mark kibort

actually, aspect ratio doesnt change efficiency.

It really is not for our cars. Ive measured 250lbs of downforce with my cup car wing at 120mph. if you look at the graph of the lift /drag, you can see that the ratio is about 10:1. so, for 250lbs of downforce, thats 25lbs of drag, and at 100mph or so, with a 24" diameter tire and a 4.5:1 gear reduction, thats costing you about 5ft-lbs of torque.
Here is a graph that speaks to a shaped wing close to what we use.
look at the lift to drag rato (far left numbers) at 0 angle of attack, its 20:1, at 12, its 10:1.
at approaching 20 degrees angle of attack you max out the lift and its starts to go negative if you give it any angle. keep in mind, the roof line will increase effective angle of attack. most roofs produce 5-10% deflection and add to the angle of attack.

mark kibort

actually, maybe for a 911, but a cup car wing on my car made it almost undriveable. The incar video shows this is obvious that the understeer it created was substantial. (vs with the prior much smaller and lower positioned wing) In fact, I often will take out a couple of degrees to get rid of a push. I want to use as much wing as possible, before it gets too pushey. I ended up running near 0 angle still producing 100lbs of down force originally, until i was able to address the front of the car. Splitters and dive planes, as well as hood vents solved that problem and gave it the balance it needed. a larger wing would just produce more downforce that I would have to match with more downforce up front. (or more power in the rear). so, wing size is important. too much is not always a good thing. Balance is really what it is all about.

Tom W

Bob: No undertray. I've wondered about trying to put a thin/lightweight cover over the bottom of the car but the Jerry Woods folk don't think it will do anything. Our cars have too much ground clearance to get the ground effects benefits that a formula car or spec racer gets. I do have cut-outs in the rear bumper and have the rear bumper reduced in size so it does not act as a sail but lets the air out easily. I think I have some pics at home and can post tonight.

My car has all RSR body work from Getty except the roof. The metal roof was removed and a fiberglass replacement added. A shot of when they were fitting the new roof (the remains of the A & B pillars and front cowl were removed when it was installed). The roof was from GT Racing and was a huge pain to fit and get to look good but it saved over 50 lbs of weight high in the car.

SundayDriver

First a minor point - I suspect you really meant sports racer, not spec racer. Spec racers are required to run a pretty substantial ground clearance and get no ground effects. Formula cars and sports racers (other than spec racers) have no such restriction.

Folks might be interested in this summary:
Ground effects work when you get down to an inch of ground clearance or less. So you do a flat bottom with diffuser and drop the ground clearance in a Porsche to about 1". My guess is that you could make 1000+ lbs of downforce at ~100 mph. But that amount of downforce will lower the car even further. And you can't tolerate any changes in ride height because that will destroy the downforce.

Think about how 'exciting' it would be to enter a corner at 130 mph - on the edge. Then you hit a bump, the ride height changes and all of a sudden the car is only capable of taking that corner at 100 mph and off you go.

So you need much stiffer springs. Probably 4-6 times stiffer than what racers are running in Porsches. But unless the chassis is REALLY stiff, the chassis flex is now your springs, not the springs themselves. AND you have given up mechanical grip because the spring rate is so high.

The are a bunch of compromises to make aero work correctly and a lot of tuning to get both aero and mechanical grip all while trying to keep this very low car driving on the tires instead of becoming a snowbaord.

wanna911

There are some manufacturers that are making rear diffusers for 911's at stock height and showing transition from lift to DF at various speeds. Do you guys think this is a bunch of fluff???




Hard to argue with wind tunnel testing eh? I can't find the graph of the claimed DF numbers though. But this diffuser doesn't even cover the whole bottom of the rear.

bobt993

Mark, That's a good point on downforce. Ironically I have lots of spring, main rears are around 1600 lbs with additional helpers. Cervelli set up the spring combos and I have taken a Radical owner for a ride and his only comment was it was the best setup 911 he has been in. I think the 60inch wing with end plates seems to be the way to go and it's going to take a full day of testing to start to get it right.

SundayDriver

End plates make a huge difference in how effective a wing is. It will be interesting to hear how this works out for your setup.

