Air under, over, or on either side of the car?
 

Ok, I've read theories about every opinion.
1) Avoid adding additional air to the side of the vehicle, thus increasing wake.
2) Use an air dam no lower than the lowest part of the car chassis.
3) You don't want any air traveleing under the vehicle.

So what is the deal? It seems to me that some air under the car would be good to clean up the trailing turbulance. If you look at this flow video from AndrewJ you can tell the air from beneath the car is moving more slowly and swirling.

I also know that air should meet at the same angle. pressure and velocity.
Anyway, crazy stuff. Just wondering if until I get a belly pan I should do a bigger air dam?

My understanding is that air under your car is alright its the air dragging against your axles/differentials, shocks and the such that is the real mpg offender.
The under tray hides what it can and the front spoiler/ small air dam keeps air from being too high up in your undercarriage. So make the front spoiler just long enough to hide your differential if you have one, or just long enough to be ever so slightly lower then your suspension/ crap hanging off the bottom of the car.
My opinion is to have the under tray 1/8th to 1/2" above the front spoiler/small air dam.. meaning you do not need a easily damaged extra low hanging front spoiler imo.

Air pushed to the sides will interact badly with nearby objects (barriers, tunnel walls, other vehicles). Air allowed under the car will wind up producing if not horrible drag, then at least downforce (drag). Lowering your air dam to cut down on the air passing under the car will increase surface area (drag).

The three laws of thermodynamics come into play here:
1. You can't win.
2. You can't break even.
3. You can't even quit the game.

There is no right answer, and the least wrong answer varies depending on what you have and how & where you use it.

Charlie my previous post must fit into your #3 law.
Most modern cars are now stock with belly pans
and front spoilers which means -
A. they are useful
B. they help sell the car
C. ( insert your answer here)

So a smooth underside sounds like the best option. Let it flow freely, and see if i cant create a lower diffuser that replicates the angle of my futur kammback. Well for now, I have no belly pan, so ill use a moderate sized air dam for now.

smooth
 

Automobile drag is dominated by pressure drag.
The pressure drag is a function of flow separation.
Anything you do to prevent separation is a step in the right direction.
The smooth belly minimizes separated flow under the car and increases the amount of kinetic energy available to inject into the wake.
If a diffuser is properly integrated into the rear portion of the belly pan,the maximum amount of energy transfer can take place,netting the greatest drag reduction.
The future tail extension will just pick up where the diffuser leaves off,allowing for ground clearance of course.

But you can still try to lose less ! :)
Optimistic people will call that winning.


Whatever air doesn't get under the car, is a win / lose less situation.
Whatever air still goes underneath should get a smooth sailing.

Yep. It's all about finding the least wrong answer.

...my mantra: "...you can seldom OPTIMIZE benefits, but you can MINIMIZE losses!"

underbody
 

I was wondering,
if you create an underbody for an ICE car, would the enclosing of the engine bay create more drag because the air coming in through the grill will only have the wheels as an exit option?