TerenceT

i know it has a 0.32 Cd

anyone knows what's the lift on the 8?

don't we need some downforce if it is a fast back RWD??

zoom44

no

TerenceT

explain

zoom44

sorry :o i was answering the thread i.e. "it is not a fastback." i don't know the answers to your other questions. as far as i know fastback has only to do with the slant of the rear glass as a styling cue and nothing to do with cd.

neit_jnf

CD is 0.31

TerenceT

fine
Cd is 0.31 ;p


anyhow
fast back is a design, but the inhanrant flaw of the design is the lack of down force at the back
example of fast back are porsche 956, Datsun 240

zoom44

so then no problem with the rx-8 then 'cause it is not a fastback

wakeech

...ya haven't quite got it Terence...it's not that fastbacks don't make downforce, as no passenger car makes downforce with the regular body shape... the deal with the fastback is that at extremely high speeds, it allows a more laminar flow over and off the back of the vehicle for lower drag, and more "fast".

with a notch-back sedan shape, you end up with a far more turbulent flow over the rear deck (the notchier the worse, obviously)... this reduces the velocity of the air, and hence doesn't reduce static pressure as much as a fast-back shape does... you trade high-speed lift for low drag.

anyhoo, there are ways around it, like the spoiler, which dams up a small turbulent air flow behind the car, which influences the laminar flows coming off the back of the car back upward (which then prevents them from exerting an upward force on the car), along with impeding the airflow over the back of the car slightly (reducing velocity, increasing pressure).

but, unless you're gonna go like +200kph in this car and expect track-like handling, there's nothing to worry about... real downforce is expensive and hard to come by (most aftermarket stuff is BS), and comes at the cost of weight and fuel economy.
in any case, the factory spoiler (referred to as "lip" here frequently... *shudder*) doesn't appear to be a poor aerodynamic solution, and is said to increase fuel efficiency (by Mazda)...

TerenceT

so are there any reference to lift or downforce (figure wise) about the 8?

since the G35 claims zero lift with a front lip (air dam)??

and the 8's rear lip increased fuel efficiency by reducing drag? making it under 0.31 Cd?

Oh, and does the rear spoiler has to work in conjunction with the front air dam and side skakes??

last thing... is it or is it not a "fast back" shape or design?

thanks

LesPaul

I remember reading about these stats in "The Book". I'll check it out tonight when I get home. (Unless anyone else has it handy today.)

wakeech

i've not seen any lift figures for the RX-8.

um, yes, the air dam must reduce drag some little amount... but a hundreth of the coefficient (0.30)?? i dunno... but yes, less.

it may be so that the entire "aero" package from the factory does help with the drag, as i know with the FC RX-7 it actually helped a LOT (-0.015 comparitively).

heh heh, no i wouldn't classify the RX-8 as a fastback: very much a notch back.

yaksplat

Actually this statement is very wrong.

Laminar flow would increase the drag, turbulent flow would decrease it. The eddy viscosity of the turbulent flow is much less than the viscosity of air in laminar flow. If you don'd believe this, then look up why a golf ball has dimples.

~Jim

Midnight Flyer

Sorry that is only true for golf ***** since they are round. On a long surface cutting through the air like a car or a wing you want the laminar flow as it increases lift and reduces drag.

A wing stalls when the laminar flow of the air is disrupted causing increased drag and turbulance above the wing and reducing the lift.

Some aircraft use vortex generators to create small vortexes above the wing . These spin faster then the wing and increase lift but only work well at lower airspeeds. At high speed, pilots are willing to live with the extra drag they create in order to get the better low speed performance.

Also, drag increases with speed but I doubt it will have that great an impact on these cars that are limited by guys in fast cars with cool flashing lights and who carry guns.

Downforce on a car is only an issue when you are really moving, say over 100mph and things start to get light. Many of the high end sports cars are designed to use downforce to keep the car on the road, but then those cars are owned by people who can afford to drive them in places where the above mentioned speed limiters are not a factor.

ChrisW

From "The Book" page 55:

"Contrary to common perception, the aerodynamics of a sleek two-seat sports car or 2+2 coupe may not be any better than those of a sedan whose longer roof works in its favor, streamlining the flow of air over and around the body.
...The RX-8 attains a respectable drag coefficient of 0.31, the same number as the RX-7. An optional rear spoiler lowers it further to 0.30.
The car's lift coefficients are 0.08 at the front and 0.07 at the rear with the rear spoiler."

Would a lift coefficient be lift as a proportion of weight (i.e. it gets 7 or 8 percent lighter)? It doesn't say what speed this is at anyway.

bwayout

I personally wish that the RX-8 was a "fastback" design, but it not, as wakeech already stated.

Also I took this from a past special Road & Track Mazda RX-8 supplement (that was handed out at the Mazda Rev-it-up event that I went to), in an article " Design Dynamics" by Sam Mitani, it's stated that the RX-8 designers felt that a fastback design would have made the car look too cabin-heavy and abnormally large for a sports car, so they went with this roof line instead.

