Relevant rule snippets below- I'm not worried about rear downforce, I can have a huge wing as long as it's single element but the front has me wondering. This would be on a rally car so stuff being close to the ground is not ideal. I'm not seeing a lot of options other than an air dam and ducting stuff through the hood to get the air to go over rather than under:
Can you do a flexible front air dam? I 'm not sure if all of the "permitted variances" are listed there or if there's something else missing.
I'd think the biggest option here is a big ducted radiator opening. Helps with cooling, also helps move air from the high pressure nose to the low pressure hood surface (aka "the front wing").
In reply to Keith Tanner :
No other variances on these parts of the car. You can definitely do a flexible air dam so when you ground it out it doesn't rip the whole nose off.
Big ducted radiator opening with a "hidden splitter" keeping all the air up inside the bumper before it goes up and out the hood is also pretty much all I see here too.
It sounds like the front underbody is not included in those rules, so can you still create a front diffuser element?
Edit: re-read the Rally part. I doubt underbody treatments will do you much good
Underbody treatments tend to rapidly self adjust, yes. Interesting thought though if I can dump air through the wheel wells.
Maybe you can get some inspiration from the WRCwings website. He looks at aero on various rally cars.
Conveyer belting from Macs Hardware... probably doesn't help until at high speeds. 
¯\_(ツ)_/¯ said:In reply to Keith Tanner :
No other variances on these parts of the car. You can definitely do a flexible air dam so when you ground it out it doesn't rip the whole nose off.
Big ducted radiator opening with a "hidden splitter" keeping all the air up inside the bumper before it goes up and out the hood is also pretty much all I see here too.
yeah, I think you're going to want a "multi-shelf" radiator opening. also, I'd suggest putting oil & trans coolers out ahead of the front wheels, and use outboard of radiator opening to feed them... then it's a choice of trying to route their exhaust air out the top of the fenders (hard / downforce-ish); or have their exhaust get smushed out ahead of the tire (easy / lower drag)
assuming you can have stuff hang down from the 'undertray', there's probably another way to make front downforce. Recall, that what you're trying to do is accelerate flow... and, via Bernoulli, you can create lower pressure by reducing area... it doesn't necessarily have to be a "vertical" constriction of area, though. This concept is going to run into a couple of problems, though, on a rally car. The constriction has be able to bend out of the way for front impacts; the constriction is not going to be sealed to the ground, so it's going to tend to wrap under around the constriction walls; and the low pressure zone going to 'act' back towards the front axle centerline... reducing moment generation w.r.t. CG compared to rear aero (and compared to 'splitters' which tend to act ~within 1" of the lip)
are you allowed to modify the back side of the fenders?
The problem with outboard mounted heat exchangers is, when you clip something that otherwise might just be a little body damage it can turn into a dnf. I'd probably want to keep them centered for that reason.
On the fenders, technically the swept in thing I usually associate with time attack cars isn't legal, BUT the factory Subaru team did it and the rules are basically written around them so, I assume it's allowed now:
You probably don't have room for it, but if you angle the radiator back, the pressure drop across it should create downforce.
A 20" wide radiator with the top 5 inches further back than the bottom would create 100lb of downforce at 1psi pressure drop.
Granted, you wouldn't see that much pressure drop at rally speeds, but it makes GM's insistence on raking their radiators back like that instead of forward make a lot more sense.
In reply to Pete. (l33t FS) :
Wouldn't that also work with the radiator tilted forward? Seems like it would be friendlier to airflow. I think that's pretty standard for mid-engined cars with big hood extraction vents.
In reply to Keith Tanner :
Yes, it works with the radiator tilted forward.... but the pressure on the front of the radiator would produce lift, not drag. I suspect that the real benefit is the ducting after it, which would have far larger effects.
In reply to Pete. (l33t FS) :
Are you accounting for the effects of the high pressure pocket on the other parts around the radiator, as you lean it forward or backward? I'm pretty sure it should all more-or-less cancel out.
Like if you lean the radiator top back by 5", there is an equivalent increase in duct area above it that the pressure pocket is also pushing up on... And if you lean the radiator bottom back by 5", there is an equivalent increase in duct area below it that the pressure pocket is also pushing down on.
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