Undertray

An undertray is a device intended to improve the flow of air beneath a vehicle. A smooth underbody offers less resistance to air movingg beneath the vehicle, effectively increasing the speed at which it can be expelled. This reduces aerodynamic drag caused by protrusions underneath the vehicle as well as improve downforce and thus handling at speeds.

Design
The key to an undertray is to increase the flow of air by reducing the turbulent surfaces it comes into contact with.