In a previous article I covered Miata fender vents in some detail, including commercially available louvers and DIY solutions. I cited Race Louvers wind tunnel data, which found that after a certain size, more vents don’t equal more downforce. What I didn’t mention was the reason for those diminishing returns.
Take a look at a Miata unibody with the fenders removed, and you can see the problem. At the top of the fender, vents can reach just about the width of the tire, but the air has to travel all the way up there in order to be extracted. A more direct route is behind the tire, but the chassis is in the way! Moreover, air needs to take a 90-degree turn to make its way out of the wheel well. On a Miata, there’s a sharp lip here where two seams come together, which makes it even harder for air to turn the corner.
In order to extract more air, the vents need to reach further inside the wheel well. Ideally the vent should be located low and behind the tire, because that’s the most direct exit route from the trailing edge of the splitter. And while doing that, you’d also design a much smoother exit. The end result would be quite different than the haphazard dumping and venting of air into the wheel well, and more of a purposeful and free-flowing duct from splitter diffuser to behind the wheel. Now these aren’t new ideas; you can see all of that on a McLaren M8 from half a century ago.
A more subtle solution can be seen on the underside of current NASCAR stock cars. Notice the air channels behind the tire that exit at the front of the rocker panel. This allows the cars to look stock-ish, but they perform much better and make a lot of downforce. In fact too much downforce, because NASCAR had to reduce the effectiveness by adding splitter stuffers.
I don’t know if these vents have a proper name, but I’m going to call them rocker vents based on their location in the rocker panels. They are a very Occam’s Razor solution to extracting air from under the splitter; of the myriad ways you might solve the problem, rocker vents are the simplest. Well, at least from the perspective of air, it’s the most direct route.
I wanted to implement something like the NASCAR rocker vents on Falconet, but hadn’t seen anyone else modify their Miata in such a manner. AJ Hartman did something like this on his Mustang, and said it was very efficient, with a L/D of 14:1.
So I knew it could be done, I just didn’t know how to do this on a Miata. I did a lot of exploratory cutting on the driver’s side, and the pictures show how I did a much cleaner job on the passenger side.
I want to thank Aussie reader/supporter Jim Grant for assistance with this project. He recognized I had no idea what I was doing and explained how cars are put together, and how to get my head out of my ass. Thanks Jim!
Step 1: Measure
My first decision was how large (height and depth) to make the vent. There are a few considerations for height, see the following image with colored lines.
- The white line is the most conservative cut, because it goes just below the curved wheel tub inside the wheel well. Cutting at this height leaves two threaded holes intact, in case you want to add bolt-on fender braces.
- The yellow line is about where the mid-body style line is on a NA Miata. Aesthetically, this is probably the best place to cut a NA Miata, but on a NB it wouldn’t matter. This cut goes between the two bolt holes, and so you could still use the upper hole to mount something.
- The red line is to the top of the wheel tub. You could go higher than the red line, but most of the air you’re trying to extract is below the splitter, so I’m not sure if a higher cut helps that much. Might look cool tho.
In retrospect, if I did this again, I’d cut on the yellow line. Mostly because I think it would look better to have the duct exit on the style line. I don’t know that there’s a lot more air to extract at that height, and it could be that a vent even half this high is just fine (the NASCAR vents are quite low). In any case, I used the white line.
The next decision is how deep to make the cut. I measured and marked a line 9.5” from the inside of the wheel tub. This is where the inner sheet metal is located within the footwell, and so the cockpit remains unmolested.
The rocker vent would be more effective if I had cut further inside behind the wheels, but then I’d need to weld sheet metal inside the cabin. Going further than that, there’s other unibody structure and cage tubing that would be difficult to work around. If I’d kept going deeper, I’d hit the clutch dead pedal, which could be sacrificed I suppose, but I do use it.
All said, I chose to make the rocker vent only about 3” (75mm) deep. This isn’t a lot of extra volume, but it does allow air to exit at less of an angle.
