radiator exit path with undertray
 

i'm wondering if i should design in an exit path for the radiator air in my undertray, wich will cover the chin in front if the engine and the wheels, but might extend below the engine at a later stage. as such it will be blocking of the most obvious exit route for the air that passes trough the radiator.

since i don't want to complicate the design beyond what i can handle i wonder if leaving a path open to the wheelwells is a good idea. this would allow for a totally flat undertray and the lower pressure in the wheelwells would draw out the airfrom teh engine bay

however haveing employed dams to keep the air away from the wheels i'm a bit cautious to this approach.

the alternative however would involve some sort of duct trough the botton of the tray, wich might add even greater drag, complexity and weight to the design.

my main goal is to decrease overall drag while maintining or improveing cooling as it is.

I think the air exiting to the wheel wells would reduce drag by stopping the outside air from entering the well by adding pressure to the low pressure area inside the well.

thats exactly what you want to do. Pretty much build an enclosure around the back of the radiator and then duct it into the wheel wells. alot of race cars do this.

you can also duct the air out through the hood like alot of race cars do as well. an example that pops into my head is the corvette c5/c6 lemans cars.

hood vents would be great, and in theory pretty easy to do, but i'm not planning to hack into my car.

so wheel well venting it is than... it's pretty much the only real practical solution anyway.

another thing that puzzles me in the original car design is the bottom of the radiator. there are large oval openings in "beam" that supports the radiator.

these enter into the area in front of the radiator, and at the bottom in a cavity behind the bumper lip, the openings being angled slightly forward there but fully shielded from oncomming air.
it doesnt look as if they're just lightning holes but even if they're just has to be some air passage trought them.

because i have a relative small exposed grill area i initially figured that when the car was moveing slowly and the ram affect would be absent this would provide aditional openings for the fan to draw air in, wich will probably happen. however as the car is moveing, the exit of this duct would be sitting in a low pressure area, and thus air entering trough the frontal grill would be sucked out again, before passing trough the radiator, i can't see that helping cooling.

om my curent tray i've made holes in the tray to not mess with the original setup to much, but i'd like to undestand more of that they're doing and how i might use them to my advantage.

for example a simple scoop would convert them to in inlet at high speeds allowing grillblocks elsewhere, on the other hand if i might use the flow comming from them to energize the boundry layer of the undertray... small vg's might aid this

any idea what the engineers where thinking when they put these holes there?

Could be Speed Holes, like Homer Simpson had. Too bad Google can't find a good image for me to embed... ;)

lol that's a good one!

https://www.imcdb.org/movie_96697-The-Simpsons.html
all cars featured in the simpsons...pretty funny.

not much aero advice to be found in cartoons though... apart form speed holes ;)

I just did some measurements on the car looking forward to build the full tray