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Miata Splitter / Air Dam

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I got bored.  Decided to climb under the car and mock up a splitter / undertray setup to mate up to the Renderos Racing air dam I ordered today (easy button, I think).

Started with stripping the lip, the undertrays, the grill, OE ducting, etc.










Initially I thought I would start at the rear most Subframe bolts or the Cross brace bolts to see how far back I could extend.  Based on other pictures I’ve seen, etc I didn’t think I’d get there with a standard 4x8’ piece of material, but decided to measure things out starting from there anyway.  These 4 bolts (M10 I think) sit “inside” of the LCA’s so I thought using them in the rear with some spacers or whatever is required would make for about the strongest rear mount I could come up with, and wouldn’t get in the way of the LCA’s.   Spoiler alert, too bad but they’re almost 5ft from where I’ll want the splitter to extend in the front, and I need to go more than 5ft wide, so I guess I’ll be bringing the “rear forward”.  I’ll probably measure / trace the template out on an angle on the plywood or whatever I use to get more than 48”.....but I don’t think I’ll get enough.





The bolt I’m pointing at I’m calling the “cross brace” bolt, which is about 7” forward of that rear most subframe bolt (seen to the right).....and it’s also basically the “low point” in there, so I’ll target about an inch behind that to extend (if possible)...

Now, for some good old Canadian (Metric) measurements.  I started off by going about 65cm wide between the control arms, which also straddles those cross brace bolts nicely.  Then I roughed out the spacing between the arms etc. for reference later to square everything up, and also check against the measurements taken before trying to hold the cardboard in place with a knee while measuring / marking 😂







Zip tied that portion into place, then moved on to filling out the corners.  Initial cut to fit stationary wheels, then measure and trim to account for steering travel.  Probably need to clearance some more, but for now left with about an inch of clearance all around.







Finally, traced out about 2” from vertical all around the bumper (the width of my level and a sharpie) to get a rough estimate of the total width and depth of the splitter.  Worked out to about 62” wide and about 56” deep to an inch past the rear most subframe bolts.  Subtracting ~7” to move up to an inch behind the cross brace bolts gets me to ~49”......without yet accounting for the cardboard flex vs ridgid plywood / frame.  So I’m in the ballpark of extending a couple of inches past the air dam, and maybe reaching that one set of M10 bolts depending on what this thing looks like all traced out.



Honestly I went about this without really doing any research, so I’ll probably do that tomorrow.  If any of you have any advice / tips / tricks, or things you wish you had noticed your first time around, let me know.

Most interested in mounting locations, and good ideas to make things removable etc.  I’m assuming that people use the frame rails in the wheel arches to build vertical supports (accessible from the side of the car to remove), and connect them up to a frame on the splitter.  I’m assuming once I look, I’ll find some “standard” locations / logical mounting configurations.


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Just a question-- are you planning to race the car in any major groups? (SCCA, NASA, etc)?  going behind the front axle centerline is a no-go for many classes because it then becomes underbody aero, not just a splitter and airdam.

If you're not planning to build to a rulebook, then carry on and enjoy the fun. 🙂

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I love your enthusiasm, thought process and energy you put into the hobby.  Also, in my sheltered life, I've never seen such a long metal ruler.  Yes, I live in a virtual world apparently.

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NASA ST rules also dictate the undertray/splitter can't be any lower than the front fascia, so look out for that too.

-Don't forget that your tires will be in a different position with the car on the ground

-A good attachment point is to bring down vertical supports from the frame rails at the front; there's a bunch of M6-1.0 holes there for various things you can repurpose.

Have fun!


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Took an hour tonight to take a look under the bumper skin.....then somehow like another hour+ googling thread shear (stripping) calcs.....

First off, this template is huge....and yeah, somehow I’m leaking oil through the press fit at the bottom of my dipstick when the car is at an exaggerated angle.  Another excuse to pull the motor and do more seals?






