Aerodynamics discussion
 

The purpose of this thread is to discuss the BRZ/FRS's aero.

The JDM underpanels help smooth out the air under the vehicle by making the underbody smoother. A smoother underbody means that there's less turbulence, thus decreasing drag. Theoretically, this can reduce wind noise, improve gas mileage, and increase top speed.

For reference, Subaru stated the following coefficient of drag (CD) statistics:
no underpanel, no wing: 0.29cd
no underpanel, with wing: 0.28cd
with underpanel, with wing: 0.27cd

Here's a picture of the JDM underpanel.
http://www.tune86.com/sites/default/...under-tray.jpg

While reading about aero diffusers, it appears that the underpanels may have another benefit. That benefit is increased downforce. The reason I say may create additional downforce is because it depends on whether or not the rear diffuser is actually functional. In other words, the air at the back needs to increase in pressure/decrease in velocity in relation to the air underneath the vehicle.

When equipped with the underpanels, air can travel at a greater velocity because there's less turbulence. When the air reaches to the rear of the vehicle, a diffuser will can slowdown the air by "diffusing" it. Essentially, the vehicle is being pulled down due to the difference in air pressure. The video below explains how diffusers work.

[ame="http://www.youtube.com/watch?v=3DIJMLrGz8Q"]Diffuser - Explained - YouTube[/ame]

Here's a picture of the BRZ's rear diffuser.
http://i.imgur.com/CDY5TQjl.jpg

From the picture above, it appears that the BRZ's rear diffuser has a small angle to increase air pressure/decrease in velocity. However, it's not clear whether or not it's functional in producing downforce.

Interesting topic!

At first glance, I think the panels are mainly there to reduce drag coefficient, evidenced by Subaru's findings, but by themselves don't constitute enough to produce noticeable downforce.

The car itself is high off the ground and a lack of side skirts would let lots of air from the side of the car rush into the bottom. Lends me to believe that the stock rear "diffuser" is a cosmetic feature more than a functional one.

What I found interesting was the TRD Griffon uses the JDM underpanels in conjunction with there custom panels and diffuser. The mid section would probably be cheap while the CF diffuser would be expensive. Mid section actually looks easy enough for any shop to fab. Only thing that worries me about the mid section is that the diff might overheat without the proper air.

My guess is the underpanels would help reduce drag and help smooth air out for the diffuser to actually create downforce

http://i2.photobucket.com/albums/y42...F55CAAB164.jpg

I'm under the impression that any difference in pressure (air over the car vs under the car) will create some sort of downforce. The greater the pressure difference (the lower the car), the more downforce created.

Also, for those that like to install super wide sticky tires, you may want to consider the aerodynamics. Tires the poke out of the fenders will create turbulence. You can Google why you don't want turbulence around your vehicle.

There are two solutions that I can think of.
1) Use slimmer tires
2) Install wider fenders

Option #1 may not be feasible for the autox crowd due to modification restrictions. However, since autox doesn't usually exceed 40mph, the extra turbulence created is probably negligible.

There's a small hole in the back. I think that may be where the differential gets cool air.

I think CSG_Mike brought that up in one of his posts. I think it would be interesting to see how much an aggressive wheel and tire package affects the aero of this car. I'd also be interested in finding out the balance between maximum tire grip while retaining (or even improving) the car's aerodynamic qualities.

with a car this slow, it is almost a guarantee that the advantage in mechanical grip gained from tire is more advantageous than whatever you are trying to accomplish with aero. downforce is drag no matter how you slice it. the net result might not always show that but thats because nobody is talking about lift. once you realize that an f1 car has a cd of 1.5-2.5 and a prius has a cd of .25, it becomes a little more apparent where the priorities are in performance cars.

An F1 car also has a much MUCH greater amount of horsepower as well as something like... 8x our power to weight ratio.

You'd be surprised how much speed the car can carry through turns. Aero is a larger factor in this than most people think.

