I would think and it seems that I've read that a bit of push is preferable at speed. Oversteer at 100+ is generally not the best feeling.

Also, what about an adjustable splitter?? or (can't remember the name) the little wings that go at either end of the front end?

I would think these observations would be what I often read about when teams have to tune the suspension for low speed stuff and then tune the aero for high speed stuff and balance the two. Not necessarily getting the front rear balance exactly even.

On the contrary. By adding more down force in the rear Jack is not increasing the mass which is what causes our "problems". He is increasing the traction which is like adding more rubber. More traction in the rear means the mass in the back is not able to throw it's weight around like it wants to. Especially in the 911 a bit more downforce in the front at speed may be just what the doctor ordered to make the car more stable.

how does BB2's current front track spoiler look like ???

I agree w/Sherwood.

Jack, have you ever taken a multivariate statistics class? I think it's time...

Also, a simple minded way to do things is to generate some X-Y plots from your data.

But I suspect the next step is already clear -- and it involves the front.

That's actually the one thing I did change. I was worried that my 5.5-inch splitter height was letting too much air under the car, so I lowered it down 1.75 inches:

http://forums.pelicanparts.com/uploa...1159645217.jpg

Although it's not an apples-to-apples comparison, it doesn't seem to have made very much of a difference.

That's a good point, and my notes include some of that stuff. But all these tests were done within 30 minutes of each other, so conditions are pretty constant.

how flat is your bottom , and do you have extra rocker extensions to trap the air

ideally , that spoiler plate would just blend over with the belly pan , and be one flat bottom... till it reaches the diffuser

bit tricky with the fan in the back doing exactly the reverse of what you need. eg , blowing air under, instead of sucking it away... but you could just as well use that to get more downforce in front, it wouldn't matter what happesn in the back, if the front is what you need to improve

Actually, I believe you want to add a disproportionate amount of downforce to the rear because of the car's weight bias. Keeping the rear end from breaking loose in high speed sweepers is where aero helps the most. Push isn't so much of an issue at those speeds.

My goal here isn't to increase downforce in absolutely equal amounts front and rear. But I'd be happy if I could increase downforce in the back without making the situation worse up the front.

I may resurrect my front wing project. Or my underbody diffuser project.

As a side note, I did a lot of searching for a street car with a front wing on it, and I couldn't find any. That wing on the Lamborghini probably isn't doing any good, but it's interesting to see that someone tried.

horn grills are they used for brake cooling ? looks like you got ducts in the spoiler ? if so , consider blocking them , you got air flow going into the wheel wells, which is technically "under" the car, and you don't want positive pressure there, but vacuum...

all the little bit's help..

EDIT , forgot oilcooler... timmeah

Would stiffer rear springs help counteract the see-saw effect and help translate more of the downforce into traction?

If you had four-way adjustable front shocks, you could stiffen low speed rebound?

how about if you took 2 hoods?

1 for daily driving, regular
1 for track use


center mounted oil cooler, get air from below, dump the air above the bonnet.... 906 style....

then remove the horn grills
and make a front plate that extends from the front spoiler , to the wheel wells , could even be made to be removable

flat bottom, and as little air under the car as possible, and trapping that vacuum, should do the trick better then any amount of "wings" in the front...

i'de go for the floor pan , should be pretty easy to optimize the air flow up front, no engine , tranny , exhaust , the floor pan is allready pretty flat, under the cockpit ,but not flat enoug for turbulence


if you take a big sheet of 2 mm aluminiym , and hang it under the nose , all the way to the tranny mount maybe some spacers so the whole thing hangs maybe an extra inch closer to the ground

then cut away where it interferes with the front suspension....

technically , you could probably make it so you can unbolt it again , so it wouldnt' compromise non track driving... it would complement the rear diffuser, then it would only be a matter of fabricating some ducting for the engine cooling, so it vents out backwards, instead of downwards...(unless you can change the airflow the opposite way , like Bernie Ecclestones suck fan F1 car in the old days )

in anyway ,the flat floor pan in front would not be affected by the fan in teh back, you'de get downforce right uptill the engine , behind he axle..

