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Old 05-19-2016, 09:33 PM   #1
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Default Spoiler height and AOA(?) advice

After my turbo install the car has been extremely fun and plenty fast for my current suspension/tire combo. I just went to the match tour event in Bristol, TN and got to see some really high level autocross cars. I think my car has comparable hp/weight to a few of the high contenders but I am having traction issues. When the car was NA it really wasn't as hard to drive on mechanical grip alone. Now the car has a bit more than double the power and I feel it needs some aero assistance in the rear. It is just a bit loose.

The car is right at 1900lbs without me in it and race prepped (not hard top, glass, passenger seat, and 3/4 tank).
Bilstein HDs with Ground Control sleeves (440/300), 1"/2.0" extended top hats, RB 1.125 hollow with HD mounts and RB endlinks, no rear bar.
Storm S1s 15x9 with new 225 Rival S

I want to ask a few things about the spoiler I am currently building and maybe spark a decent discussion along the way.


I am currently looking at going about 14" in height (12" supports) and a maximum AOA of ~50 degrees. Measured like this...



I am making 3 supports out of 1.5 x 1.5 x1/8 inch aluminum L stock
3 home depot turn buckles rated at 150lbs each
Lexan or some other quality brand of poly-carbonate (transparent so I can still see out the back since I drive this car on the road to events and some mountain fun.)
Closest one in the photo was a mock-up for proof of concept.









Cardboard is about 18" tall.








So the further I get into this the more I question. I don't have any front aero, outside of a GV rep lip and I am concerned on how large I can go before I need a splitter. Is there any testing with this? I have read a few things on the BlackBird Fabworx spoiler and a few have commented on how nice it is but I see little autocross data with it.




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Last edited by 90civichhb; 05-19-2016 at 11:23 PM.
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Old 05-20-2016, 10:28 PM   #2
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I know you're asking about the spoiler, but additional wheel rate would help your traction as well. Those are some seriously soft springs.
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Old 05-21-2016, 02:57 AM   #3
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That's about the limit the HDs can take from what I have read. I'm limited to a 200tw tire but I agree, a 700/400 setup would be preferred.
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Old 05-21-2016, 08:05 AM   #4
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Excuse my ignorance but what autox class limits you to 200tw tires yet allows turbos and 14" spoilers? Hoosier grip is your solution here.
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Old 05-21-2016, 12:14 PM   #5
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Quote:
Originally Posted by hector View Post
Excuse my ignorance but what autox class limits you to 200tw tires yet allows turbos and 14" spoilers? Hoosier grip is your solution here.
It's just the class I run. It's a local class that my club made for people that don't want to follow a restrictive rule set on their car. The only way to make it somewhat fair was to keep everyone on a street tire. We are usually the largest class of cars and run times around that of pro STX/STR cars.
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Old 05-22-2016, 05:49 PM   #6
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Progress continues. Now is the time to get the poly-carbonate for it. I want to keep around 2 inches of space around the outside of the supports. I am hoping that 3/16 thickness will be adequate for the job. Really happy with the final product but mounting is going to be tricky. I am thinking about using some blind rivets (nutserts) in the trunk for ease of install. I think some M6 inserts will be fine for strength...and that's the largest manderal I have. lol







Lowest AOA
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Old 05-24-2016, 06:24 PM   #7
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Quote:
Originally Posted by 90civichhb View Post
That's about the limit the HDs can take from what I have read. I'm limited to a 200tw tire but I agree, a 700/400 setup would be preferred.
That's simply not true, spec miata runs 700/400 on them I'm running 800/450 with no problems. Would different valving be better? Absolutely, but you'll go much faster with stiffer rates. The smoother the racing surface the better they'll do until you get them revalved.

I would find a way to seal then spoiler to the trunk and get it scooted back as far as possible. You're missing quite a bit of airflow pushed up that far.
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Old 05-24-2016, 10:51 PM   #8
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Quote:
Originally Posted by Bronson M View Post
That's simply not true, spec miata runs 700/400 on them I'm running 800/450 with no problems. Would different valving be better? Absolutely, but you'll go much faster with stiffer rates. The smoother the racing surface the better they'll do until you get them revalved.

I would find a way to seal then spoiler to the trunk and get it scooted back as far as possible. You're missing quite a bit of airflow pushed up that far.
Wow, that goes against everything I have read but I don't doubt the spec Miata guys are doing that. I feel like the lots that I race in are not near as smooth as a track but I've never been on a racetrack so I have no idea. I will probably just save up for Xida's and go from there, honestly.

