mark kibort

splitter could possibly help the rear too, as you divert the air from going under the car to the sides. makes everything work better.

You got plenty of power, so it probably is not a big issue. I had no lightness feel at Road America, although the fastest turn is before the kink at 100mph through a constant radius high speed sweeper. the kink is about that speed too. I still have a wing, just not a big one.
keep in mind that VIR is a high speed track too, and lots of speed can be traded for some much slower turn grip.

I think a general splitter will work fine.

Keep in mind, when the corette teams went to the wind tunnel with big wings and big front splitters, they started at where they thougth they were good. (probably much better than we would start at) they began with 60lbs of lift in the front and 120lbs of down force. they ended up with 120lbs of down force in the front with 250lbs of downforce in the rear. (and thats with all the right stuff!)

one can only imagine that we can make things a whole lot worse without these tools if we dont take it very slow.

I would say that splitter is a fine idea!

Mk

DR

Tom,

> it's hard to know whether a splitter is even needed

The front you are using now is actually sort of a splitter and it works pretty well, I think Joe has the same one. That is a replica of a Porsche designed and tested spoiler/splitter used on some of the special factory "Exec"? versions. I spoke to the guy who had the mold made from the original part.
I used one myself for a while and the difference in front stability was dramatic over the original S4 setup.. that is before someone I know and love crashed into a curb making a quick u-turn :-(

Regardless, the shape of that front spoiler/splitter you have now makes it very easy to add a true plate type splitter if you feel you need it.

Joe, you are using that same setup with a BFW, how does it work for you?

mark kibort

Joe uses a simular splitter, but also runs 12" wide slicks up front. he is also very low to the ground.
mark has the splitter and less of a wing than Joe and about the same hp now. so many factors, hard to list and think about them all. big birds wing is larger than Joes and marks and more effective as it is higher too.
those factory splitters work very well on cup cars too, but notice the small rear wing to match? the ALMS cars with the big wings use a real splitter up front. (still smaller than this wing on big bird)

I think at 150mph Id rater be a bit light in the rear, than have a push that just drives me into a wall.
MK

DR

Mark,
>big birds wing is larger than Joes and marks and more effective as it is higher too.

Actually I think you may be wrong on that one, the wing Tom is currently running on Big Bird is our small street wing. It is only 60" wide and a 10" profile (front to back edge). From the pics I have seen Joe and Marks wings are a lot bigger than that, but only they could measure and verify this.

The larger wing is coming that is 70" with an 11" profile that is a more modern airfoil design and makes a lot more downforce with less drag.

FYI, the wings I got for Dave and Stan are 70" wide with 12" profiles.

Ketchmi

Why wud us'ns ned any skoolin?

My message was not a personal jab, just showing my disinterest for your unwavering stance. The most intelligent people I know listen more than talk and actually try to learn something from what they hear.

BTW, I gave up a thriving electronic engineering/consulting business to do this. I am not exactly the dumbest kid on the block, didn't ride the short bus. Save the personal jabs for beers and dinner when I can tell face to face you are kidding.

Jim bailey - 928 International

Joe has a bigger engine,more power,wider tires ,wider car,bigger wing and is ALWAYS slower..........but always LOOKS GREAT, the car too Joe actually has become a very good driver he just does not race enough !!

mark kibort

Yes, we are all jabbing, and I know your done with the topic. all this stuff is MUCH MUCH easier with a white board too! Ive taught that stuff for so long, and my frustration rises exponentially, when i cant reach that kid in the back of the class!! (kid term used loosely).
I have an unwavering stance, as im only repeating generally accepted engineering principles. as shown by many micro views of the question, folks werent able to grasp that torque is only part of the equation. Even after the simulation that someone posted and my manual integration of the torque values in all the gears for both GTS and S4 boxes, still there were many that didnt understand the point. Ive just posted the actual numbers, and still folks dont understandthe point. gearing doesnt creat HP. gearing only effects efficiency for which the HP is put to the ground. PERIOD. thats all ive ever MENT to say!

If you are educated, and I know you are, it gives you the abilty to learn, not just remember. (memory always seem to fail eventually). Im sure over beers and dinner, most of us woud be saying "yeah, thats what i was trying to tell you !" (but the words got in the way!)

when im in a class room, i dont know about you, i want the instructor most of the talking. I listen to the questions and then , carefully listen to his/her answers. If i dont understand or agree, i ask questions.
I gave you and others more information than i normally provide. charts, facts, figures. Proving, even in your terms that the overall torque to the ground, over an infinite speed range, doesnt change with gearing. However, i had the comebacks of ("you start in 1st gear, i start in 5th gear and see who wins", or "acceleration is instantaneous, speed/distance is not a factor", and countless others) this told me that there was a clear misunderstanding of the facts and theories.

I showed you and others the effects of gear ratio changes on TOTAL acceleration.
whether you choose to accept them or question them intellengently, is up to you . thats really what we learned, or didnt learn, in school.

you know what, prove me wrong, I WOULD LOVE to learn something new.

Mk






Mk

worf928

D.R, Tom, Mark, Bigun and all with Big Wing envy...

If new wings are going to be produced and new struts designed - if all the trouble's going to be gone to... consider a couple of items. Tom's BFW setup is fairly typical - there's a pair of main struts and a couple of support struts to keep the wing from vibrating sideways since the main struts flex because of the thin hatch metal.

The main strut: It may look cool, but all the voids in the middle and the exposed nuts and bolts, create a LOT of drag. That strut is just begging to be sheathed with a skin that has a passing resemblance to an aerodynamic surface.

The support struts: The best thing would be to figure out a way to solidly mount the main struts so that they are not needed. But, if that cannot be done in a cost-effective way then make the support struts as short as possible. And, if at all possible, do not give the supports a round profile.

