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Old 07-16-2014, 05:18 PM   #1
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Default Front fender aero testing

I found myself with a bit of time and a magnehelic gauge yesterday, and did a bit of testing. I'm trying to get more ventilation around my brakes, and I've been curious about playing with the front fenders behind the wheels.

This was tested on an NA (the Targa Miata) with a front splitter and undertray and stock plastic near the rad. It's not completely sealed off in the nose because that's never been necessary for cooling. Tires are worn 225 NT01s on 15x8 6ULs.

I strapped the high pressure hose for the gauge to the upper control arm near the shock - it was an easy place to anchor. Ambient pressure was taken inside the cockpit. Fender liners were in place. Driving down the road showed that the wheel well is definitely high pressure. It gained 0.25 inches of pressure at 75 mph.

I then pulled the fender liners. Interesting result - 0.30" of pressure over ambient. Didn't see that coming. The fender liners do cover the front of the wheel well somewhat, but only at the top edge. Nothing like an NB setup. I'm not sure if the trailing edge of the fender was pulling air in.

Then I pulled the rear of the fender away from the door using a high-tech and carefully made set of precision brackets. Fender liners were left off. At 75 mph, it was back to 0.25" of positive pressure.

So, opening the back of the fender didn't cut pressure at all. It's possible that I've got airflow through there, I didn't have any yarn on hand for tuft testing to see what airflow looked like.

Anyone else experimented here? I've seen what ThePass has done to his poor fenders, I'm seriously considering giving mine some surgery in the lower half but only if it gains me something.

Precision bracket.


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Last edited by [email protected]; 07-16-2014 at 07:28 PM.
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Old 07-16-2014, 06:35 PM   #2
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We did just that on the weeken. Pulled the fender, cut the bottom off. And used a piece of aluminium sheet curved to extract air. We have not measured any gains yet. But will be wool tuft testing the area.


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Old 07-16-2014, 06:40 PM   #3
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Quote:
Originally Posted by [email protected] View Post
I've seen what ThePass has done to his poor fenders
Hey, mine my be a bit on the extreme side, but I cuddle with those fenders and bring them flowers, they live a good life

From everything I've seen, venting higher rather than lower along the rear fender edge is better. I think this has to do with proximity to the open bottom of the wheel well, only venting down low just pulls air from the lowest section near where the air is moving under the car and doesn't affect the rest of the wheel well much.

I also think there are different goals with different setups..

For just reducing pressure if you don't have a lot of flow into the wheel well, your best bet is louvres up top forward of the wheel centerline. This is the least turbulent area where the louvres can do the most good.

In some more specific set ups, like with mine (3" brake ducting and diffusers in the splitter which feed air into the wheel well) the goal was to give the air an exit otherwise I would probably have much higher pressure than what you were seeing.

I need to get a magnahelic gauge... wondering if the $50-60 ones on amazon are decent enough to trust their readings..
All that said, even what you've experimented with so far saw a 0.05" change. That's a result..

-Ryan
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Old 07-16-2014, 07:14 PM   #4
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i was just going to suggest what the Pass just suggested, louvered vents at the top of the fender. ALA a smaller version of what you guys sell for hood vents.
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Old 07-16-2014, 07:33 PM   #5
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Interesting, I hadn't considered that. I need some sacrificial fenders to test.

I do have brake ducts feeding into the wheel well as well as some spillage from the "mouth" of the car. I imagine it's pretty close to overall underhood pressures, given the fairly open nature of the V8R front subframe. The wheel well is basically open to underhood from the bottom of the frame rails.

The gauge I'm using runs from 0-3" of pressure. It's big, too, so it's easy to get 0.05" precision. We have another with a different scale, but I forget what. Numbers were very repeatable.
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Old 07-16-2014, 07:47 PM   #6
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Quote:
Originally Posted by [email protected] View Post
The gauge I'm using runs from 0-3" of pressure. It's big, too, so it's easy to get 0.05" precision. We have another with a different scale, but I forget what. Numbers were very repeatable.
What's the brand?
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Old 07-16-2014, 08:04 PM   #7
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I'll check tonight. At least, I plan to check tonight. My memory may have different plans. Feel free to harass me until I give an answer.
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Old 07-17-2014, 12:36 AM   #8
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Dang, these discussions keep popping up at just the right time. I measured and marked cut lines on my hood and fenders last weekend, but wasn't completely happy and decided to hold off and do a little more research before cutting. Glad I did.