the weight should not be to bad, and it's down low anyway

Jack's already got the floor pan smoothed out.

any pics of that?? i remember the diffuser thread, but haven't seen anything of that floorpan flattening work

this is just a thought that my not be of use but is interesting.

increasing the front spring rate should reduce the lift......seems contrary to logic at first

i shoud say it will reduce the amount of ride height change due to lift

Stiffening the front spring rate would have little effect on the cantilever-effect created by the rear wing; at least that's what I'm interpreting from the posts. I think Jack realizes there's a need to balance both ends so they act harmoniously.

That means either reducing the downforce of the current rear wing and/or increase the downforce of his front end (or perhaps reduce undercar air flow to prevent front end lift).

I believe Jack has tried vehicle skirts, that in conjunction with the air tunnel experiment which, as I recall, created some engine airflow issues. If skirts around the periphery helps (I don't know), they should be made from a rubberized material (Jack used?) to increase longevity. There are too many inflexible obstacles on a track (or off-track) surface to expect a rigid, metal material to excape damage.

Sherwood

I previously had a flat undertray, although more recently I cut a section out for airflow for the AC. Flattening the bottom out helped reduce turbulence under the car, but there was no attention paid to the actual angle of the flat surface relative to the ground. If the big plastic sheet actually contributed anything significant to negative lift, it would have been pulled off.

My next big project is to do a more sensible aluminum undertray and diffuser. It will be removeable.

Today I'm going to get the sand bags out again and try to get some real calibration for the amounts of actual ride height change that's taking place relative to the numbers the sensors are giving me.

If I get the time, I might try another run with the side skirts in place, and also a run with a skirt/air-dam hanging down even lower in the front. I have a skirt piece that can attach to the underside of the splitter and reduce (by another inch and a half) the amount of air getting under the car.

Here are some pictures of previous aero efforts. I wish I'd had the ride height sensors back then. If I put all that stuff back on for tests now, it'll cost me another Saturday.

http://forums.pelicanparts.com/uploa...1111613203.jpg

http://forums.pelicanparts.com/uploa...1111613229.jpg

http://forums.pelicanparts.com/uploa...1112290827.jpg

http://forums.pelicanparts.com/uploa...1112209185.jpg

Jack, instead of holes in the tray for the cooling air exit , how about a duct to the back

lower the diffuser a tad, and have an opening for the cooling air to come out in there, the fan would push it out anyway ,so for cooling flow , it wouldn't matter, but the cooling air would not arrive below the car (opposite of Bernie's F1 hoover) and it wouldn't upset the flow of air from the front...


http://forums.pelicanparts.com/uploa...1159739852.jpg

think of a somewhat reverse air shroud at the bottom



http://forums.pelicanparts.com/uploa...1159740387.jpg

I am by no means trying to be a wise guy...however...has any thought been given to what the current NASCAR stock cars look like for instance the current Monte Carlo, etc...they have some VERY fluid looking front, side rear and allaround body mods to allow 200+mph at some tracks!! Is suggesting this a no-no in the Porsche world?
just trying to help... What may be best to know, if not already stated in thes thread, what are the tracks, places that a"911" will be driven that want these aero body mods? strip? oval? enduro? street?... all of these type of road courses will require different set ups to achieve best "overall" performance/lap times... just a thought

Jack -

i haven't followed your aero progress completely and it might be a little off-topic from your ride height experiments, but have you contemplated the use of strain gauges on the wing struts to measure drag vs lift? it might answer some of your questions regarding the rear wing's effectiveness.

I love this stuff.

