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Old 09-02-2012, 07:51 PM   #1 (permalink)
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The Underbody (flat belly pan is suboptimal - explanation & solutions offered)

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.

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Old 09-02-2012, 08:38 PM   #2 (permalink)
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Old 09-02-2012, 09:27 PM   #3 (permalink)
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so your saying that if i make a belly pan i should make those vertical ripples like at 1:59, im doing at least past the transmission for protection and picking up a little bit of mileage helps
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Old 09-02-2012, 10:34 PM   #4 (permalink)
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If you have to make a flat underbody, then yes, there should be some sort of golfball effect like what is used on that Toyota.

EDIT: Another example of the dimple used... (Audi A8)


All of the parts that get hot are allowed to breathe, and the attachment points are dimpled to prevent the bolts from creating drag, as well as reduce the inherent drag of a flat underbody.
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Old 09-02-2012, 10:39 PM   #5 (permalink)
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Another Audi example (RS4):

Notice this underbody employs the tactics used in the Toyota video.
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Old 09-02-2012, 10:50 PM   #6 (permalink)
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yes i do see i plan on doing this more for protection but the mileage is nice, dad was forced to run over a semi truck tire that had just blown up because he was trapped on the hw ended up costing him a new transmission which is not something i can afford to do
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Old 09-02-2012, 10:58 PM   #7 (permalink)
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I believe it, because its a huge aero mod that covers a lot of square footage and only gets you a minimal 3% or 4% fuel economy improvement.
While lowering the front bumper or putting in an air dam seems to have the same effect and does it with a tiny fraction of the work and materials.

Given that we have a pretty small sample size to compare the trend would lead me to test an air dam first.
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Old 09-02-2012, 11:00 PM   #8 (permalink)
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Another thing that the dimples do is increase the strength of the panel! See the sides of new cars, they all pretty much have some sort of crease that makes them harder to bend. Win Win.
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Old 09-02-2012, 11:11 PM   #9 (permalink)
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oil pan 4, after going through the list of aero mods you guys have on the forum, I've come up with a solution that may be the optimum for ecomodding.

--> Front air dam to move the air around the car
--> Underbody that slopes upward for the airfoil shape in the back
--> Kammback extension to match the airfoil shape from the underbody

Important about this design is where the air starts to move under the car to make proper use of the underbody. The air dam will greatly reduce the amount of air entering the bottom of the car from the front. This will create a low pressure under the car. By leaving the sides of the car open, air will rush in under the car from the sides. Controlling where this air enters is vital, because if you cover too much flat area, you won't get much benefit. If you can get the air to enter right before the beginning of the curve of the airfoil profile, I believe you could achieve massive gains.

Not to mention the front air dam will reduce the drag caused by the front tires, and help the airflow around the car.

I have some drawings of this, but don't have access to a scanner right now.
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Old 09-03-2012, 06:17 AM   #10 (permalink)
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i currently do have an air dam but belly pans also add for protection who knows when somethings gunna jump to your underside and do some damage. I also just added an air dam last tank gunna fill it by the end of this tank we'll see what my mileage does

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