure there was more than a little thought behind it. Being that my car could still be considered "low/mid power" with 250whp, every bit of drag reduction helps right?

The CCP fabrication fastback for the NC is a pretty penny at $3,400 shipped. However, I could sell my current hardtop for more than half of that easily, and would be doing all of the prep and paint work on the new top and trunk myself. The rest of my car's paint is far from pristine, the top wouldn't need to be show-worthy.

Does anyone have input on how a longtail would interact with my current wing and a fastback? Apologies if I'm asking a stupid question without enough forethought, but I gotta assume I'm not the first person to ponder this. I'm also aware that some (or a lot) experimentation will be required since I don't have CFD software to toy with. From what I've read, the fastback will effectively increase the downforce created by the wing, while the longtail will decrease the wing's efficiency. And both accoutrements should decrease drag.

Anyways, yeah, any input or dialogue would be greatly appreciated. Both for insight and entertainment purposes.


Renderos Racing Longtail


Drag wing/longtail inspiration. Not sure on what the ideal length would be, but there's a guy in my area that makes these custom for lots of different cars. At any rate, I think I'd have something built like this C6 wing, spanning the entire width of the rear end instead of just trunk width. Seems like it would be more effective, in addition to just plain looking better.


CCP NC Fastback. Ignore the bag on the back...

 

I can't help, but I had no idea someone already made an NC fastback. It looks cool!

 

I don't have much to add either, other than the fastback aero is proven and looks the ****. If there's an option, and you can afford it, go for it. It's always been a bucket list item for my car, personally.

 

I almost wish they didn't. Then I wouldn't be making this thread lol.

Say no more! I'll do it for the community then hahaha. In all seriousness, I always wanted a fastback for my NA as well. Just icing on the cake that Occam's CFD shows substantial drag reduction too. I'll likely put my hardtop up for sale later this year and place an order with CCP once it sells.

 

Yeah, the fact that you already have an OEM hard top to sell to put towards the fastback would seal the deal for me. If I get my old NC back from my friend, it has a very old, glue-stained OEM soft top on it that has some small holes in it. When I finally would need to replace it, this fastback would be the top option for sure. Thanks for the heads up!

 

I would be interested in knowing how aerodynamic the NC removable hardtop is compared to the NA/NB hardtop.

While the NA/NB hardtop does make for a great drag reduction compared to top down, that's a pretty low bar. If you've ever driven with an NA/NB hardtop at highway speed through a rainstorm (starting and ending in the dry), you'll note that the back window never gets wet. Go through a lot of spray from other cars and the dried road goo is like low dollar flow-vis. The airflow going over the center of the windshield breaks at the roof/windshield joint, and rather than going straight back from there, is pulled from the center towards the side windows. The joint acts like a trip strip, and pretty much any convertible is going to have that seam, but it can be done a lot better. By the time the airflow reaches the back window, there's no question it's turbulent and detached. The fastback shape does a better job of keeping the airflow attached past that point, which is why it has such a dramatic drag reduction even compared to an OEM hardtop. It leaves a much tidier and smaller wake. The NA/NB is not exactly an aerodynamic triumph. I don't think the NC removable hardtop was designed with aero in mind as a first priority, but surely it must be better with 15 years of aero knowledge gain.

Just looking at the NC hardtop profile, I think it looks better than the NA/NB. No break for a recessed window, and probably a bit more of a fastback angle too? I've not studied one in person.



Flow-viz is a green liquid used for aero testing, for those who don't know. You put it on the car and see how airflow drags it along the body.



TL;DR: I wouldn't expect the NC hardtop leaves as much room for a fastback to give drag reduction as the NA/NB hardtop. I have zero actual data though and would love to see someone come through with real facts.

 

Option, thanks for the well-thought out response! It's funny, I've looked at my car nearly every day since I bought it but never took the time to compare the hardtop design to the NA/NB. You're right, just eyeballing it, the angle of the rear window on the NC hardtop looks much better.

