mark kibort

As some of you may know, im running a used cup car wing on my 928 racer.
I did some intial comparison between the downforce of a stock 928 wing with risers and tilted 10 degrees vs a GT3 cup car wing from a 997, set at 7 degrees.

My first rough measurements (done on a freeway at speeds from 60 to 120mph) were done on the tilted stock rear wing/spoiler. it got 80lbs of downforce at 120mph.

using the same tests, the cup car wing got 130lbs of downforce at 100mph. however, it was maxing out my spring scale set up.

I just confirmed with a better measuring system the downforce of a stock GT3 cup car wing at 100mph. Its around 165 bs of downforce, and it produces near 65lbs of downforce at 60mph
No wonder ive developed a major "push" with my car, rendering it almost undrivable, even though ive clicked off my fastest times at laguna and near my fastest at Sears. I took 2 degrees out before my last race a week ago, and it didnt feel much better.

This week, i made a special mount so that i could take all the tilt out of the wing. (making it level at 0 degrees.) I went out and did my test runs. amazing, still near 140lbs of down force at near 100mph and the wing was not even tilted! It just goes to show that a high lift air foil like the GT3 cup car wing, can produce a tremendous amount of lift (downforce) even though it has no real angle of attack. i wanted to find out the angle of which there is no lift (downforce ) produced, but i need to make taller adapters.

the point is, im almost positive most of the cup car teams and car owners havent done much in the way of wind tunnel testing. And, since my car, which is the only street driven and track raced car with a GT3cup car wing, i thought this imperical testing would be interesting to those playing around with settings.

ive done some things to clean up the front of my car, making the small splitter more effective and changing some airdam configurations that cause lift up front. hopefully, taking the 5 more degrees out of the wing, loosing the swaybar up front and making my splitter more effective should bring back some of the handling that has been sacrificed for the rear being nailed down!

I know the GT3 cup car and the 928 couldnt be more different as far as racers, but with the cup car being rear engined, and the incredibly effectiviness of the cup car wing, im surprised the most common handling issue with most cup cars is oversteer!

Be interesting to hear about some handling stories regarding the GT3 cup car wing interacting with the half a million other set up factors!

MK

A.Wayne

Mark ,
try lowering the rear of your car for a better balance, i would also increase the front splitter. you did not mention so i will speculate that your push is at low speeds, or are you experiencing this push at 100 plus , i would have to believe if below this your push is mechanical more than aero.

Good info on the wing , i would like to know in more detail, what is your actual setup to determine the aero download generated by the wing.

mark kibort

the low speed push was not really an issue. (laguna turn 3-4 are 60 to 75mph turns) high speed push is the issue and it is really bad, unless i drive around it by tossing it into a slide. (turn 6 and 9 are 80+ turns) part of the issue at laguna was the new pavement, but sears was a better comparison , and there was only a slight push on the slower turns and a big push down the carocel which was easy to drive around.

so why would lowering the rear be the solution? wouldnt that put more weight to the rear, or does it follow the same logic of when we lower or raise corners of the car, by reducing the force to the road, while other corners increase force. (like lowering the front right corner and it becomes lighter, as does the opposite rear, and then the other two corners get heavier). If so, im out of luck as the rear is as low as it can go, but may be able to change that. certainly, will be making the splitter more larger and more effective up front

as it is now, the weight balance is 54/46 front to rear .

thanks for the help!

mk

edit: ive added a picture in the previous post of the calibrating weight 160lbs shown here to match up with the spring/scale movement at 100mph.

A.Wayne

Mark you can lower the wing , this will reduce it's effectiveness way greater than changing the angle...

JackOlsen

I've done some testing along these lines with a wing I built myself out of aluminum.



First, I did wool tuft testing, including some tufts suspended from wires. The thing you're not seeing with your wing is that you are not getting airflow at the wing that is anywhere near parallel to the ground. On my 911, even with the wing as high up as you see in the picture, the airflow is parallel to a line you might draw between the upper rear edge of my roof and the top of my ducktail -- it's flowing down at an 11 degree angle. So when I want to test an effective 12-degree angle of attack for my wing (roughly where separation begins on the underside of my wing), I have to position the wing at 1 degree relative to horizontal. You could probably run your wing at a negative angle of attack (relative to horizontal) and still be generating downforce.