I'm with leafy and thepass- louvers on top of the fender seem to be the ideal approach. All the off-the-shelf bolt-on louvers I've seen on miata fenders look like ***, though. I had decided to do simple vents behind the wheel with the "cut 3 sides, bend metal in" technique, but I decided that would be tough to make look good with the curve of the stock fenders. This is what I ended up with. The plan is/was to just cut these sections out and then epoxy some black mesh to the back side.



The basic concept is pretty common on pro cars.



However, after learning about the fab technique for those louvers being discussed in the cooling thread, plus the fact that my boss just bought a waterjet, I think I'm going to take some measurements and make some miata-specific fender louvers. I'm not going to have time to mess with it for a few weeks, but I think there's potential for a really good looking solution.
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Front fender aero testing-20140712_191709_zpsioby9nki.jpg   Front fender aero testing-fender_vent_-_ferrari_575_gtc_evoluzione_race_car_-_1_zpsnmrbh3hx.jpg  
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Old 07-17-2014, 01:06 AM   #9
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Isnt there a home depot dryer louver or something thats like the perfect size (like 6" x 3") to go above the wheel? I swear thats the louvers people used before FM started selling better ones.
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Old 07-17-2014, 09:16 AM   #10
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vents go on top of the wheels





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Old 07-17-2014, 09:18 AM   #11
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Or more on our level of aerodynamics
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Old 07-17-2014, 10:59 AM   #12
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Le Mans prototypes are required to have fairly large cutouts above the wheels to prevent inverted flight, BTW. You can see them grow over the last few years as the regulations change.

It would be hard to make a louver panel that dropped on top of the Miata fender, given the curved shape of the fender. It might be possible to cut and bend some, but again the louver is going to want to straighten that part of the panel. Should make for some interesting experimentation. I might do some pressure testing to find out where the biggest pressure differential is.

My gauge is from Dwyer Instruments. As I noted, I was using a 0-3" gauge but I think the other is a 0-1". I'll see if I can find it, it'll be more precise.

Last edited by [email protected]; 07-17-2014 at 12:02 PM.
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Old 07-17-2014, 08:38 PM   #13
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Wouldn't a reverse naca duct be suitable...?

maybe with an oversized trailing edge lip...

keeping with a previous comment "more in line with the Miata community..."

might be an easier test method at least....one or two along the top edge or top side....
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Old 07-17-2014, 08:40 PM   #14
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If by "reverse NACA" you mean simply turning a NACA duct around, they don't work like that.
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Old 07-17-2014, 08:45 PM   #15
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Do you mean a NACA duct with the inlet inside the fender well, or a NACA duct with the inlet on the skin of the car but facing reward? The first one wont work, the 2nd might.
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Old 07-17-2014, 08:45 PM   #16
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Let me make this very clear....You are far more knowledgeable than I !!!

but...

I "thought" I had read a few books that say they work both ways.....in particular a British book called the "Race and Rally Sourcebook"...an old book but one that I was sure discussed NACA ducts in this context...

Anyway don't want this to go off topic so will take your word for it!
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Old 07-17-2014, 08:45 PM   #17
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Drill/punch 1" holes and adjust the edges with a broom handle.
Testing is all about getting it done
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Old 07-17-2014, 08:48 PM   #18
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Sorry Leafy, I don't quite follow you..I "think" I meant the 2nd option but I cant quite distinguish the difference of the two you said...

It's alright though, just wanted to put the thought out there for consideration...that's done now...rightly or wrongly
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Old 07-17-2014, 08:50 PM   #19
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NACA ducts work by generating a vortex along the edges - well, properly implemented ones do, and they're amazingly effective. They're very cool. Inverting or reversing them "breaks" them. There are more efficient ways to make an outlet.

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Old 07-18-2014, 12:47 AM   #20
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Perhaps I have the wrong end of the stick, but I think kiwi was suggesting that the inside of the fender contain the NACA duct (think of the picture above being the underside of the fender). Would that still break the functionality of the duct?
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