I'm glad you mentioned the flow-vis. Next time I drive through some rain on the highway (probably about 7 months away for us ), I'll have to see if my rear window gets wet. I'm sure the result will be the same as with the NA/NB hardtop, but I've never paid attention before.

Would love to see someone come through with real facts/data as well, but I'm also aware this is a very small niche question and there might not be anybody coming to the rescue on this one haha.

 

Would second Option's thoughts. Linseed oil paint (a couple of drops) and any low weight oil make an excellent DIY flow-viz "paint", albeit a bit messy to clean though. Alternative is water with some sort of food colouring in it. You could even use the tuft method with tape, plus an action cam, to see if flow is attached on the glass and/or where it detatches and becomes turbulent..

 

I just want to add, I spoke to Mario (author of the Occam's Racer site) with some questions about hardtop drag reduction on the NA/NB and his experience in windtunnel testing after buying his windtunnel report. I believe he worked with Johnny from 9 Lives Racing on his last test that was Miata specific.

Basically, there's no replacement for a full fastback panel on the NA/NB hardtop by his testing and simulations. A hardtop "spoiler", even one that doesn't kick up and instead follows a fastback profile, does not keep flow attached any further or reduce drag. I put spoiler in quotes to clarify the difference between the typical hotboi kicked up spoiler you see regularly, and more of a turned down roof profile extension like I was thinking of. Vortex generators don't help either.

I can't find the thread right now, but I believe the Lightyear NB fastback began as a simple large piece of plastic, bent and cut to tightly fit to the window and bodywork. That was tested and became the form of the fiberglass top. You may be able to do something similar and get most of the technical benefits for significantly less than $4k.

Unless you're really in love with the look, I'd look for other places to spend that money to get lap time improvements.

 

Interesting, thanks Option.

One question that I think is important in this discussion: What use?

If the car is regularly seeing high-speed tracks, drag minimisation will pay greater dividends than slower circuits. This will be enhanced for the high-speed tracks if the fastback feeds cleaner air to the rear wing, allowing it to trimmed down to generate the same DF at lower AoA, generating less drag as a result.

As well as cost, I assume these fastback HTs weigh more, exacerbated by being mounted high on the car. This makes it more important that they produce good results to offset the weight/cost.

Instead of messy flowviz, try tuft testing. Surface flows can be seen at varying speeds, and by using strategically placed masts etc, flow can be seen in 3D, particularly useful to check the flow immediately in front of the wing for direction and stability.

 

I didn't mean to emphasize or harp too much on a messy flow vis. I just drive my car in most any conditions in the southeast and don't wash it regularly, so I am used to seeing the results.

I found the post in question within two minutes of being on my home computer. I read forums way too much...

For ease of reference, here are the pictures, just like what you're talking about except all home brew. @Z_WAAAAAZ Imma be real with you, if you're willing to daily a trash can bumper this shouldn't be too out of line to have on your car for track days. The NC may not have as natural a place to make the break between hardtop and fastback extension, but it could be worth a shot. Hardest part would be finding a sturdy and reliable way to attach it without being permanent.

 

Yeah, thanks for that Option! I thought I'd read previously that the roof spoilers do a whole lot of nothing, but it's good to know they he and Johnny actually tested them. As far as the modified HT goes, you're right, the $ to benefit ratio has to be much better. I'm not spectacular at making projects like that look presentable, though. Might have to put my thinking cap on if I were to attempt something like that haha.

Well, it's a dual duty car but all the tracks it does see are at least moderately high speed (top speed of over 100mph), with one being very high speed (top speed of 123mph and average speed of over 100mph on the fastest one), so I think the car stands to benefit a good bit from any drag reduction possible. I need to go back through the forums, but I remember seeing that the CCP NA/NB fastback weighed about 47lbs with windows/hardware installed. Not sure if the NC fastback is a substantially different weight from that. The NA/NB fastback is definitely a bit heavier, but I think the lexan windows offset the weight increase to a certain degree. My current hardtop should be around 40lbs, so the weight increase might not be too much of a factor. That being said, I could also get a Treasure Coast or CCP minimalist hardtop, which only weigh 13-14 lbs according to their websites. More to think about, thanks for the input.