Crude example: I can take a high speed sweeper faster in our BRZ with 225 tires, than I can in my S2000, with aero, with 255 tires. The S2000 has more power, and similar weight.

i am not downplaying the importance of aero. if anything i know how significant it is since i ride road bikes. there is no better way to come to appreciate that than being your own engine. at speed you can tell that like eighty percent of your work goes into fighting the wind. i was simply saying that if i had to chose between sacrificing mechanical grip for a reduction in slipperiness, i would do it. especially with 200 hp.

If it works anything like an airplane wing, a smooth underside means higher pressure air going by, as it is smooth, this would decrease the wind resistance and promote lift, not downforce.

To have a proper discussion about aero we need to clear up a few things.

First, we're dealing with lift. 'Downforce' is negative lift. So until all aero enhancements are working together to create negative lift, there is zero downforce. Downforce and lift are not independent forces. You have coefficient of lift. Once it goes negative you get 'downforce'.

Second, drag and lift have a complicated relationship. Sometimes one can be adjusted without the other being affected. Sometimes not. The drag to neg lift ratio is referred to aero efficiency. So while an F1 car has high drag, it also has ludicrous downforce. But that doesn't mean that we can't modify to get both a lift AND drag reduction.

What we've found in our testing is that additional mechanical grip generally is wasted, when you're already WOT (which is a LOT with this car).

For an extreme example, would you rather have 225 or 285 tires on a Miata? For a more realistic one, 225 vs 255? 195 vs 225? Of course it all depends on the situation...

the only reason i bothered mentioning the lift/downforce thing is because cars are shaped like aerofoils and have lift inherently. i think its important to talk about them separately because at any net amount of lift (positive or negative) you can have any drag coeffecient. adding downforce and removing things causing lift can be different.

there is some confusion going on here. we arent talking about the same things. i understand the importance of downforce and the grip that is provided. i was addressing the thought that running smaller tires would be a good idea in order to keep the drag coeffecient down. i was just saying that i would rather have mechanical grip over dropping a hundredth or so off the cod.

i also think aero grip is equally wasted if you are going wot. there are many people running around in miatas with 275s and in nearly every situation the miata with 225s is faster than the one with 195s. i do get your point though but wonder if it might be turning into a historical kinda myth. i think newer compounds are making the optimal tire width greater than they used to be but thats just speculation on my part.

I think we're getting at the same point from different perspectives :p

No. There is only a single force: lift. If it is positive it pushes up. If it's negative it pushes down. It's that simple.

You can't 'add downforce' with a giant wing if the net lift is still positive. It may seem like semantics but it's important to think of the whole system.

Is it safe to assume that the FRS/BRZ has negative net lift? If there was positive lift, then the tires would lose more traction as speeds increased.

Almost all cars generate positive lift.

Those large Voltex wings suddenly don't so bad now. :popcorn:

In @CSG Mike's s2000 CR example, he said that the BRZ could take the sweeper faster in the BRZ than in the CR. Keep in mind that the CR has wider tires and front and rear aero.

CR:
http://www.motorstown.com/images/honda-s2000-cr-06.jpg

Based on this, it appears that the CR could potentially generate more lift than the BRZ even with all the "downforce" parts. I wonder how a base S2000 would handle on the same sweeper. :popcorn:

My unsubstantiated belief based only on anecdotal evidence is that the BRZ generates less rear lift than the my CR.

Honda's own documentation states that the CR wing eliminates 70% of rear lift [with hardtop].

The front of the s2k is proven to make substantial downforce via windtunnel testing.

The center of pressure can affect that. If that wing is reducing lift at the back, but not the front, the rear will grip better than the front at speed, meaning high speed understeer.

Keep in mind that all aero lift does is change loading on tires at speed. Tires generate grip based on load. So balancing which end of the car sees more or less load from aero affects under/oversteer relative to speed.

there is only a single force but there are multiple things that affect that force. i think were talking about the same thing. i just think too many people get fooled into thinking that you can get downforce without consequence if you dont acknowledge that more than one thing may be happening at the same time when you refer solely to the net results.

