95MX6

I agree that balance is important but this wing does not really change the balance of the car at speed.

Baseline RX-7
Front Lift coefficient 0.16
Rear Lift coefficient 0.08

Front weight balance at 100mph 45%
Rear weight balance at 100mph 55%

Front Weight balance at 130mph 39%
Rear Weight balance at 130mph 61%


99+ Spec Wing on Setting 4
Front Lift coefficient 0.053
Rear Lift coefficient -0.075

Front weight balance at 100mph 44%
Rear weight balance at 100mph 56%

Front weight balance at 130mph 40%
Rear weight balance at 130mph 60%

The balance is largely unchanged, with a base RX-7 at 130mph your effective total road pressure is 1700lbs almost 1100lbs less than at 0mph, but with the 99+ spec wing (and I assume R1/R2 front lip) your road effective total road pressure is 2900lbs a 100lb increase over 0mph. We can't make any solid guesses at the different levels of grip since rubber friction coefficient is not linear with pressure, but it's not a huge leap of faith to assume that it's a large difference between these setups. Sure the balance at speed isn't 50/50 like it is at rest but given the choices the 99+ spec wing seems like a win all around assuming you have the power to afford the increased drag.

FYI, make sure the 99+ spec wing is well attached from my calculations it applying ~650lbs of force at 130mph, I wonder if racing teams attach any bracing inside the hatch.

P.S. My numbers may be inaccurate (due to not knowing the correct lift surface area), but they shouldn't be off by too much. As stated before wind tunnel testing is the only accurate way measuring this. Wish I had access to a wind tunnel.

FDNewbie

iTrader: (10)

PM Jimlab. I remember him saying something about having a hookup on a wind tunnel, and he's local to you IIRC.

I have "access" to a wind tunnel here, but it'll cost me $20,000 - $50,000 EASY lol.

jimlab

I think you'd better check your calculations again. A Carerra GT only generates ~340 lbs. of downforce at 150 mph.

Not anymore.

FDNewbie

iTrader: (10)

Hey Jim, you know what the '07 911 Turbo #s are? IIRC, it's even faster than the Carerra GT, so I'm curious if it's aerodynamics surpass the Carrera GT as well.

Oh and just for fun... This is WRT to the Chrysler ME 4-12:

"The computer controlled active rear spoiler articulates rearward 100 mm to increase down-force to a total of 925 lbs. (421 kg) at 186 mph (300 km/h), while achieving a competitive coefficient of drag (Cd) of 0.358."

That's a shame. Seriously.

jimlab

Same jerk that strung me along for 5-6 months on my last bushing group buy before he finally finished them. No big loss.

95MX6

A simple misunderstanding, the rear of a stock RX-7 generates ~325lbs of lift at 130mph, put the 99+ spec wing on and you have a total of 325lbs of rear downforce. The lift didn't disappear, the wing just generates enough force to counter the lift and still apply 325lbs of downforce. -325 + X = 325, solved X = 650. So the net force is 650lbs which the mounting points of the wing must support.

And overall the RX-7 with 99+ spec wing still only generates ~100lbs of total downforce (front lift + rear downforce) at 130mph, even at 150mph the total downforce is ~170lbs. So the Carerra GT is generating more total downforce with a big wing.

Also the Carerra GT has to brag about it's top speed, and downforce raises the drag coefficient of the car. So Porsche keeps the downforce moderate while still allowing an impressive 205mph top speed. Silly as it sounds most people think top speed is more important than 0-100 times, anyone remember the Hennessey Viper being clamed faster than a CART Champ car in Motortread? Guess which one covered a mile in less time from a standing start? Can you imagine the reaction to a 600hp supercar that only does 190mph? Hell the '07 Corvette Z06 can do that and Porsche doesn't want that kind of press. I don't agree with that type of thinking but the general public does.

chingon

I can see that being true for the lift coefficient but not the drag coefficient. Coefficient of drag or lift depends on air velocity, the force component (in drag the horizontal one), the density of the fluid, and the reference area.

95MX6

The creation of downforce or lift typicaly causes turbulance and localized seperation of flow which increase the drag coefficient. There are times when you can improve downforce with little effect on the drag cofficient but in general the creation of lift (negative or positive) increases the drag cofficient.