Once I pulled all that off, I sized up the crash beam, and considered repurposing the (4x) M8 bolts that seemingly just hold that plastic bumper insert in place.  The plastic tabs on that white pool float could be broken off by hand, so I don’t understand the need for (4x) M8 bolts - but I’m referring to the two protruded bits on either side at the front of the car.  There are therefore (4x) M8 threads pointing down.






I’m not sure “how” I’d plan on using these, unless it would be to support the splitter frame first, and THEN install the bumper cover (reverse order to remove obviously)....but it would provide a lot of strength out at the front of the car rather than back inside the wheel wells.....obviously by giving up some “easy access” / “ease of disassembly” in the process.

So anyway, I saw a “6” stamped on the bolts, which I assumed worst case class 4.6, and did some more “Canadian” (Metric) math:



I don’t know if any of that is correct, or applied in the right way - but basically I calculated the Proof Load based on worst case class assumption (~1,850 lbs); then calculated Thread Stripping (Shear) Proof Load [if that’s actually a thing].....which assumed “sloppy threads” rather than “perfect / ISO class threads” (~1,210 lbs).

Finally, applied a 0.50 S.F. on top of all that, and considered all 4 bolts.  Look reasonable?  Looks like (4x) M8 x 1.25 Class 4.6 bolts with thread engagement ~= to thread diameter could support ~2,420 lbs with a 0.50 S.F.

What I didn’t find or explore is something I remember about a bolt in single shear being valued at about half of that same bolt in tension?  Assuming that with adequate engagement, “thread shear” kind of just works like a crappier version of bolt tension, and isn’t the same “shear” as that same bolt in say single shear?  

Yes, I’ve gotten too deep into this already, shut up and rivet some stuff on already, and so on.

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Yup, those are the holes I was talking about.

The balance here is strong enough to not come apart through normal use or gentle offs, but weak enough to break and tear away if the splitter catches on something. It's a tough call. If you go off a lot, it's not a terrible idea to carry spares for easy replacement at the track.

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I think everything turned out ok (my son flipped the headlights up before I disconnected the battery last week, which explains why they’re up in all of the pics):



I made the splitter removable from the bottom - basically installed two “hangers” on the rear, to hang the back of the splitter from, before raising the front up and bolting into the supports hanging from the front frame rails.  Everything is accessible either sitting in front of the car (for now, without a grill or radiator ducting), or through the wheel wells. Overall, it gave me about 3-1/4” extended out past the “vertical-ish” air dam.  I read a “rule of thumb” for this level / design of “HDR Aero 😂” which said anything over 3-4” is wasted when talking about an air dam on a production car.




Overkill on the mounting / hardware, but I’ll use the “garden trim upright” bit to support some of the load forward of the brackets, and avoid (whatever the turnbuckle / wire things are called) on the front for now.  I may consider drilling at the steering rack to install the two support bolts, just to avoid flex from the dP across the undertray.

The Air Dam is currently bolted to the bumper skin (wasn’t sure my 1/4” x 1/2” rivets would span some of the locations based on a test piece).  I actually had to trim an inch or so from the Renderos Racing NA air damn to sit on the splitter at about 3-1/4” off the ground (to match the ground clearance at the back and center of the subframe at the cross brace and main span).  I did this by fitting the splitter in place, and then measuring up to the body line that the NA’s run around the bumper cover, then tracing onto the piece.






In building it out, I riveted the brace uprights to the frame rails (plus recycled one bolt location), then bolted the “L-brackets” in place for ease of replacement / removal if required in the future.





I also sorted out that my 5x Racing tow hook won’t reach anywhere near the front of the air dam, so I guess I’ll have to find a soft strap type tow hook.



And while I was in there, I bought a spare trunk lid, riveted whatever aluminum braces I had laying around to the bracing on the underside, and mounted a rear wing.  The wing is kind of borrowed, and the mounting points had already been riveted onto the underside, so rather than run Miata style uprights at the edge of the trunk, I just reinforced what I had, and went for it.  (Cue another Larry Enticer gif).