I also got some ($4) ride height sensors from an old Lincoln Continental and ran them into my data logger. I get noticeable ride height changes (both on track and in flat, 100-mph road testing) for 4 degree changes in the angle of the wing. Downforce continues to increase well beyond the point where separation becomes pretty dramatic on the underside rear section of the wing -- but drag also increases. At any of the three settings for my rear wing, there is a ride height change in the front as a result of a cantilever effect.

Lowering your wing would probably reduce its effectiveness because of the increased amount of turbulance closer to the car.

I experimented with some crazy ideas to try and get more downforce in front. No good solution yet.

kingkyle

Mark,

Please give some more details on the your spring scale setup measuring the downforce. I've been trying to find a way to do this. Thanks...

Larry Herman

Jack, that looks like a mid 70's F1 car design. Boy was that an ugly period.

Mark, I don't know if it is possible, but it would be interesting to know how much the drag changed from your 928 wing to the cup car wing.

As far as adjusting the car, I have always been told that you adjust the suspension to get good balance at low speeds, and the aerodynamics for high speeds. Just seems to make sense to me, but I'll bet SundayDriver could weigh in on this with a lot of practical experience.

My big aerodynamic experimentation was at Daytona, and that was limited to yanking the wing element out completely to get a few more mph on the banking.

SwayBar

Nice fabrication!

It appears you have more of an air-damn on the front versus a splitter which is more effective. With your excellent fab-skills, you can easily come up with a flat-bottomed splitter whose lip extends out 4 inches from your existing air-damn, and is as close to the ground as practical; the closer the better. The net-result will be a greater pressure drop beneath and behind the existing air-damn which will help suck the front-end down, as well as generating higher pressure on the top of the properly-braced splitter, pushing-down the front-end into the partial vacuum giving the front tires more grip.

Depending upon how effectively everything works with your big-wing on the back and an equalizing splitter up-front, you may have to install stiffer springs to compensate.

analogmike

I found that I could feel the difference when laying the rear wing flat. I normally run it full tilt for maximum downforce.

At Mid Ohio, the car was too darty under braking at the end of the long straight when I put the wing down. Put it back up and the car was much better while top speed was the same.

mark kibort

Good points. actaully, as far as downforce being tremendous at near 0 angle of attack, that was my point. the deflection off the roof line is known from some wind tunnel test charts for the 928. it looks to be about 12 degrees as well. so, the true angle of attack is based on the angle of the air off the roof line. so, at 0, i (we) could still be at near 12 degrees angle of attack!

good idea about lowering the wing. i never counted on the GT3 cup car wing as being SO effective. knew it was a high lift air foil based on its shape, but never counted on such a huge difference vs my tilted 10 degree stock GTS wing.

as far as drag, this is a commonly misunderstood number as well. I was getting 80lbs of downforce at 120mph and with the GT3 cup car wing , getting 160lbs of downforce at 100mph.
assuming a 10:1 lift to drag ratio, of most wings of the GT3cup car style in aviation, the 80lbs of additional drag only cost the 928 8ft-lbs of torque at the rear wheels at 100mph. (based on a 1 ft or 12" radius tire) this means that through the gear box at 100mph, and at a gear ratio of 3rd gear of 4.5:1, the net effect on the engine is less than 2ft-lbs of torque. The only other variable here is the extra force on the tires, causing increased rolling friction, but i imagine that force to be less of a factor. 2ft/lbs of engine torque at 100mph!!! thats near 2hp lost! thats it.

the other thing that i should do as mentioned, is fix up my front splitter. this way, if i can somehow match the new downforce in the rear, with downforce up front, i could get my handling back.
as a side note, you do want the splitter as low as possible, but with that, you increase frontal area and increase drag too. also, since my car has the turn in problem and high speed push, and since my air dam almost touches the ground during the high compression turns, not having it too low may be good too, as on the straights, that air that would be used for downforce, would be traveling under the car, down the straights where i dont need downforce. underbraking, the front of the car drops by a few inches and the air is routed around the car via the splitter and is low enough to not let much of the air travel underneath the car. kind of like having a variable wing , like in the '60s before the racing community disallowed variable air flow devices!

Mk

edit: Here is the aero profile for the 928 and the lift to drag curves for a flat bottomed air foil basically like the GT3cup car wing
You can see on the graphs that the lift to drag ratio curve goes down with angle of attack. you can see the other two curves for lift and drag both going up with angle of attack . angle of attack is the x axis and lift, drag and center of pressure values are on the y axis.