I used a sharpie to draw a line back to the hinge at my desired height. I’m not going to use the lower hinge (my doors are only half height), but there’s a vertical chassis member here and I wanted to keep that intact. So at this point, I have defined the area I want to remove.
Step 2: Surgery
I used a grinder with a cutoff wheel to cut along the lines. There’s a horizontal shelf inside that has to be removed. It’s spot welded in various places (you can see the dots) and I tried drilling those out and in some cases getting in between with a cold chisel and breaking the welds. This little shelf is a pain in the ass.
I continued to cut out all of the sheet metal and spot welds until I had a rectangular-ish hole like this.
The interior footwell and exterior wheel tub are two pieces of sheet metal with an air gap between them. I used a cutting wheel to make a slit in this area, and then used a reciprocating saw to cut out the sheet metal. I ended up with a cavity or “smile” between the interior footwell and the fender well.
To get a smoother exit for air, I used a cut off wheel to cut several straight lines through the bottom sheet metal. I then bent the sheet metal upwards to close that gap.
I used a hammer to tap these into a more graceful arc.
I then cut the front of the rocker panel off at an angle, so that air can make less than a 90-degree exit. Notice the rocker panel is made of a few layers of sheet metal, and plays a significant role in chassis structure and stiffness.
Next I’ll weld in some sheet metal to cover all this ugliness, and provide a smooth path for air to flow.
Step 3: Welding
Welding Mazda sheet metal kinda sucks. I was once a certified structural welder, but you’d never know looking at the shit job I did on this. I got the best results by spot welding pieces together, and then joining some of those to create stitches. Note that continuous welds are unnecessary, as you can fill the gaps with seam sealer.
First step was to bend in a piece of sheet metal to make a smooth curve, and then cut it to size.
Next I welded in a horizontal flat that makes the ceiling part of the vent. Otherwise there would be an open cavity that might collect gunk inside. I welded that to the curve I had cut.
Next I welded the tabs I cut in the footwell area, that I had bent upwards with a hammer. I ran a cutoff wheel into the overlapping pieces of metal and pulled the little triangles out, then smashed it all flat again. This way the seems were pretty tight and easy to weld inside and out.
Now the fun part, bending sheet metal into curves and tacking it into place. I used aviation shears to cut sheet metal into various shapes, and then tack welded it all together. This process is more art than science, and satisfying work.
The next step was to add external bracing to the shotgun panel (the frame member that connects the door hinge area to the shock tower). There are several commercially available fender braces, but I chose to DIY my own and weld it on rather than use bolts.
I used 1” square tubing and 1/8” steel spreader plates to make a triangular brace. Weight weenie that I am, I was pleased they only weighed 3 lbs apiece. I then welded the braces in several spots, choosing places where the sheet metal overlapped and was doubled in thickness. I also seam welded the entire front of the chassis for good measure.
I ordered a new set of fenders, which I’ll cut artfully to expose the rocker vents. In the meantime, I took my existing fenders, which have a large fender cut, and mounted them up to see what this all looks like.
When looking at the rocker vent from the rear, you can see that air will exit much lower and smoother behind the wheel. No more 90- degree bend or having the air exit between the chassis and the quarter panel.
Step 4: Finish
The final steps are to fill the gaps between the spot welds with seam sealer (automotive caulk). I have also heard that you can fill the rocker panel and front cavities with expanding foam, which supposedly adds more rigidity. I don’t know that expanding foam from a can (Great Stuff) is appropriate for this, but it would certainly be easy to do.
I’m not going to cover the rocker vents with quarter panels; the area will be entirely exposed. So it needs Bondo, primer, and paint to match the bodywork. I haven’t done all that yet, but it’s just regular bodywork, and I’m the last person you want to watch do that.
When that’s done, I’ll also add some vanes under the car. I’ll take my inspiration from NASCAR again, and guide the air out the new rocker vents with a pair of strakes.