I did some calibration. It involved getting a 'before' measurement and sensor reading at resting ride height, then adding 200 pounds of sand to lower the end of the car down, then getting sensor readings for every 1/4 inch step as I raised the car back up with a jack. That gave me a chance to compare the initial ride height sensor reading (at rest) with the ride height sensor reading when the ride height was brought back to that level with the jack and the weight. I also measured a couple of additional inches up above that point in 1/4 inch increments to see if the readings I was getting from the sensors were linear or not.

Does that make sense?

It ended up looking like the sensor's output was pretty close to linear -- meaning the voltage readings proceeded in equal increments as the car raised up. The front and rear increments weren't the same, though. I'm sure that has to do with the way the sensors are mounted. But in the front, 1/4 inch of suspension travel changes the readout .07. In the back, 1/4 inch of suspension travel changes the readout .108. This lets me connect real suspension travel distances to the sensor data numbers.

I also went through the data itself to make sure the numbers I was having the computer average were from portions of the circuit where the car was always going 100 mph. In some of the data, I found the speed dropping earlier than in others. I narrowed the segment lengths so my data would have less of the data that was from not-100-mph travel.

And I also found one anomaly that I won't be able to explain until I do more tests. The at-rest ride height readings from the calibration check don't sync up with the at-rest readings from the day I did the test. Worse, the at-rest readings from that day don't really jibe with the other sets of readings I got. Something funny was happening that day when the car came to a stop. I don't know if the sensors were shifting position, or what, but everything looked much more credible and consistent when I used the no-aero-devices attached run as a baseline for comparison instead of the at-rest readings.

Still with me?

With all that said, it looks like my initial findings were not 100% right, and they weren't completely wrong, either. In other words, the front end isn't actually rising up when the rear wing is working. But the rear wing is reducing the amount that the front dips. The front drops more with no wing in the back than it does when there's a wing back there.

Also, the blue 'starting ride height point' lines in the graphs I posted were not correct. They made it look like the front end was riding higher in every test (except when the rear wing was removed) than it was when the car was standing still. This wasn't the case.

Here's the more accurate information.

If you use the run with no wing and no splitter as a baseline, the following happens:

When I put on just the front splitter, the front end runs .41" lower, and the rear runs .04" higher.

With both the rear wing and the front splitter, and the wing set at 12 degrees, front end runs .24" lower, and the rear runs .01" lower.

With both the rear wing and the front splitter, and the wing set at 16 degrees, front end runs .09" lower, and the rear runs .17" lower.

With both the rear wing and the front splitter, and the wing set at 20 degrees, front end runs .08" lower, and the rear runs .26" lower.

Now, because of my sand tests, I can ballpark actual pounds to go with these figures. But I don't know if the data is meaningful or useful, since a ride height measurement can't distinguish a force pushing the front down from a force that might be pushing the back down so hard that it works to lift the front up, in a teeter-totter effect, giving a number that's just the sum total of all the forces involved.

But my math indicates that the splitter is pushing the front down to the tune of 103 pounds when there's no wing in the back. Once you add the wing, the change up front appears to decrease (in spite of the fact that the splitter is doing the same work in every run). With the wing at 12 degrees, the front registers 60 pounds pushing down. With the wing at 16 degrees, it's 22 pounds in front. With the wing at 20 degrees, it's 20 pounds in front.

With no wing in back, there's actually 13 pounds of lift showing in back, which is probably the teeter-totter effect working in reverse, with the front 103 pounds of force lifting the back up a little. Once the wing goes on, the rear starts coming down, to the tune of 31 pounds at 12 degrees, 55 pounds at 16 degrees, and 82 pounds at 20 the degree setting.

I don't think the rear wing is only generating 82 pounds of downforce, though. Partly because the data I posted earlier says the car itself with the ducktail is generating 75 pounds of rear lift at 100 mph. So maybe that means the wing is doing 157 pounds of work to get the ride height to that point. I suspect it's even more, since the wing stretched its base much more after a lap at Willow (going up to 125-30 mph) than it did with 250# of sand loaded on it for an earlier static test.