Mike: "full tilt" at mid ohio??? now you know why you were so darty! heck, if im getting 160lbs at 100mph at 5degrees , just think of the downforce at 140mph. it goes up with the square of speed, so you could be in the 400lbs of downforce range! no wonder your car was darty!

fatbillybob

Downforce was always an interesting issue to me. A car is about 3000lbs and you add in 160lbs of downforce which is 5% of the weight. Is this significant? I bet the gt3 wing has some NACA spec and from it you can calculate the downforce without having to do the weights etcs... Although your measuements are more accurate since they take into account the wing position and flow sep etc... that would be harder to calculate. A car is a huge wing that generates lift. Are we really getting downforce or is the wing spoiling the lift which is much more significant? The wing is perhaps 4 square feet and calculations put 80-160lbs as well within the range of downforce/lift for a generic NACA airfoil and the car is huge by comparison and I bet at 100mph make way more lift than the wing can counter as downforce. But the wing can effect flows which spoils lift. I'm not an engineer do I have any of this right?

mark kibort

I guess its all how you look at it. a car really has 3000lbs of downforce, but as it goes faster, lift, downforce and drag are all produced. however, keep in mind, the downforce im measureing is exactly for the wing. as without the wing, the 928 has the ability to prove this as there becomes lift, as the rear hatch will float in the air the faster you go. the purpose of the rear spoiler is to spoil the attachement of the flow over the car, and this produces less drag (for better gas mileage) tilting the rear spoiler and moving it out of the boundary layer, creates downforce. about half as much as a real wing with half the angle of incidence.

Now, the front of the car. most cars will naturally have some lift up front. in order to have downforce that works at highway speeds, the drag would be high enough as to negatively effect fuel economy.

With the cup car wing, im getting 160lbs of downforce at 100mph, this grows to 400lbs at 160mph. with 400lbs pressing on the wing, this is over 100lbs lifting up on the front wheels! (no wonder Analog Mike said his car got squirrely down the main straight at watkins Glen!!) and my wing is only set at 5 degrees, analog mike was probably 15 to 20 as he said "full tilt".

as you mention, you can find a naca number and approximate value with, however, due to the other points you mention, it would be totally useless unless you can determine the angle of attack. I havent even found 0 yet! i suspect its going to be near 15 degrees pointed up ! (but i will find it soon)

now, having seeing results in a wind tunnel of one of the speedGT Vets, it is proven that the cars can have some lift naturally too, but its going to be uneven front to rear. the race cars that were tested started out with some lift up front and some downforce in the rear. they left the wind tunnel with downforce in the front and more downforce in the rear. then its back to the track to make sure its the right combo.

The entire idea of the wings and splitter, to increase downforce or like having weight in the car, without the momentum/centrifugal force costs. The reasons that formula 1 go so fast is due to fact they are stuck to the ground with huge amounts of downforce and the power to push the drag both rolling and aero based on the downforce.

Mk

JackOlsen

Thanks. The wing itself is just $25 worth of aluminum -- but $50 worth of structural adhesive. It weight 4.5 pounds, has no fasteners, and has supported 300 pounds of sand bags in a static strength test.

Not bad for a guy who just looked up some stuff on wing construction on the web one day in order to not have to pay another $1000 for a wing to do some tests with. I got input on the Pelican BBS on what specific airfoil profile to use, and I got some very helpful guidance from some actual aerodynamicists. Those guys can't be blamed, however, for my misbegotten front wing idea.

I've tried a handful of front end designs. The straightforward low splitter is the best, of course, but it's also the one that gets torn off when I go off the track. The one in those pictures has since been replaced by one that drops two inches lower. Here's a picture of it at speed, you can see it flexing:



In a burst of backyard-fabrication exuberance, I even took a stab at an underbody treatment. This was before I actually had a clue as to how the underbody stuff worked. I'm working with a professional, now, to see if something like that can be made to work on a street-driven car.

Here's a look at the old, shot-in-the-dark version, before it got paint:



Ugly? Well, yes it is. But that's part of the design. It comes right off for the drive home, breaking into two pieces to easily fit inside the car, where no one has to look at it.



I don't mean to hijack Mark's thread. I'm interested in how he's using a scale to test his wing. Before I got the ride-height sensors, I came up with an idea using cheap veterinarian syringes and a piece of an old tire pressure gauge. I never got to implement it, but here was the basic concept: