For some time the ideea of the 4c making "lift" instead of "downforce" is bothering me so I studied a bit the shape of the car and some basic aerodynamic concepts.
First I believe the car is making lift due to the following reasons (there may be others but I don't remember now):
1) Almost all street cars are making lift, apart from supercars and exotic ones that have big wings --> Alfa has only a small tiny wing on the rear that I will discuss further on
2) The car feels very agile, gripy and it is extremely fast on low speed corners, however when you get on the high speed corners the car feels unstable and the rear end tends to loose it's grip at considerably lower G forces.
3) I compared the corner speeds on similar tires with my previous track car (VS Scirocco which is known to make some little downforce ... while the same platform golf makes lift)
4) I analyzed the shape of the car --> more on this later
There is one easy way for the car to make downforce: adding a big wing on the rear and a lip / canards on the front. Below are the reasons I will not take this approach and probably 99.9 % of the owners will not do it either:
1) The car looks great in it's oem shape. It's an exotic, streetable, less racier design and adding a big wing will damage it ... not to mention that it will look odd when taking the car for a ride in the town. I drive the car both on streets in town, outside town and racetracks. I enjoy the car on all sorts of roads not only the racetrack.
2) You have to make wholes in the boot cover for a big wing and when I want to remove it I will have problems and additional costs in putting the car in it's original state.
3) Adding a lip on the front will create problems on approaching ramps or street imperfections.
4) When you go to the racetrack is one thing to set a very good lap time with an oem looking car with a 1.74 liter engine and one thing in a car with big wings that it's clearly intended to work on the race track only! Beating a gt3 rs is far more impressive when driving an OEM looking alfa romeo 4c (with not so good online reviews
5) We can create more downforce with what we have (OEM car design) and less drag --> Because the 4c is relatively low on power the car can get close on lap time with a 4c with big rear wing because it would have much less drag.
Now why do I want more downforce (or some downforce instead of lift): Because the car is quite light and making downforce will be extremely well felt in the high speed corners --> These will make the car a lot more faster than it currently is and safer both on street and track. It's enough to see how much faster was that scara 73 alfa with aero than rudi's car (it was considerably faster, much more stable and controllable and this with inferior tires).
Below are my findings and some ideas to improve the aero package of the car while maintain the oem aspect:
1) I installed the autogiano rear wing which looks quite oemish (I opted for the cheaper fiber option and painted in the same color as the car) --> I am in no relation to them apart from buying one rear wing.
On the race track I go to, there is one high speed corner (whith speeds around 190 Km/h) that is extremely tricky to approach --> The corner also have some slight deviation that tends to unsettle the car --> This corner is so tricky that if I not approach it in the absolutely perfect angle I may loose the car --> on several occasions with even stock power I loosed the rear end but managed to get it back on trace (at these speeds you can imagine what I felt).
After installing the autogiano rear wing I didn't feel a major improvement however on this corner (even with wind from the back) the car was more stable and easier to approach. I didn't have any more drama --> I felt something similar on other corners (high speed corners): the rear end felt a little bit more planted: Now the difference is quite small and some may not feel it but it's there and it helps and the car became safer.
Another bonus: the wing looks more OEM than the OEM one --> It follows better the line of the car with less protrusions
The OEM rear wing: I don't know who designed it but it's bad because it has a thick margin in the place of contact with the rear hood and it also has a lip (that margin on top of it): these elements will create turbulence's and the air flow is detached from the rear wing and basically it doesn't do anything good.
2) I will test these days the air flow along the rear boot window --> if the flow is not laminar then we can add some vortex generators on the rear window to create vortexes --> This will keep the air flow tied to the rear hood and into the rear wing making it more efficient. I have some suspicions that in OEM format the air flow is not optimal (see the pictures below in wind tunnel --> not much air is getting into the rear wing).
Btw, adding a scope on the rear boot for cooling the engine bay is a bad ideea.
3) The rear diffuser:
This part is more interesting and here are the biggest gains to be made.
The diffuser I believe is well designed and with some minor tweaks the aerodynamics can be dramatically improved:
- We have a high angled diffuser (around 19 degrees on the margins towards the fences and around 14 degrees in the middle) --> this type of diffuser is more effective with high ground clearance. However our car is too high --> even with OEM sport suspension the diffuser entry point sits at around 130 mm above the ground and at this "height" it have almost no effect (you can clearly see this from the graphic attached). If we manage to lower the car then the diffuser should gain much effect (I'm sure that those who lowered the car with a few centimeters while maintaining the rake angle did feel something) -- with adjustable suspension we can lower the car on the race track and lift it back for the street (it takes some time and those who did it can share this info).
- The car bottom is "relatively" flat and it has a rake angle (which again is very good) --> The bottom have some protrusions where the carbon fiber sheets meet and these are creating turbulence. Turbulence is bad and it detaches the air flow from the bottom of the car and ultimately from the diffuser. We need somehow to smooth those parts
- The diffuser is only a central piece --> This is the most effective zone but we can extend it towards the rear wheels and I believe this will add some downforce (see the undercar picture).
- On the front of the car we have a lip under the front spoiler: The reason for this I believe is to keep some of the air away so basically there will be less air under the body so it will move faster and should create some downforce. However, this lip will also create turbulence's and I don't really know if it's effect is good or bad --> I could not find any example online with a similar approach.
I will try to add a bigger lip (4cm or so) more towards the front wheels --> This should deviate some of the air towards the front brakes and also away from under the car --> Will see what happens