Just as a point of reference when I talk about an aero car being stiff, let's compare your very stiff setup with what I had on my Stohr. Let's say your car weighs 3200# with driver, fuel etc. 40/60 weight. So the static weight at one rear corner is just under 1,000 lbs. 1,600 lb/in springs with a motion ration of .85 (a guess) gives a wheel rate of about 1150 lbs/in. Or a wheel rate/weight of ~ 1.2

The Stohr was 1,000 lbs fully loaded- 40/60 goives 300 lbs on one rear corner. Spring rate as 1200 with a 1.0 motion ratio so the wheel rate was 1200. Wheel rate/weight was 4.0

So if you ran rear spring of about 5,300 lbs/in you would have the same stiffness.

Just some interesting tidbits for everyone. And, IMO, those diffusers are all show. Those cars are too high and the bottoms are not smooth enough for them to do anything.

JackOlsen

Well, at smaller tracks with tighter turns you don't often have 150-mph straights. Some autocrossers swear by their enormous wings, which never see 60 mph in use. I don't know what to make of that, but I'd say expect modest returns on aero pieces on slower/tighter tracks.

Sunday Driver/Mark hits it right on the head with underbody aero. There are some benefits to keeping air from getting underneath the car (low nose, side skirts), and there are some benefits to smoothing out the bottom of the car for the air that is passing under there. But in my opinion, you're crazy to try to chase serious ground effects with any kind of a 911. You've got to get very low and maintain a very precise position relative to the track. Corners are where you'd benefit from this, of course, but keeping a car completely flat in a corner is next to impossible with our spring options.

The aftermarket pieces might produce some kind of effect in a straight line (the one I made did). But there's no benefit to downforce when you're going straight, and they're going to go 'all out of adjustment' as soon as your car leans through a corner. They look cool, I guess. But most people who claim benefits from their diffusers are simply benefitting from having smoothed out the underbellies of their cars. (In my opinion.)

Big wing. Big end caps (go up above the wing and down below it). In front, get 5" of splitter extended out parallel to the road and down low. That's my recipe.

C.J. Ichiban

but they're carbon! and pretty! hahaha

claykos

That's why I said as big a wing AND a splitter as practical - I said you couldn't have too much downforce, not taht you couldn't have too much rear downforce.

Also thought I would share an interesting point to somewhat illustrate the futility of really trying to optimize aero within the frame of club racing. I happened to listen to a presentation this afternoon about some wind tunnel experiments on airfoils. A piece of scotch tape placed on the leading edge of the airfoil made close to a 10% difference in Lift/Drag ratio....

Not to say people shouldn't add aero to their cars, it helps - A LOT! But, without serious testing and engineering quibbling over a few extra pounds of drag force, etc is not worth it. So....if the rules allow run a big *** wing and splitter. If the balance is off try adding downforce to fix it - if you can't, then remove downforce from the offending end of the car.

Tom W

Mark: Yes, of course - and I was thinking of you and your car as I wrote it.

Wanna: Yes, I think it's fluff. While it looks cool, I see no data that actually shows it creates a difference in downforce.

Bob: My car's rear end. You can see the vents behind the rear wheels the big vent where the license would be and what's not as obvious is that some of the lower bumper material has been removed too.

wanna911

More pictures and a wind tunnel graph. FYI, I'd never consider a part like this, but I do think Porsche bottoms are pretty smooth, how smooth does it need to be for desired effect? I'm more concerned with actual functionality because I'd like to add one that's not so flashy.

JackOlsen

Yeah, so long as you never have to turn, it's awesome.

I got better straight line numbers with this.





But it didn't help my lap times any.

Made the car a little less stable, in fact:

wanna911

WOW!! Now that is beefy.

Did it cause any excess engine heat?

Less stable huh? That's not good.

mark kibort

1000lbs of downforce for being 1" off the ground?

What do you base this on. curious. usually, the downforce you produce with lowing the chassis, takes the air that would normally go under the car, to go over or around the car. Ive taken some pressure measurements under the car at various spots , as well as around the splitter tops and bottom and the bottom area always stays around ambient pressure. to get 1000lbs of down force the differential pressure would be tremendous, to do it strictly with ground effects. my wing (GT3cup) puts out near 175lbs at 100mph. in order to get 1000lbs, you would need near -.3psi for a near 3000 square inches. very unlikely, plus, it would actually need to be quite a bit more, because it is differential pressure and the top of the car will normally be in the -.15psi range (vacuum).

Now, venturi tunnels, etc, yes, downforce from ground effects is possible.