Thanks for the clarification.

I still wonder if the OEM "diffuser" is actually decreasing lift, or if it's just there for aesthetics. For a sub 30k car, I have a feeling it's the latter.

Even something like the OEM diffuser is probably for aesthetic purposes as well.

http://i.imgur.com/yWPDb.jpg

This is what I could find for wind tunnel testing on the S2k. It, like most stock cars, generates lift front and rear.

http://www.c-westusa.com/labs/fudo1_s2000spec.asp


But it does show that you can get a lift AND drag reduction with a good package.

This makes sense now when you say that most cars will generate lift. The shape of a car is similar to the shape of an airplane wing. The air above the car has to travel a longer distance than the air below the car. This pressure difference generates lift.

[ame="http://www.youtube.com/watch?v=YQwh2fdfYiM"]Audi: Aerodynamics of planes and cars - YouTube[/ame]

So in theory, if you install panels underneath the vehicle and speed up airflow, then the net result should be an increase in lift. However, if you have a rear diffuser, then that should reduce lift. Is this correct?

http://www.maxrev.net/Icons/Voltex/voltex2.jpg

from http://www.maxrev.net/site/index.php...blog&Itemid=54

Faster air is lower pressure.

What we want is the ability to transfer high pressure under the car to above.

What a diffuser does is speed up the air going under the car in the front, then slows the air down as it tapers up in the back, but in the back is a big low pressure section. So the pressure difference also 'pulls' the higher pressure under-car air to 'fill' the low pressure behind it. The reduction of high pressure under the car reduces lift, and the reduction of low pressure behind reduces drag.

Ok so we have one saying CL ~-25kgf at just under 200km/h, and another saying CL ~+24kgf at 40m/s (144km/h).

Were there any changes through S2k models that could account for the differences?

You mean this...?
http://imageshack.us/a/img560/8663/1004126h.jpg

Not the optional diffuser. I was referring to the rear oem bumper/faux diffuser.

nice

I'm just gonna put this out there...that rear diffuser wing thing is actually just for looks. The leading edge is almost flat, it breaks up any of the potential of the rest of the "diffuser" by blocking airflow rather than guiding it. I have it on my car and love the look but it's function is purely cosmetic.

AP1 vs AP2 bumper :)

Diffusers and under body panels are cool because they're one of the few mods/designs that can simultaneously reduce lift (or create down force) and reduce drag. By giving the air under the car a clean path and smooth exit, the pressure differential caused by the shape of the car can be reduced. Depending on the car, it could be negative. Not likely with the bubbly roof line of our cars.

Overall these cars are pretty solid aerodynamically compared to many others, but the traditional coupe shape isn't as inherently down force friendly as something like a McLaren 12C or other wedge like supercar.

So in the spirit of DIY, has anyone done seen the guys at Ecomodder and their "tuft testing"?

LINK to AutoSpeed
http://us1.webpublications.com.au/st...108656_6mg.jpg

Similar to the wind tunnel tests summarized above, it's a really simple way to visualize flow directions and turbulent spots. Painter's tape + yarn/string + video camera = data. From there, I'm sure we could pinpoint the easiest gains.


Alternatively, I know that we could develop a solid model and run a CFD analysis to get those beautiful clown puke pictures... but at this point, tuft testing is much more accessible to the DIY community. If someone wants to volunteer for CFD duty, I'd be glad to help, but it's beyond my experience.
http://www.fea.ru/spaw2/uploads/imag...W-SauberF1.jpg

It's the old F1/racing way, too.

Another is paint streaks, where a series of dots are painted on the car which then makes a few runs, they then look at the trails left on the car.

Analysts still use a similar technique when they can get pictures of race cars that run in wet conditions. They have very distinct streamlines left on them. They are particularly useful for analyzing inlet effectiveness.

Does anyone have more pictures of the TRD Griffon underbody work? I'd like to try and replicate their work since it's probably the most extensively tested stuff for our cars so far.

@serialk11r Don't know if you've seen this thread. Thought you might be interested.