Oooh, don't tempt me. You're right, I'm not above running something like that on my car. At first glance, that solution actually looks cleaner than what I was expecting lol. I'll add that link to my reading list tonight. Double interesting as that car also has something similar to the drag wing I was proposing. Thanks for digging that up!

Edit: Do you have the post number for the pics you're referring to? The link just directs to page 2. Can't find said post nearby.

 

It's post #75. Not sure why the link I copied from the post number didn't work, sorry about that.

 

Hot damn! Lightyear is in my backyard, he is getting a phone call! That fastback shroud is just what I was looking for, but didn't know it ... have a cat Option!!

ETA: Reading on from that link, Lightyear has described how he made the shroud - he makes it sound pretty simple, I am sure it wouldn't be quite so simple for me but I am keen to see how it goes. The fact that I will be using a 'race-only' boot lid with high-mount wing will simplify some aspects, especially the mounting.

 

I've got a CCP NC fastback on order, should be getting it sometime in July. I plan to do some aero testing on it when it arrives.

 

The added benefit for Zak is that he's in CA and can pick up from CCP. When I priced them out the shipping was crazy (as expected), I figured if I ever decide to buy one that I'd plan a trip out west and pick it up.

Edit: CCP is in FL. Nevermind. I could have sworn one of the fastback manufacturers was in CA, maybe is was the Renderos fastback I was looking at?

 

Oh heck yeah. Good to hear someone else is getting one of these made too. I'll stay tuned for updates.
Curious, how long ago did you place your order and what roll bar are you running in your car, if any? I was told the fastback might need modification to clear the BBFW roll bar.

This would be such an easier purchase if they were down the road for me. $445 for shipping to my work is kind of a bummer haha. Yeah, Renderos is just a couple hours up the road from us. Actually almost on the way to Willow Springs.

 

I ordered it just a couple weeks ago, their website was saying 15 weeks lead time.

As far as roll bar, I contacted them to ask if they knew whether it would clear Hard Dog M3 Hardcore, and they said they didn't know, they did say it has a slightly different profile than the stock and or their copy of a RHT, implying that it might have less clearance. In any case my tentative plan is to find someone local to build me a custom 4 pt roll bar, might also see if a single diagonal door bar will add some stiffness similar to the one Hard Dog does for the NA/NB roll bars. If someone is in the Des Moines area with an NC roll bar that I could test fit once I get the fastback that would be a fun experiment.

On a related note I recently purchased a 3d scanner so that will probably be enlisted to get information out to the community.

 

That's the same answer I got when asking about my roll bar. My first reaction was "ok that for sure means it doesn't fit", but I'm interested to see if it happens your Hard Dog bar. Also looking forward to any of your other findings and info provided by the scan!

So at any rate, the fastback is obviously a long ways off for me (and MrJon too), but I tried doing a little more digging into the trunk extension/drag spoiler idea in the meantime. I messaged Ryan Passey on FB today regarding the trunk extension he ran on the HyperMiata since the early 2010's and surprisingly got a quick response back.







Good enough for me. I'm going to start brainstorming/looking into the trunk extension next. While it sounds like it's not going to be a total game changer, I also like how it looks. So it's a win/win.

Also while perusing pics of the HyperMiata, I came across a pic of the rear wheel spats. I've been cutting out a set of rear wickers/spats for my car (just simple ABS ones like you'd find being used to cover the front wheels). After looking at this pic, though, I think I can do a little better than that, and make a set that flare out from my side skirts and direct air around the side of the tire instead of simply blocking the tire itself. More to come..

 

I have a NA/NB CCP fastback sitting in the garage... I should really get around to installing it (and the wrap also sitting and waiting). I don't have the number, but I was surprised how much heavier it is than their "race" hardtop. Could probably cut a bunch out if you don't need the stock mounting points but I don't think it'd make too much of a difference.