I've met both opposition and curiosity to my info on underbodies, and MetroMPG kindly asked me for a new thread on it, so here we go.
I will discuss why:
- A flat underbody probably does no good.
- Downforce provides just as much drag as lift.
- Diffusers increase airflow.
- The golf ball effect is usable.
- Any other questions that need to be addressed.
Now, I hate saying it, because you guys have put a lot of effort in to it, but just putting a flat sheet under your car isn't going to do much good. In certain cases, it may improve a little bit over the pumpkin and other bits that hang from the bottom of your car causing drag, but it's not the best solution ever. Here's why:
Ok it won't let me put in an image right now, i'll edit this later. (Done)
Flow over a flat plate is laminate for only a brief length. It then becomes turbulent. This occurs because at the surface of a flat plate, the velocity of the air is zero. This is because the molecules get caught in the little crevices on the surface of whatever the material is. This is why we have airfoils.
The difference between downforce and lift in relation to drag is a negative or positive sign. If you're driving a vehicle, lift is bad. I know how you guys feel about the rolling resistance thing, but I would never in my life try to reduce the grip of the tires. They are the only thing connecting you to the road. For me it's a safety thing.
Again, with the 'Downforce = Drag' thing: Diffusers help racecars produce downforce by making airflow more efficient. Efficiency works with economy as well as performance, it's all about the application. Why wouldn't you want to smooth out the air behind your car with some vertical slits? It's probably the only aero mod that would look cool!
You all probably know how golf balls work. You've also probably heard about the Mythbusters episode where they did that to a car and it increased mileage. Wellll, here's the thing, with an airfoil, you can achieve laminar flow across the whole thing. With production cars, you generally won't be able to achieve it (see Aptera). However, attached turbulent flow is better than straight turbulence. A sphere will lose attached flow, but put dimples on it, it keeps turbulent attached flow. Laminar attached flow is better, but if you can't achieve it, do the next best thing. MetroMPG posted this video of the new Toyota EV. Notice the underbody is flat... BUT WAIT! MTrenk says flat underbody is BAD, why would Toyota engineers do that?
Notice the dimples in the underbody.
If you want an effective underbody, it needs to resemble a golf ball design, or be shaped as if the bottom of your car is an airfoil.