roof aero/trunk aero
 

Bored at work, I was thinking about the sti roof spoiler and evo vortex generators and what effect something similar added to a hard top might have. This is assuming there is a active wing on the back.

Also do the "duck tail" style trunk lids offer any worth while down force? and if an active wing were already installed would it be pointless to have a "duck tail" trunk lid?

-Spencer

I am pretty sure they do. Im not sure exactly how much but if the flow if air is altered, more/less force will be put on the car.

The purpose of vortex generators are to bring clean air flowing from the rooftop down to a lower surface area. Now that being said, if properly placed they will do what they're meant to do. Yes it will work. Ducktails dont do much. More of just an asthetic piece

Thats kinda what I thought but just wanted to confirm all that cause my knowledge on aero design is very very limited.

Upon further searching I found these two photos. It would seem the evo's is more effective based on someone once telling me that the air is supposed to go under the evo's wing and suck it down as opposed to pushing down like a normal wing (which makes no sense to me and in our application that wouldn't even matter) but I'm sorry I couldn't live with a Sonic the hedge hog thing on my hardtop.

Do you know what a splitter does and how it works? Think of the carbon fiber piece in the middle of the Evos spoiler as a splitter and the trunk as the ground. It pulls the wing towards the trunk to make downforce as a splitter on the front of the car does pulling towards the ground.

Also if you were to put vortex generators on your car, do it like this
Attachment 203058

My understanding was on a splitter that the down force effect is created by a vacuum generated by starting with a small void expanding to a larger one with the fin sections there to stabilize the air. I'm probably wrong though.

That is an awesome looking car btw. I'm assuming that's the GTC300 wing? Can you give me any specs on that splitter? Been racing autocross and want to start doing time attack, but I want to put together an affective aero kit together first

The downforce is created by an area of low pressure below a surface and an area of high pressure. That creates a vacuum under the surface. I like to call it the frisbee effect since you can feel it on a windy day while holding a frisbee close to the ground. That isnt the GTC300. Its the GTC200 from the elises. They didnt want a big wing for some reason. Do you mean specs on the diffuser? They are called splitters too but be specific. ALSO DO THIS NOW: go to the meet and greet section and introduce/de-noob yourself.

I meant diffuser, sorry about that. I was looking into the Elise S2 diffuser upgrade a handful of people have done but abandoned that idea thinking that a purpose built diffuser would be worth the fabrication time in the end.

I posted in the meet and greet section a year ago but admittedly probably could have done a better job. Most of the time I just lurk around read and every once in a while post a question. It would probably be worth doing again though.

I'm just using this as imagery for my point, Sam. Also do not forget that using a diffuser creating such downforce (especially with strakes) means that you could add a wing to the diffuser to create extra downforce.
A wing placed at 50mm from the ground creates twice the downforce it could produce in free air (boot mounted). If mounted at 152mm from the floor the 50mm downforce is 50-60% larger than the 152mm downforce. Unfortunately this discovery also led to the active suspension era in F1 because as the chassis dives and rolls around the track as well as increasing and decreasing in velocity the pitch and height of the wing changes. This can create significant changes in downforce across the wings area and make the car become unstable.
My effort would be to make in at around the 150mm mark then any downforce changes wont be as dramatic.

Numbers and placement information taken from Simon McBeth's book Competition car aerodynamics.

Ask and ye shall receive... BTW, AutoSpeed is an awesome web publication site.

Part 1
Part 2
Part 3
Part 4

Great information. These pages made it very clear to me about the principles of how they work. Reading about flow separation makes me want to attach wool string to mine just cause i think it would be cool to see the way wind travels around the car.

Sam: After rereading the information you posted, it is much more clear than when I was reading it last night. Thanks again for the information.

Not a problem on sharing the information. They have quite a few other articles on aerodynamics as well that are worth a read.

Old thread but I recently tested VGs (same design as the low speed VGs pictured above) and it actually decreased the effectiveness of my spoiler (at about 60 degrees). However, I had them mounted more forward. I wish I had the time to test them at different locations, becuase I have a feeling that they are more effective mounted right in front of the seperation point.

does anyone have an idea where the separation starts to happen, and where the optimal VG mounting locations would be?

I remember reading a post by Keith@FM that the flow separation starts 1/2 to 1/3 down from the end of the roofline.

Thanks! BTW I'm the same guy making those aero posts on CR and M.net

I did some very rudimentary track testing with new placement of VGs. 1.1% increase in grip, calculated from sector times. Its not conclusive. I am going to do another manometer run sometime in the future.