Wind tunnel test?
To find out how much downforce and drag the rocker vents make, I’ll fabricate rigid covers that approximate the shape of the original bodywork. I’ll then remove the covers and A/B test the vents back to back in the wind tunnel.
The A2 wind tunnel has a static floor, and so it’s not as accurate as one with a vacuum to remove the boundary layer, or a rolling floor. So while the numbers won’t be 100% accurate, there will be a useful delta value, and a way to compare the rocker vents to other vents.
Caveats aside, I’ll update this article with those results sometime after 6/20.
Now it’s your turn
If I’ve inspired you to make rocker vents, consider the following:
First, do you really need rocker vents? A splitter alone makes enough downforce to offset a low-angle single-element wing. Add splitter diffusers and vent the fenders, and you may get another 50% more downforce. Add spats, side plates and canards, and you could double the original splitter’s downforce. At this point you’ll almost certainly need maximum wing angle and a Gurney flap, and even then the front aero load distribution may be too high (oversteer in fast corners).
But if you’ve already done all the tricks and still need more front downforce, then the rocker vent is perhaps the next step. But you’ll probably need to add some combination of spoiler, second wing element, and rear diffuser.
On the other hand, maybe you’re not after maximum front downforce and are simply after better efficiency, or maximizing the effectiveness of the undertray. Or perhaps you’re dodging the points taken for using a splitter, but you want similar downforce. That’s a pretty clever use of rocker vents, and I would definitely get on board with any of that.
Next question, do you have a full cage? You’re removing important structure from the unibody and a full cage is arguably a requirement for rocker vents. Without a cage, you’ll need fender braces or other supports that help support the shock tower (no, not a strut tower brace).
As a practical matter, most commercially available Miata fender braces use the two threaded bolt holes in the wheel tub for mounting, and the location of those bolts limits the size of the rocker vent. If you recall the previous image with the horizontal lines, you’ll want to cut on the white line, or lower.
Final question, if you’re racing, do your rules allow modifying the unibody structure? For any Spec racing series, that answer is certainly no. For other series, it will depend on how they evaluate such modifications.
I’m building Falconet to the NASA ST/TT 3 rules, and changes to the unibody require the “non-production chassis” mod, which is a .4 points penalty to the lbs/hp calculation. If I was building to the NASA ST/TT 4-6 rules, then this modification would be illegal, as the rules state you can’t alter the unibody.
My car also fits into SCCA Time Trials Unlimited 2 class, and this would be legal. But not in any of the Max classes (in fact Falconet isn’t legal in Max for other reasons as well).
I didn’t look up the SCCA road racing rules because it’s a 1000 page rule book. Nor did I look at the 400 page autocross rules, because this is an aero mod, and unlikely to help much at 40 mph. But there’s probably some flavor of unlimited class my car would fit into if I wanted to roadrace with the SCCA or dodge cones in a parking lot. (I don’t.)
Most of the Grid Life Trackbattle classes state that you can’t make modifications to the chassis, and that includes GLTC. But you should be able to get away with rocker vents in Street Mod, Track Mod and the Unlimited classes.
Most endurance racing rules would allow rocker vents, and with their high efficiency, it would be a good idea for AER, 24 Hours of Lemons, Lucky Dog, Northeast GT and WRL. Champcar would assign material points (2 points per square foot of metal), and that might be 4 points total. But a clever team could reuse sheet metal taken from various weight loss trimmings and/or use the lower quarter panels and do this mod for free.
If you aren’t racing or tracking your car, one could argue that rocker vents are a lot of work for Racing Inspired Cosmetic Enhancement (RICE). But if that’s the way you roll, you’ll one up everyone else’s fender vents at the local cars and coffee.
All told, this project cost me maybe $40 bucks in materials (8 feet of 1” steel tubing, welding wire and gas), so if you have more time than money, I say why the fuck not? So now it’s your turn. Let’s see some rocker vents!
Occam's Racer 