It should also be noted that these numbers all rely on my ability to do basic math. I assure you that the odds of me making a key error at some point in all of this are very high. I haven't taken a math class since high school, and that was back in the eighties.

But it's more data. I want to try repeating it after I make sure that there's no slop in the way the ride height sensors are mounted. I want to figure out why I was getting the goofy at-rest readings during the test. I'd also like to test the car with a flat rear decklid, and the 3.8RS lid I used to use, and also with the big wing I used to run. Now that my 'Poor Man's Wind Tunnel' is on the verge of being reliable, I want to tie up all the loose ends and see if it will generate consistent and repeatable data.

Of course, I've got to fit it into my spare time. So this thread will be popping up again in a few weeks. :cool:

Subscribed!


Jack,

I made a set of "Poor mans" DTM style front winglets a few weeks ago in hopes of reducing some of the understeer my car was experiencing...

BOY did it! If you have the time I'd recommend investigating those pictured above. Took me about two hours to make and mount. First attempt had too much angle. Second attempt worked nearly as well and gained me about 100 RPM on the main straight.

Mine pictured below:

http://forums.pelicanparts.com/uploa...1160058615.jpg

It doesn't take all that much of an excuse to get me to drive 300 miles on a Saturday, does it? I took the 911 out again this morning to test some ideas for bringing the front down. The drive was fun, but I still haven't found a great way to get the nose lower.

My first idea was to extend a curtain below the 5-inch spoiler to reduce the amount of air that gets under the car. I built a pair of bolt-on pieces that would be easy to replace after an off-track exctursion. It wasn't pretty, but it kept the clearance to a minimum.

http://forums.pelicanparts.com/uploa...1160272005.jpg

I also threw together a mount for the front wing I'd built. I had read the concerns that the low pressure zone under the wing might cancel out the wing's downforce as it pulled 'up' on the car below it. I also had my own concerns about how ugly a front wing would be on a 911. Those concerns became worse when I came up with a scheme to extend the wing a little bit further forward.

It was might ugly.

I'm serious about hat. Cover your eyes if you're sensitive. This is going to sting.

http://forums.pelicanparts.com/uploa...1160272140.jpg

http://forums.pelicanparts.com/uploa...1160272162.jpg

http://forums.pelicanparts.com/uploa...1160272182.jpg

http://forums.pelicanparts.com/uploa...1160272197.jpg

Well, the nice thing about a wing up front is that you can see when it's working. At 100 mph, it was pushing down on the support rods enough to bend them. At the same time, though, the lightweight hood was actually fluttering in the turbulence (or the low pressure area, I guess). It actually pulled part of the rubber seal out from under the trunk.

The good news? The front wing didn't work. The wing is pushing down, but I think it's also pulling up on the car in an equal amount. This means I don't actually have to put it on at an event where other drivers might see it.

The not-so-good news is that with or without the wing, the vertical front curtain had almost no effect at all. According to my ride height numbers, the front is riding less than 1/16 of an inch lower with the curtain under the splittler. That's maybe in the neighborhood of 4 pounds of downforce -- less than a gallon of gas in the tank.

So, no solution for the front end, yet.

I'll keep thinking about other possible solutions.

Jack and Chris have you seen the "winglets" that Mitch Rossi put on the front spoiler of his car? They are on shown on the picture on the front of his new book about modifying a 911 for street/track. He claims they are effective and they dont look too bad.... Jack you have probably seen them live in So cal.

Everything I've read about the front corner winglets is that their effect is pretty minor. But I'm going to look at the Rossi book and see what he says about them.