Ever notice how jets have to take off and land at high speed? Ever wonder why? To allow the high speeds they travel at in a fuel efficient maner the wings are designed with a low drag coefficient. The side effect of this is they generate low lift, flaps are used to increase the surface area of the wing during takeoff and landing but even with this speeds are typically 100+mph. If engineers could design a low drag wing that didn't require 100+mph to takeoff they would.

SystemsWizard

I've been trying to get a better understanding of the FD's aerodynamics and have to say - what a great thread! Thanks everyone, esp. 95MX6, for so much in-depth information.

Neo

iTrader: (4)

Nobody got their shipment yet??

I have yet to order mine... lack of funds...

SystemsWizard

Slightly off-topic, but do you think an aftermarket wet-CF rear hatch from Seibon would be able to handle the downforce from a 99-spec wing?

FDNewbie

iTrader: (10)

More like completely off-topic. No CF hatch will safetly handle the drag and/or drownforce of a functional wing at high speeds. They need to be securely fastened to the chassis of the vehicle or some other form of support. Otherwise, they're likely to rip right out of the hatch.

Oh and please, in the future, 1) Search, as this topic has been addressed, and 2) Post in the appropriate thread.

Thanks,
~Ramy

SystemsWizard

Give me a break. The talk had evolved (not devolved) to measures of downforce and how good rear wings were. I was asking whether a good rear wing could be placed on a CF-hatch. I said "slightly off topic" to be polite. I could hurl something back, but choose not to...

SystemsWizard

Ramy,

BTW, thanks for your opinion on the subject. I take it that it's good to stick with the stock hatch for the '99-spec wing.

By "I could hurl something back", I mean that I sent you an email long ago about the weight of your Spirit R Type-A rims and never heard back.

The real point is that I've read this forum and many, many of the links in FAQ sections across-the-board to learn more about the FD3S. I'm also not new to automotive engineering or, even, the computer science behind search engines and have done better than I expect most new-comers will.

Yet, this is twice I've received a chilling welcome from the RX7 community here.

I can accept it from a moderator. But, not from a salesman...


Roy

chingon

The angle of attack to create downforce or lift is what causes turbulance, not the force itself. The increase in drag force affects the drag coefficient, not the downforce (which is a vertical component) like you state.

No matter what your improvements on downforce are, it won't affect the drag coefficent, because downforce (a vertical component) only affects the coefficient of lift. The creation of lift is caused by the angle of attack on a wing. The angle of attack changes both the lift and drag on a wing.

For take off they need to build a high pressure zone under the wing relative to the lower pressure zone on top of the wing. To overcome the weight force of the plane, the F=PA under the wing must be greater than the weight of the plane +F=Pa on top of the plane.

On landing, they must keep a high velocity to keep lift prior to touching ground, otherwise they'd fall out of the sky. They must land at a high rate of speed to gradually reduce lift.

I'm not arguing the merits of the low drag coefficients of wings, I'm just stating that lift force does not affect this number.

FDNewbie

iTrader: (10)

NP.

And I'm sure I sent you an email back, or it wasn't "that long ago."

Because you have a low post count, a new membership, and fail to follow forum rules...like not thread hijacking and posting off-topic. It wouldn't be a bigger issue if ppl knew you or who you are, but no amount of degrees or vast knowledge base can make up for the phenomenon of being an outsider...so you're just gonna hafta be patient on that one (or follow the rules, but c'mon now, I can't ask too much...lol).

Wow...I REALLY tried to be offended from that comment, but unfortunately I failed. "I could hurl something back" by telling you that this is nothing more than a passion and a hobby I do on the side, and that my real profession is just that - a profession, but then again, I think you've done a good job of displaying how outsiders are too hot-headed and quick to jump to conclusions and throw jabs at ppl they don't know...but then they have a hard time taking those jabs themselves

Oh and welcome to the forum. Looks like you're off to a great start when it comes to making friends...

~Ramy

95MX6

There are costs to increasing lift (downforce is negative lift), the thought that you can get "free downforce" is laughable and my comments on airplane/jet design were trying to show a real world example of the trade offs between high lift and low drag.

An aircraft requires a pressure differental between the top and bottom surfaces of the wing, this doesn't require high pressure under the wing but many modern wing designs generate a high pressure zone under the wing, flaps help with this effect during take off and landing.

I'm not sure were you got the idea that aircraft need to gradually reduce lift, prior to the tires making contact with the runway the wings of a plane must support the entire wieght of the plane. There is no "gradually" about this, 1 second you need 100% lift, the next you need 0% lift.

My mentioning of modern high speed jets was to point out that a wing can not generate high lift while having a low drag coefficient. Many very intelligent and knowledgeble engineers have worked on this problem (for decades) and they have yet to design a fixed wing that is high lift and low drag. Boeing would be very interested in a wing design that can lift a fully loaded 747 at 50mph yet still provide a range of 7,000+ miles and a crusing speed of 500+ mph.

There is a cost for lift (downforce is negative lift), it takes energy to provide that force. The energy used to provide lift is paid for by the car/aircraft in the form of increased drag.

This thread has wandered too far. I'm not interested in arguing the finer points of aerodynamics, for one I'm NOT an expert in the field, two the subject is huge with hundreds of books covering it. I think I provided enough information in previous posts to allow forum members to make semi-educated purchasing decisions in regards to aerodynamic aids for their car(s) without having "marketing hype" completely fool them. A good book on car aerodynamics is "Race Car Aerodynamics: Designing for speed" by Joseph Katz ISBN 0-8376-0142-8, and I suggest anyone trying to modify the performance of their car using aerodynamic aids read at least part of this book. For anyone adding wings/diffusers for visual reasons you can skip the book it doesn't add any "bling" to your car , just be sure to mount the parts well so that you don't "give" the car behind you any of your expensive parts.

chingon

ugh, it's pointless... pick up fluid mechanics by cengel or gas dynamics by keith...

afterburn27

iTrader: (5)

Look up induced drag, I think you'll find your answer. Long story short, drag does increase with greater amounts of lift.

The overall drag coefficient is equal to the base drag coefficient (at zero lift) plus the induced drag coefficient.

I'm not sure how much it all changes when talking about cars (I'm an airplane guy), but the general idea should stay the same.

chingon

I think it's all a misenterpretation really. To achieve lift, you change the geometry or angle of the wing, this is what changes drag, not the lift involved. I know what you are saying about induced drag, and I know the coefficient of drag is directly proportional to the coefficient of lift, but that is due to the angle of attack affecting both drag and lift which allows for a direct relation. This however does not change the fact that the coefficient of drag ais dictated by only four parameters, of which lift force is not one of.

afterburn27

iTrader: (5)

Ah, I see what you are saying.

Yes, a vertical force does not automatically create a horizontal force perpendicular to it. But to create a lifting body you need a geometry that also creates some drag, and generally higher lift devices create higher drag.

FDNewbie

iTrader: (10)

I'm def. an aerodynamics noob but that makes complete sense. That's the same tradeoff jetl have by designing the jets for maximum fuel efficiency due to the long flights they make, and thus they expend considerably effort to minimize drag. As a result, the aerodynamic design generates much less lift. Consequently, the jets have to reach very high speeds in order to generate enough lift to take off...

In simple terms, I think one can oversimplify it by saying that in order to increase lift, one must increase the angle of attack, the result of which is increased drag (and the opposite holds true as well). In relation to automobile aerodynamics, in order to maximize downforce, the angle of attack of the wing must be increased. Consequently, the drag also increases.

SystemsWizard

I gave it a lot of thought and have to say, sorry for the attitude...

I do have some experience and have read more than I expect of most first posters including Racecar Engineering magazine, the _Mazda RX-7 Performance Handbook_, and searches and FAQs here for a couple of weeks, before posting a single comment. I'm no expert and obviously am not taking in the full history here, but you guys have got to work a little more with newcomers to the platform, IMHO.

Damn, I didn't ask what BOV sounds coolest.

FDNewbie

iTrader: (10)

LOL don't sweat it man. Simply said, we don't know anything about noobs (ie ppl new to the forum). And this forum is NOTORIOUS for it's hazing of noobs' every mistake. So while it's great that you've did your homework (something most ppl don't do unfortunately), just keep in mind that it's part of the "culture" of this forum for you to get VERY little lattitude for anything "wrong" that you do. Not saying I agree or disagree w/ this approach, but this is simply how it is.

Having said that, welcome

~Ramy

PS: If I may, please read this post: https://www.rx7club.com/showpost.php...59&postcount=2 There are some things that are CRUCIAL to you keeping your FD in one peice which aren't found in the FAQs. Hopefully you'll keep this in mind, as many ppl learn about this the HARD way...