Presented by Nine Lives Racing
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Dusterbd13-michael MegaDork
8/15/19 7:33 p.m.

so im second guessing myself. 

This is my autocross challenge car spoiler. Its 18 inches tall from the decklid, 49 inches wide. Bottom/rear of the spoiler rests at 3 points on the decklid(edges and center) and also bolts the the 1x1/8 steel that is retained by a nutsert in the rain gutter at the rear corners (between the trunk weatherstrip and quarter panel). Top will be retained by wires (two fron the centerpoint, one either end) to tabs under the Frankenstein bolts. 

Spoiler itself is made from 2x4 for reinforcements, an aluminium tube at the bottom, and a scrap of 1/8 luan. 

Is this too much? Too much downforce? Gonna tear itself to shreds at 30mph? 

I know its not class legal (xp, can only be 10 inches high i think) but this car is so far from competitive in class i dont think anyone would care (10 seconds behind 3rd place last event). 

Im more concerned about structural integrity of the spoiler, making the car an understeering pig, or causing damge to the body of the car itself. 

So, thoughts? Anyone able to do the math on how much downforce this monstrosity will make? 

Robbie GRM+ Memberand UltimaDork
8/15/19 9:10 p.m.

I heard too much was never enough.

I cannot do the math on that thing. But you can go test it. Go to an empty parking lot and do circles. Time yourself with a friend and a stopwatch if you want to get fancy. Then add or remove the spoiler and do more circles. If time/feel improves, you know it's working.

maschinenbau GRM+ Memberand SuperDork
8/16/19 8:02 a.m.

If you're worried about understeer, just build an equally ridiculous splitter.

jharry3 GRM+ Memberand HalfDork
8/16/19 8:09 a.m.

Is that what you call an "air brake"?    Just flop it up when you want to stop faster and it automatically folds down at speed?

Dusterbd13-michael MegaDork
8/16/19 8:16 a.m.

Im hoping to test tomorrow or Sunday. Two day autocross, so i should be able to almost do back to back before/after testing. Im just concerned about possible damage from it in the process....

AngryCorvair GRM+ Memberand MegaDork
8/16/19 8:27 a.m.
  1. get a junk trunklid
  2. remove nice trunklid
  3. mount spoiler directly to frame rails in trunk
  4. cut junk trunklid to fit around spoiler supports
  5. be disappointed that you don't have enough power to push that much drag
sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 9:16 a.m.
AngryCorvair said:

      5. be disappointed that you don't have enough power to push that much drag


Drag Force (i.e. how much the spoiler will try to slow down the miata.) = Df = 1/2*rho*(V^2)*S*Cd   sometimes simplified as Df = q*S*Cd

rho = "density of air", using 'standard day conditions, english units  = 0.002377 slugs/ft^3

Coefficient of Drag of a Flat Plate
Cd_flat.plate ~= 1.5

Area of Wing (S) = 49*18 = 882 sqin = 6.125 sqft
Speed looking at V1 = 60mph = 88fps
Speed looking at V2 = 80mph = 117fps

S*Cd_flat.plate = 6.125*1.5 = 9.2 sqft

"Dynamic Pressure" (short hand)  q = 0.5*rho*V^2
q1 = 0.5*.002377*(88^2) = 9.2 slug / ft*s^2   (units iirc)
q2 = 0.5*.002377*(117^2) = 16.3 slug / ft*s^2


Df1 = 9.2 * 9.2 = 85#s
Df2 = 9.2 * 16.3 = 150#s

I think the extra power from the V6 will trade that drag off

edited to clear it up some; with caveat that I'm happy to be proven wrong if someone else comings along and disputes my numbers

Dusterbd13-michael MegaDork
8/16/19 9:18 a.m.

In reply to sleepyhead the buffalo :

English? What am i looking at?

AngryCorvair GRM+ Memberand MegaDork
8/16/19 9:30 a.m.

In reply to sleepyhead the buffalo :

Ok, I’ll accept your drag numbers.

IIRC duster said 800 lbs of rear downforce.  Is that correct?  

Dusterbd13-michael MegaDork
8/16/19 9:43 a.m.

In reply to AngryCorvair :

That was my guess based upon claimed numbers from a spoiler half this size. So, im probably blatantly wrong.

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 10:44 a.m.

The data I have is based on 20deg from vertical, and tops out at 100mm tall (~4in).  I’m going to do some work after dinner and see if I can do some extrapolation.  Then I’ll tie this all back to “power required”

Justjim75 Dork
8/16/19 10:46 a.m.

All spoilers and wings add drag, right?  Seems like nobody runs that big of a spoiler because the cost of the drag out weighs the benifit of the downforce? Unless its part of the concouse, if that's the case, carry on.  It looks like a freaking parachute to me and for the sake of the trunk lid id chop it in half

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 11:05 a.m.

Yes, all spoilers & wings make drag.  Spoilers make more drag than wings, generally.

and nobody runs spoilers that big because the only SCCA class that allows spoilers also restricts their size.  The other classes that are more open, the higher downforce capability of moving to multi-element wings makes running a comically large spoiler moot.

however, when budget, time, and the finesse required to get a multi-element wing right... massive spoiler could win-out

iirc, ymmv, #becausechallenge, etc

edited to fix my reading over 'spoilers *and wings*'

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 11:52 a.m.


slowbird Reader
8/16/19 12:11 p.m.

Not that you would try this, but I wonder if, with the spoiler on for drag racing, could you get enough grip to do a wheelie? laugh

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 2:10 p.m.

Alright, so I took the data from Katz, page 226 which has variable height spoiler set 20deg from vertical, which only goes to 100mm of height (~4in) and gives a downforce increment of -0.4.  I estimated off values from the graph provided and pulled them into a cell-based quadratic regression to spit out a 2nd order polynomial approximation.  This is probably not sufficent, and I should have gone 3rd order... but I manually tweaked things a little bit, and think I have a "ballpark" downforce increment of -0.7.

simple english...
I took the graph of spoiler downforce that doesn't go far enough, used a bunch of maths and tried to use it to predict what the downforce would be for an 18" tall spoiler (~450mm).

I'm going to use a "worse case scenario estimate" of a drag increment of 0.15, and a base drag coefficient of 0.43.... for a total drag coefficient of 0.58

As opposed the calculation I've done above, all of these are based on the Crossectional-Area of the miata, which is ~18 sqft.

so, our Cd*A = 0.55*18 = 10.44

and our Clf*A = 12.6


plugging this in to an acceleration estimate sheet... in 4th gear, at 60mph

your base drag and weight would mean about = 8.76 ft/s^2 of acceleration

while the spoiler would mean about = 8.38 ft/s^2 of acceleration

so, you'll loose 4.5% of acceleration due to the spoiler's drag, while getting 100-200lbs of downforce for 'upper autoX speeds'

* "Total Spoiler Force" applies a pythogorian assumption to the drag and downforce forces to estimate the total load from the spoiler onto the trunk lid.

One of the interesting things to see is how closely matched the Clf*Across and Cd*Across values are, although it's important to note that most of the drag comes from the size/shape of the miata's body.  I'd have to dig back through Robbie's thread to try and make a drag comparison... but it's Cd*A would probably more like 8.5sqft

standard caveats:
I'm pushing the data I have a bit, and using some guesses.  It's possible I've guessed wrong, or that I've screwed up a calculation somewhere.  I retain the right to be wrong, and or be corrected.

Dusterbd13-michael MegaDork
8/16/19 4:00 p.m.

20, 200lbs at 80mph roughly, and 120 at 60?

Also, its done.

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 4:12 p.m.

In reply to Dusterbd13-michael :

that's my guess until stafford1500 comes in here and smacks me upside the head.

also, I really need to go buy Hucho... I think this is like the fifth time I've batted around with spoiler questions in the last 9months-year.  Supposedly it has more answers/data on this.

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/16/19 4:17 p.m.

I'm kind of tossed between suggesting you write this:




"Becauser GRMChallenge Car"

should be scrawled on there to head-off any "protesting"

Patrick MegaDork
8/17/19 7:49 a.m.

In reply to sleepyhead the buffalo :







My biggest fear is it’s going to rip out the nut serts and destroy your trunklid and possibly mangle the quarter panels.  Build what you want to own.

Robbie GRM+ Memberand UltimaDork
8/17/19 10:15 a.m.
sleepyhead the buffalo said:

Erm, what book is that and what other books do you suggest for aero?

Also, nothing beats


I really need to get on making my t shirts or stickers with that one.

mazdeuce - Seth
mazdeuce - Seth Mod Squad
8/17/19 10:17 a.m.

I bet a taco at the Challenge that it rips off the car before 50mph.

Dusterbd13-michael MegaDork
8/17/19 10:20 a.m.

Well, it stays on just fine to the rev limitef in third gear! Have no idea how fast that actaually is, but way above 50....

So, taco bet it is!

sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
8/17/19 10:26 a.m.

In reply to Dusterbd13-michael :

1)  Pfewww

2)  But, how did it feel!?

Dusterbd13-michael MegaDork
8/17/19 10:42 a.m.

Way more planted than before. I can use a lot more throttle now! And the squirmy feeling over transition strips/camber changes under throttle with a turn going on is gone! So, exactly what i hoped it would feel like.

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