I have thought about pushing the spoiler back as far as I can but a mounting solution is hard to come up with unless I add more structure to the trunk. I felt like a 14" spoiler would be high enough to help compensate for the lack of more rearward mounting.
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Old 05-25-2016, 12:31 PM   #9
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center AOA lower than the other two so its slightly concave will yeild more than 50 deg across the board
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Old 05-28-2016, 06:19 PM   #10
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Still some minor finish work to do on it but pretty happy overall. I am using 1/8th Lexan right now and plan to transfer the template over to another sheet of 1/4" that I picked up. This is about the max AoA I can achieve with the current design. I picked up a ton of Lexan from a dude out in the boonies on Craigslist, so I will have plenty of material to play around with.

The mounting was a bit tricky and I would have liked to get it further back. I think the next one will be 8 to 10 runners with 12-14 inch supports. I may need better hardware for that though.







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Old 05-31-2016, 02:10 PM   #11
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The trunk is sloped down, measure spoiler angle relative to horizontal so you're speaking the same language as others when discussing spoiler angle. "0 spoiler angle" would be horizontal with the ground.
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Old 05-31-2016, 04:18 PM   #12
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Quote:
Originally Posted by ThePass View Post
The trunk is sloped down, measure spoiler angle relative to horizontal so you're speaking the same language as others when discussing spoiler angle. "0 spoiler angle" would be horizontal with the ground.
Really happy you took the time to respond to my method of measure. That's honestly why I put that image up there. I'll be sure to measure horizontal to the ground. I think it may be significantly less than 50* AoA measured that way.


I was able to take the car for a test drive the other day and noticed a significant difference in power. My wife even noticed it was slower pulling through second gear. I hope the added drag is giving me some downforce as well. At speeds above ~45mph I can see the polycarb flexing and reflections distorting. Also, heads up for anyone looking for a similar spoiler, if the sun is pretty high in the sky expect your rear view mirror to beam that **** right deep down into those optic nerves. First autocross with it Sunday. The best part is that it is the same course we did two weeks ago in reverse. I am sure times will be different but, the elements will be very similar so it's a great opportunity for testing. Had a ton of positive feedback with it so far.

Last edited by 90civichhb; 05-31-2016 at 04:28 PM.
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Old 06-03-2016, 02:05 PM   #13
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The spoiler works by increasing air pressure to the top surface of the trunk lid and the slope of the back window. By mounting the front edge of the spoiler 1/4 to 1/3 of the way forward of the back edge of the trunk you are giving up a lot of increased pressure area on top of the trunk and adding decreased pressure area on the top surface of the trunk behind your spoiler. This is far from Ideal. The thing to remember is you are always trying to increase pressure on upward facing surfaces.
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Old 06-03-2016, 02:13 PM   #14
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Also worth noting is that bro-spoiler on your hardtop is reducing the effectiveness of the actual spoiler on your trunk. You want airflow to stay attached to the car so it actually gets to your massive trunk spoiler.
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Old 06-03-2016, 11:10 PM   #15
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So, just to be clear, the top does not stay on the car when I race it. It's hard to get past the looks of the Miata, for me. It's got great engineering but the aesthetic leaves so much to be desired. I put the hard top spoiler on to detract from some of the "bubbly" lines of the car. Do all agree, no, but it's a personal feeling towards the car.

So am I better off having this spoiler or not? I would love to have a place to test it against but autocross isn't a road course and collecting data on what works is difficult, for obvious reasons. I'm not looking to compete in SSM or XP, this is a street tire class with cars that are in the same RAW time as top STS/STR drivers.

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Old 07-14-2016, 07:21 PM   #16
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Got a chance to test this out finally. Although I only autocross the car so a direct comparison is almost impossible, unless, I was able to swap trunks or something between runs... which...I may try.

Set the spoiler at max AoA, still not sure what the total number is based on the ground level, and set up launch control. These were the only changes made from my autocross last month to the autocross this weekend. I won my class by .8 of a second and was 5th fastest in RAW time this weekend. The fully prepped STX FRS that beat me was one of our local PRO guys and he only got me by .057 of a second the rest were on Hoosiers. The improvements were staggering and the car was much more neutral. I was having a very difficult time trying to control the oversteer in the car at my last event. I wish I could get better data and see how much the spoiler improves times. It feels much more compliant and performed great. I feel the spoiler was very helpful in balancing out the car and look forward to developing more aero on the car.

5th entry down.
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