In the past I have experimented with diffusers and undertrays on my RC cars, and they DO work (depending on design of course).

However, now all sanctioned race meets have banned them, so there was no point in me continuing.

And for those wondering, the bad part, aerodynamicaly speaking, is all the holes and cutouts underneath the chassis creates drag and turbulence. If you ever get to look underneath any groung effect racing car, or high end road car, you'll see that the bottom is very smooth, with no gaping holes,(and if anything, naca ducts for cooling on some road cars), with a rise, generally restrained by the rear suspension, so that when the floor clears the rear wishbones, the rake picks up further. Also, once the diffuser section becomes too large, you need to add spill plates to stop the air from getting too messed up.