Here's a graph that shows how acceleration effects front and rear ride heights. Part of what I'm seeing with front ride height is from the fact that the air is pushing the car backwards as the car pushes forward. In the graph, the car goes up to about 133 mph and then deccelerates. The front ride height line trades places with the rear ride height line. (Remember, with these sensors, a low point on the graph means that end of the car is high, and vice versa.) So under throttle, the front is high and the rear is low. Then the front dips down and the rear comes up as the car slows down.

http://forums.pelicanparts.com/uploa...1160335308.jpg

I'm wondering if a portion of your front end lift is coming from the decrease in rake that pushing the rear down causes. The reduction in rear 'under tray height' would reduce the scavenging of the air under the car but the front would be allowing the same quantity of air under the car. (or increase the quantity of air from the cantilever effect...) This would cause the air flow under the front to slow down which means increased air pressure.

You are having so much fun with this. Why don't you make a multiport manometer rig and collect air pressures from different location on the car. (such as under the front)

On a different subject... A suggestion on your rear diffuser arrangement. You are pushing the cooling air down into the diffuser's airflow. I would suggest a NACA duct or a stepped outlet kind of like the cowl flaps on an airplane. I would think it would smooth the airflow and possible help the cooling airflow.

BTW - I think the to Lamborghini’s front wing was actually for US bumper height laws. (someone know for sure?)

i'm sorry i posted that photo of the countach - i was joking. i did not realize you would actually build one to test. i'm glad it didn't work. i guess you could sell it on ebay so some kid can put it on there corolla.

the front skirt idea is interesting, i guess the volume of air splitting under the car is important for speeding up the velocity of the air under the car. i would think that the right amount is nessesary for a rear diffuser to work.

No apologies necessary; the craziness was all mine. I'd built the front wing weeks ago. I originally came up with the idea back in July, but wasn't able to find an example of anyone actually trying it on a production car.

Here was my original drawing:

http://forums.pelicanparts.com/uploa...1160330035.jpg

Based on the airflow over a 911, I also thought one could be put right above the windshield (C), although it might suffer the same 'cancelling out' problem. In front (A) would probably work the best, but it would be hard to maintain its height relative to the ground because of suspension height variations. It would also be a nuisance to other drivers (and pedestrians).

There is no reason that you can't mount a wing in front like example "A" but make it higher above the ground. For stability's sake I would think you would want it about 1-1/2' to 2' above the ground. If you put it close to the ground the downforce will oscillate in an unstable fashion as the height changes.

In the case of a rear wing, when the wing pushes harder the rear of the car lowers and the wing's angle of attach slightly decreases. This is a stable reaction because it tends to regulate the force. In the front this will be unstable because when it pushes down the wing gets closer to the ground and increases the venturi effect, which pushes it even harder down. When the wing's height goes up it loses downforce, which deceases the venturi effect and makes it go even higher. This creates a situation where it wants to create an oscillation instead of stabilizing.

To combat this winged cars have insanely high spring rates, which you don't want to get into on a 911. (unless you want to build a whole new chassis that can deal with the pounding) The car will have more of a tendency to porpoise over any ripple in the road anyway.

I think you have two conventional solutions you can use.
- One is extend the splitter and change the spoiler so it isn't so vertical but more angled like a snow shovel. This would help keep the high pressure in front of the car from pushing more air underneath the car.
- The other idea is to manage the air around the front tires to get the air scavenged out through the sides. The tire is going to squeegee all of the air off of the road and you want as much of it as possible to go around the outside of the tire as opposed to the inside.

Anything you can do to let the low pressure area in the rear of the car extend as efficiently as possible all the way to the front spoiler will get the job done.

Some quick and dirty origami-style canards:

http://forums.pelicanparts.com/uploa...1160516408.jpg

http://forums.pelicanparts.com/uploa...1160516418.jpg

http://forums.pelicanparts.com/uploa...1160516428.jpg

I have no idea if these will do me any good, but I'm going to try them out tomorrow.

Slick!

I'm really curious to know what their impact is... Thanks for doing this research Jack!

These are /were used on ground effects cars (ie the IMSA GTP cars of the 80's). What they accomplish is creating a vortex down the side of the car, filling the low pressure area that exists along the sides of the car, and that kept the air entering the underside of the car from the nose under the car and effectively increased the down force with a low drag penalty. Aero guys are always looking for something for nothing (ie downforce for no drag penalty) and this was a good find.