Still need to paint the splitter and then drill/connect the bottom of the air dam to the “upright” portion of the splitter.  Also, build some radiator ducting, and figure out some hood vents.  And I guess figure out how to get some thread inserts for the end of the wing, and build some end plates.  Anyone have good luck with rivet thread inserts, or should I be thinking some kind of composite glue (or both) here (to build threaded inserts in the end of the wing for end plates)?

Overall I would say a pretty good week of progress.



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We can splice some plate into that tow hook pretty easy with the MIG if you'd - like to keep the bright red bolt on part you already own :) I'm jealous of how clean your setup turned out. I am still nto happy with the level of "coherence" that I have with my splitter / air dam / front bumper integration.

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Good progress. Looks like you still have ducting for front bumper to work out. What materials will you be doing for that portion?


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I saw some guy post about using the “thinner” duct work sheets that HD sells in like 24” x 24” sizes with success.  Might have been the same post where he learned that it doesn’t make for a good (non-collapsing) air dam 😂.  I’ll mock it up with cardboard first, then maybe try that stuff.  Easy enough to bend up and much cheaper than lower gauge material.  I think I’ve also seen one guy use like an ABS plastic sheet to make a “box” (cylinderish, heat-gun massaged thing) out of one piece but I’m not going to do that.

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>Investigates bolt shear strength 

>uses bent right angles with no bracing and attaches them 6+inches behind air dam

You're killing me dude! If it's doing what it's supposed to, the downforce will be concentrated right at the splitter/air dam interface, in addition to the forces on the air dam. That's where you need to support it! I've gone through development and destructive testing of 15+ air dam/splitter assemblies for NA Miata, and I see you making some of the same mistakes I made along the way.

Do yourself a favor and properly support that splitter and air dam at the bottom, as close as possible to where the two meet. The way it is now, it will tear off and mess up your session, maybe break something else along the way, and then you'll have to build another splitter anyway.

Again... ask me how I know these things...




And how many time's I've spent Saturday evening after a race piecing together/repairing splitters and supports...


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I never claimed to be logical anywhere in here 😂.  But in all reality, I could definitely use a good suggestion on how to best support from the front a little better.  I backed off of building it super robust, and the brackets that you see on there were basically a compromise between “don’t make the structure too strong, you don’t want to damage the car during an off”, and placing them as far outboard as possible while hanging from the front frame rails.  

With 4 of the upright components perpendicular to each other, they should hold decently well vertically, and have about the same amount of resistance to lateral movement as the example that was posted above to hang from the frame rails?  The L-bracket bits were just to reach the height needed, and distribute the load connecting to the splitter over a larger area.



They should also be weak enough to fail before damaging the frame rails - I can’t claim that it was a good approach, but it seems like a good start / compromise.

I also kind of approached it with a “how much downforce can this thing actually generate” attitude.  Actually what amount of force should I expect?   I’ve seen that 949 racing basically suspends their splitters from wire rope with automotive washers, and figured that the (4x) vertical supports with the garden edge bit of the splitter connected to the air dam and most of the OE bumper in place would all work together to not let any more than about a 3-1/4” span go unsupported.

I have some wire rope and turnbuckles, would you recommend basically running them from near the front edge of the splitter back through the opening for the rad, puncture the yet unbuilt rad ducting, and then connect to the rails also?  I haven’t really come across a good reference for that bit?


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It's probably not necessary to support forward of the front air dam, but using the wire rope/turnbuckles just behind it would be a good way to add some support. You can loop the wire rope through the almost-hand-sized holes in front of the radiator, or find another anchor point in that general area. The nice thing about using flexible supports is that they won't bend when you impact something.

I'm anchoring mine currently to the outer mount for the steel bumper... bumps? But I'm running a steel "drift" bumper, so it might not be accessible with the big plastic bumper in place.


You're on the right track balancing rigidity with self-destruct capability.



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Thanks.  If I’m staying behind the front air dam, then I can probably piggy-back a couple of those 5/16” bolts used for the garden edging thing, and then yeah tie into that front beam near the other braces.  Maybe use that M8 bolt on top of those bumper things.  Something like this:





That would keep the “rigid” supports a couple of feet apart, and about a foot or more away from the centerline of the car to either side for off purposes (although I assume they’re bending themselves backwards in any case).

Alternatively, I guess I could add one more center brace and just commit to breaking shit in an off, if that’s the assumption anyway.  Something like this:



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Finally got an hour tonight to look at the cooling ducting.  Seeking input here:

- Radiator frontal area is 24” x 12”

- Air dam opening is 20.5” x 5.5”

- “Easy box” behind the air dam is 24” x 7” at the radiator

- Should I expend much energy trying to build it out closer to the 24” x 12”?

I’m thinking no.  Car had no cooling issues right through June with the stock undertrays on there an no ducting.

I also decided to leave the upper part of the bumper in tact at the top of the air dam opening - to maintain a little “strength” in the bumper cover, and bonus, a place to add another bit of ducting, back to the crash beam.














Also decided to do the o-ring on the dipstick in the car rather than pull motor to do all seals.  For anyone interested, it seems like 5/16” x 7/16” (1/16” X/S).  Looks like that was the leak point (so far).




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Not super happy with how this turned out, but I think the recipe was pretty good, and besides some visual things that will bug me, it should be pretty good functionally.

First, I mocked up the lower portion of the ducting.  Trimmed to the shape of the air dam at the front, increasing in width as it extends back to the radiator, and including both “sides” trimmed to fit, and overlap with the top bits later to build out the “box”.





The lower portion was made to bolt into place for ease of removal, while two additional upper sections were either permanaently riveted in place, or tied into the bumper / air dam and overlapped.

The first, permanent, section was riveted to the crash bar, extended back to the radiator, and had about a 1” “lip” at the back to sit against the rad and help with directing air flow.





After the permanent piece which basically connected up the crash bar to the rad, I went ahead and built a second upper section.  This one tied into the air dam at the front, overlapped the lower section on the sides, and overlapped the permanent section back toward the radiator.  It bolts up behind the air dam in the front, and is metal duct taped over the permanent riveted section in the back.




Finally, I riveted some straps to the lower section since its relatively thin ducting, and spans a decent area (~24” x 16”), and then fixed it in place.  It bolts through the overlapping side sections from the upper pieces, has a metal duct taped 1” lip at the bottom of the rad to keep that in place, and will also bolt through the front of the air dam once I get a little more time to play with it.  Should be relatively easy to remove, and strong enough to hold together.  Momentary evacuation of my brain when I did the straps, and although I had all intentions to rivet them from the inside, with straps sitting on the outside.....After marking and drilling, I riveted from the outside-in.  Functional, but less visually appealing than it should be.






I also got a chance to paint the splitter today, I think it turned out alright.





Next up, I need to install the splitter and make the holes to connect the bottom of the air dam to the splitter upright edging piece.  

I also need to figure out a hood vent solution, and figure out how to make some anchors in the rear wing to accept end plate bolts.  I’ve signed up for this weekend at MSR-H, and so far timeline looks ok, I think...

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I had something similar to that on a E36 M3; mounting and wood type. Don't be like me and test with a 70mph run oh highway without a second person following.  The down force was ridiculous (noob design) and riped the bolt heads off holding the 1 in wide 1/4 steel flat bar.

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Nope.  Got out of the garage (barely), but no-go on the driveway.  Guess I’m making a ramp, or notching some hockey pucks 😂



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8 hours ago, Solo_S14 said:

Nope.  Got out of the garage (barely), but no-go on the driveway.  Guess I’m making a ramp, or notching some hockey pucks 😂



Been there 🙂

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