Aerodynamics discussion

[troek]

Originally Posted by churchx

troek: if that matters that much, one can also think of adding rad to cool diff oil (as i've seen for eg. GT86 CS V3 Cup track cars). Still imho nothing that would prevent making these by vendors. If even having or not stock underside panels slightly affect mpg (if that is any of criteria to judge aero efficiency ), then imho full/even flatter one should do even better, easing high speed for underpowered NA.

I agree, but its enough to deter me from fabricating my own without any equipment to monitor temps in my transmission tunnel. it would be nice if someone did though.

[lamawithonel]

I noticed these square-tooth wheel spats on a Maserati Ghibli the other day. Anybody know why they're like that? The only thing I can think is maybe something to do with noise, like the exhaust outlets of the 787.

[J-W]

Originally Posted by lamawithonel

I noticed these square-tooth wheel spats on a Maserati Ghibli the other day. Anybody know why they're like that? The only thing I can think is maybe something to do with noise, like the exhaust outlets of the 787.

In simple terms, they allow a smoother separation for the air as it flows along the sides of the engine where turbulent air and smooth flowing are meet. 90° angles and perpendicular angles to air flow have typically higher drag.

[Pug_BRZ]

Originally Posted by churchx

Low weight/high power needs aero to add grip, especially on such gravel. Imho we don't see that big wings in several other motorsport areas like WRC/F1 only because these days their maximum size/area is limited by regulations, not because teams chosen to go for lesser ones.

Right on. If you want a perfect example of freedom, look at time attack. Very limited regs, massive aero.

[lamawithonel]

Originally Posted by J-W

In simple terms, they allow a smoother separation for the air as it flows along the sides of the engine where turbulent air and smooth flowing are meet. 90° angles and perpendicular angles to air flow have typically higher drag.

Yes and no. The jagged edge helps transition the hot, highly turbulent exhaust stream into the cool, relatively smooth air surrounding the engine. The major result is a reduction in exhaust noise. I imagine the reduced turbulence has some effect on drag, but small enough that it hasn't been mentioned in any of the Boeing literature. The picture is a link and has more details.

Anyway... The Gihbli picture somehow didn't make it in my original question, but that's what the question was about. Here it is again with the wheel spats highlighted and enlarged-- uploaded this time. I found the answer, though.

The serrated edge stabilizes shear waves, according to this Mercedes CLA press release. That would reduce wind noise, though it's unclear if it that's a side-effect or the direct intent; the press release is about Cd. It also doesn't explain how it works exactly, but I'm curious to find out. Anybody with a CFD capability care to model these and test them on our platform to see? My best guess is that it creates countervailing waves that cancel each other out, but I'd love to see some real data.

[Gunman]

My speculation is the saw tooth in the tire blocker, is to reduce wind noise. The blocker itself is reducing drag.

[Shankenstein]

So I just learned about the "Tail as Turn" mod that's available from Diode Dynamics. It allows your turn signals to be reverse lights, and incorporates a flasher function into the red LED brake lights.

Link to Diode Dynamics site

If this frees up the lower reverse lamp, is there anything preventing us from running a larger diffuser? Major structural members, etc.

Examples:

[lamawithonel]

Originally Posted by Shankenstein

So I just learned about the "Tail as Turn" mod that's available from Diode Dynamics. It allows your turn signals to be reverse lights, and incorporates a flasher function into the red LED brake lights.

Link to Diode Dynamics site

If this frees up the lower reverse lamp, is there anything preventing us from running a larger diffuser? Major structural members, etc.

Examples:

I've been thinking about that, too. It looks like the biggest limiting factors are the actual bumper bar and the muffler. Here are a couple pictures to llustrate.

The first one is a street legal track car. He smartly retains the bumper bar for safety. The second one says is a show car.





I think about the best shape we could hope for would be similar to the rocket bunny diffuser/replacement fairing. With the right axle-back, it might be possible to raise the center section a little more.

[churchx]

I'm not so sure that deleting center light assembly will pass through inspection. Probably like with 3rd brake light, it's dependent on local rules/laws. We here have one saying that if car by factory had it then it should be kept and has to be working to pass. For 3rd brake light i could relocate to top of window, but for reverse light .. not sure if replacing with mentioned 'tail as turn' mod will be street legal in all countries. I rather bet that only in very few if at all. For rest - usual "offroad use only".

[Fenix]

Originally Posted by Tcoat

I find this is much more effective!

The best post ever. lol

[neilvosko]

awesome job and experimentation.

i really like the idea of removing the Reverse light,
4th brake light in the rear bumper to allow air out of the parachute..
i have the ARK Grip exhaust, so as you all know what happens without the OEM muffler, so i removed that piece as well and used the DiodeDynamics kit of tail as turn and rear reverse in turn. the ONE thing i do notice is that the car is much quieter on highway. otherwise not sure if this helps or hurts aero? i read and saw other cars doing this venting, but each car is so different. Oh and i have the BRZ limited spoiler with gurney flap. will post pics in the next few days.

CURIOUS to hear your opinions on deleting this lower piece to put a hole in the parachute?

[QUOTE=Wonderbar;894378]Hey guys! So, I'm not an engineer or anything, just a self educated hobbyist and diy'er. I hate to see issues on my cars that could be fixed with a few hours and a hundred bucks.

I also hate to see fake or nonfunctional parts on cars. Our "rear diffuser" drives me nuts and in my opinion, it's a critical piece of both performance and visual on a sports car.

Here is my baseplate/prototype/version 1 of my diffuser for my car. I haven't riveted on the vanes for channels yet. The "R" cutout is to clear my Perrin exhaust, the front tabs bolt in the lower bolts on the rear sway bar, this closes nearly the entire open space in the rear bottom bumper and extends about 2 inches past the rear of the car. It doesn't follow the lines of the fake diffuser on the bumper, instead it creates a clean line on the bottom, where the OEM additional "diffuser" bolts on. Putting the exhaust inside of the air free cavity.

I built a similar panel for my 2006 eclipse using the tissue paper "tabs" and a gopro for testing.

It isn't close to finished, I'm doing a wrinkle coat black on the top and I'm planning on a gritted bed liner or flat black on the road side. Might leave it bare aluminum, haven't decided. This one is all built and bent by hand. Once I'm done tweaking, I'll do a nice laser cut one and anodize or powder coat it.

[Gunman]

Originally Posted by neilvosko

the ONE thing i do notice is that the car is much quieter on highway. otherwise not sure if this helps or hurts aero?

Generally, wind noise is turbulence, which means drag.

[neilvosko]

i dont have a way to test this definitively, maybe a MPG over a X amount of miles?
car is a DD with occasional autocross here and there, i figured this may help release some of the air captured in the rear bumper area.