[Sven7]

Originally Posted by expensivehobby22

If the bottom of your vehicle is flat bottomed, then yes, an aero feature in the middle of the underbody can be beneficial.

-It can deflect flow that has crept in behind the front tires, or around the front airdam back out from under the car before it hits the rear tires, axle, etc
-It can stop crossflow in yaw, ie air coming in from the left of the vehicle and impinging on the right rear tire

So, it can work in the right situation. However, it's probably simplest to add small, local tire spats in front of the rear tires, or skirts along the sides.

If the bottom of the vehicle is flat, a spoiler/dam would just add drag. Why push air away from a smooth surface? Add wheel strakes instead and it'll be ideal.