Assuming that the BFW itself has an efficient angle of attack and creates down-force as a result of (upside-down) lift all these struts create an amount of drag that is on the same order - perhaps more - than the wing itself. The round profile support struts create 5 times as much drag as a strut with a almost-smashed-flat profile. Each main strut with all the voids and exposed hardware probably has as much total drag as the wing.

For perspective: at an efficient angle of attach a BFW might create 200 pounds of down-force at high speeds and create 50 pounds of drag. Add in 100 pounds of drag due to the strut support setup with exposed bolts, voids, and round profile supports. Or, flatten the supports, get rid of the exposed hardware and fill the voids in the main struts and add 20 pounds of drag instead of 100.

The exact numbers are outta my butt, BUT the relative magnitudes of drag between the wing and the support structure and the magnitude of differences between the 'bolted together' supports and 'more thoughtful' supports is from experience. And this easy stuff (but perhaps not cheap) does not need a wind tunnel for tuning. On the other hand if you run the wing at 20 degrees the extra drag from the struts is small compared to the drag the wing creates when it's being dragged through the air rather than flown through the air.

Of course, if it's going to cost an extra $1000 to an otherwise $1000 setup to win back those 80 pounds of drag then I'd rather spend the money on more horsepower or bigger rubber or more seat time.

shaaark89

dave,
thanks for your comments. one of my ideas for doing it differently will completely eliminate all the hardware and struts. just need the time and fabrication to do it. i really am not crazy about having all that sh*t back there either.

p.s. when do i get to see carchicks ride?

mark kibort

Great points. (even the outta ya butt numbers)

I think that the fact that a round pole has as much drag as an air foil 10 x its size, brings home the fact that all of that mumbo jumbo holding the wing up, needs some aero shapes.

I dont know what a set of those places and poles would have as far as drag, but it sure would be interesting to find out.

sure looks fast though.. (matches the car!). I dont know how many have seen this thing on the track, but in my first race in my 84, Tilo(lapped) passed me with this car running at about 140mph compared to my 110mph on a back straight at Buttonwillow. I think i tossed that set of underwear after the race. its a fast car!!

MK

worf928

Ay yi yi! I wasn't talking about you Tom! You could hang a parachute off the back of that thing and still run down the turbos on the straights! If you get rid of all that parasitic drag you'll need bigger brakes.

Current plan is SITM

Pierre Martins

I'm fascinated by aerodynamics and I've put a lot of research into my car's design. I spent hours and hours on the net reasearching stuff. I also have a contact at the CSIR, a local testing facility with wind tunnels. Some F1 teams have tested there in the past. Anyway, i've incorporated a lot of advice from my CSIR guy into the car and i hope to get a slot in the wind tunnell later this year. That will be the real learning curve i guess.

In my opinion, Mark K is right in a lot of ways in his opinions regarding BFW's. F1 cars produce so much horse power and are so light they can run upside down given the down-force they produce. That's old hat. We don't have that kinda power to weight luxury with our cars, and more often than not, we try to make up for lack of mechanical grip by slapping on bigger wings and in the proccess produce more drag than we need. Fast in the corners, but we give away straight-line speed down the straights.

My CSIR guy tells me rear wings are supposed to add downforce and create turbulance behind the car. The trade-off is drag. You don't want drag. So, there's a fine line to be found between mechanical grip (suspension, tyres etc) wing angle and drag. This is different from car to car, track to track and day to day, given the variables of weather conditions and the car's overall performance and handling...

To cut a long story short - I've bought a wing with a gurney flap on top. My car is not strong enough to handle a lot of drag so i'm gonna set the main wing for turbulance only, and use the adjustable gurney flap for downforce when i need it. Here's the clincher - the flap will be spring-loaded and cable operated from inside the car so i can adjust it when and where i want.

Let you minds flow on that one for a while... You can even have a servo motor to electronically adjust downforce in the corners you want, and minimum downforce down the straights.

Just my 2 cents and change.

Cheers,
Pierre.

blau928

Pierre,

THAT's REVOLUTIONARY for the 928 gang...! Keep it coming man...!

An electric motor set to run on a program linked to a gps device that can tell the spoiler how high to go based on position on the track...? Or is it one tied to a gyroscope thing, or just a switch on the steering wheel..?

The GPS thing would be verrrrrrry cool indeed.... As a cable op'd one...!

mark kibort

Interesting idea.(and you will be joining the cheater club with Anderson.. ha ha ha)

Ive always wondered about the gurney flaps. ive heard, they effectely change the angle of the air foil by the amount that they are tall. at a cost of drag, vs just using more angle with the wing. which is it? Ill have to go back to some of the aero racing books to learn more about the. Ron had Joes old GTS wing, and he had a little custom gurney flap.. Looked cool, but wonder if it works better than just getting more angle. Also, I think your adjustable gurney flap is a cool idea. (i work with servos every day, so let me know if you need something small and need a deal on it.) the problem is the one more thing to think about going into the turns. Or, you can have a controller programed and tied to brakes or g forces.

mk

worf928

A long long time ago in a galaxy far far away I did the drag vs. mass vs. strength analysis of round-profile suspension components on an open-wheel car vs suspension components of equal load capacity with a basic low-drag profile. The aero-shaped member needed to have about 50% more cross-section but the drag was something like 20% of the round one. The difference is greater for a longer component due to edge effects being more dominant on the shorty components.

In that same universe we did the analysis of round profile support wires on long wings. The drag penalty was a jaw-dropper. But, in that case the mass and cost penalty of doing something different was too great.

Don't ask me to repeat those analyses; I killed those neurons a decade ago with beer...