The primary function of the devices (dive plains, as they were sometimes called) was not to create down force by forcing air up over them and pushing down on the nose.

They were very misunderstood in their function.

For some good reading on aerodynamics (and race cars in general) explore these two great sites.

http://www.mulsannescorner.com/

http://www.suzukaracing.com/ (The owner of this site is a well respected aerodynamicists who has designed many Le Mans cars. He brought the dive plane back in the 80's. It had not been used in a number of years.)

First, let me echo the other posts which have said "thanks for doing (and sharing!) this research." Second, I have some specific questions about equipment. First, what datalogger are you using? I've been gathering notes to build my own from scratch, and I thought it would be useful to learn what others are currently using, especially if you have opinions on what features are especially useful/useless. At this point, I'm sure I'd like to gather data, but there seems to be a wide divergence of opinion as to what data channels are useful. For instance, I'm not so concerned about engine management, since I don't plan to redesign it, but I might be interested in suspension monitoring, wheel speeds, tire temps and steering angles to monitor things I might want to change. Anybody have words of wisdom there?

Ed

From the Suzuka Racing site:

http://www.suzukaracing.com/Aerodyna...LipVortex3.jpg

I'd heard that the canards didn't do much for downforce. Now it makes more sense to me. I'll run a test with these and see if they make any difference, even accidentally.

Maybe I should try wings off to the side of the headlights.

I use a DL1 data logger, made by Race Technology in Britain. I like the multiple inputs for different sensors (I use throttle position and brake as well as front and rear ride height. But I also like the integrated GPS and accelerometers. And of course the software allows a short-bus guy like me to actually make some sense of the data.

One question on this line, though. Why do data loggers generate such high lateral G numbers? I show sustained g's in the 1.4 range, maxing out at about 1.6. Common sense tells me my car isn't generating 1.6 lateral g's. I know how to recalibrate the accelerometers, but that won't change the lateral g inflation that all the different mainstream units seem to share.

what is the sample rate? that might be the cause...

I think it's due to where the unit is mounted. I've played with the location of where I install my unit (Traqmate), and it seems to make a bit of a difference. Not sure if it's just relative to the CofG or how level it is, but it seems to have an effect of some sort.

Assuming I'm not riding on the same short bus, I would think that a unit mounted on the inside radius of the turn would tend to have less G's measured than if it were mounted on the outside radius of the turn. Differences with front-to-aft mounting could also come into effect, I'd imagine, but nowhere near as much.

With my original placement (basically right of center in the front trunk), I found that my G's at Seattle (left-hand turns) were substantially higher than my G's at Portland (right-hand turns). It seems that my left hand turns were being recorded higher.

I've been meaning to go out to a skid-pad (or parking lot) and test the measurements from both directions (left and right turns) and see if they measure "off" equally for both, or if they're biased to one direction over the other.

I've also been tempted to mount the unit 90 degrees to normal, just to see if it's a difference between the lateral and longitudinal accellerometers. My bet is that the same "errors" would be shown.

Well, differences left to right can be diagnosed more easily. Just slide the unit out of its harness and rotate it 180 degrees along all three axes. I did it with mine and got different readings, which led to me learning about resetting the accelerometers.

But the artificially high numbers come from all the big brands of loggers, as far as I can tell. We're all getting numbers with our production cars that are better than Le Mans GT cars that cost 300K and have full undertray aero.

It could just come down to the cheapness of the parts... after all, the typical 2-axis g-meter found in these systems are only about $25, 3-axis are about $45, and they probably have issues with sustained, accurate readings. Maybe it over-reads the initial load, and then settles down to something reasonable, etc.

Data is only as good as the sensor.

set it to avg. over some time period -- 0.1 to 0.4 sec

I think it it s sample rate issue. Your instantaneous G may well be that high.