I completely disagree - but who am I ... I wish when they upgrade the RX-8, Mazda makes it a fastback (... or when ever they introduce their RX-3 or RX-6)



On my Photoshop fastback version - I also extended the rear-quarter window (glass only) to the door's edge to help narrow the appearance of the rear pillar, so it gives the illusion that the B pillar doesn't look as thick as it really is ... still maintaining their "freestyle door system" rigid structure (at least i think it does).

wakeech

actually, i'm very right, and cars aren't golf *****. the principles which golf *****, baseballs, and other high-speed spheres or sphereoids (like bullets) operate on depends upon one very key dynamic factor: they're spinning as a free body moving through the air, something which is not so desireable when trying to operate a motor vehicle.

what this spinning does, in conjunction with the irregular suface of the sphere, is (as you've said) create a thin, turbulent layer of air around the object which has a few neat features about it. primarily, it helps maintain an even pressure around the object, preventing it from "knuckling". secondarily, this turbulence especially on the lee side of the sphere (the "back") helps to create "artificial" streamlining, resulting in (SHOCK AND HORROR!!) smoother laminar flow around the body as it moves through the air.

if these spheres moving through the air were smooth, even with very very high rotational speed, the impact they have on the air they're moving through is almost entirely determined by their shape, which is only slightly more aerodynamic than a flying wall; on the lead side, air dams up and slows down, resulting in a high pressure zone, where on the leeward side there is a much smaller zone of irregular turbulence and much higher velocity flow, creating a lower pressure zone behind the ball. as an added bonus, the high pressure on the front of the ball exterts a positive force on the ball (toward the rear), and the low pressure zone exterts a negative force (towar the rear) adding up to some very nice drag. this pressure difference (when not spinning) is what makes a knuckle ball knuckle, and why fast ***** go real fast when you give 'em a good rifling (lower drag).

...so, as you can see i'm quite familiar with these principles, and am not wrong.

...i was gonna go over the car again, but i guess i already did that. thanks midnight.

wakeech

wow, that's a whole lotta drag reduction. shweet.

anyways, no, that's not quite how coefficients work... they're a fixed proportion which is entered into an equation that takes your net pressure effect and surface area the pressure acts on, with the coefficient consideration to determine what the upward force generated is.

anyways, that is not very much lift at all, but without the spoiler it could be significantly higher (maybe as much as 0.09 - 0.10 as a guess)...
but yes, certainly not a problem at road legal or autocrossing speeds.

pelucidor

So now you know... this is more fun than studying Bernoulli's equations and turbulent boundary conditions for two years...

TerenceT

duh
i guess i've never really see the car from 90 degree on the side

LOL

TJRX8

So after all this...the spoiler is actually functional. I wonder if Herc and the rest of you wish you had ordered the spoiler now. My car is going to be more aerodynamic than yours and have less lift at speed. :D

Wakeech, Thanks! I have been suffering from insomnia for the past few days....you cured it! J/K good info.

Schneegz

Whadayatalkin' about!? I LOVED fluid mechanics! Ok, not really. I hated that class. Tough as hell. But I did learn a lot, and I learned how to apply it to cars, which was the best part of all. And yes, wakeech is right, laminar flow is usually the goal when you're talking automotive bodies.

wakeech

...i said spoiler from the beginning...

(direct links to my posts which apply to this thread)

from the thread "what about a wing?"
https://www.rx8club.com/showthread.p...=6717#post6717 <- note the rediculously low number.... this current post is #58106 for reference

https://www.rx8club.com/showthread.p...=6953#post6953

https://www.rx8club.com/showthread.p...=9513#post9513

heh heh, i just love this one... from "to spoiler or not to spoiler"

https://www.rx8club.com/showthread.p...1588#post31588

...damn, too tired to keep lookin'...
gooooood night.

pelucidor

Good point - I have been wondering if I should leave my car butt-naked or go with the 'arched' spoiler, and the fact that it actually does something functional (as opposed to look good/ricey) has persuaded me to go for it. I still have plenty of time to order this unfortunately. Is the correct term an aerofoil or spoiler?

Hehe - I flunked 1st year fluid dynamics twice (even after passing 2nd year fluids)... Third time was the charm (or perhaps the marker felt sorry for me).

Schneegz

pelucidor:
"Aerofoil", or "air foil" (depending on who you ask), is a correct term for any device that is shaped like the cross-section of a wing. An air foil can be designed to speed up the air flow over one side of the device, or it can be designed to keep velocity equal on both sides, depending on what you want it to do.

On a car, you'd want it to speed up air flow on the under-side of the the air foil, so that it generates down force. But you probably already knew that, since you've taken classes in fluids. :D

For all who HAVEN'T taken fluid mechanics (and those of us who have, but barely passed), here's a good explanation of how a wing generates lift from howstuffworks.com.

http://travel.howstuffworks.com/airplane2.htm

Just flip it up-side-down and you get down-force.

What I would like to know is how Mazda managed to reduce drag along with generating down force. Whenever you generate lift or down force, you ALWAYS generate some drag. My guess is that those clever fellows over at Mazda shaped and placed the spoiler so that it makes air flow over the tail-end more laminar, so that the drag reduction over the tail is actually greater than the drag generation of the spoiler itself, yielding a net reduction in drag.

Bunch'a smartypants, they are!