Rear Rock Guards...An Interesting Observance Today

I've watched these a lot. If the shield is mounted hanging just above the road it drags as you go over bumps, etc., and kicks up rocks and muck. Just look at the beat-up bottom for proof.
If it hangs too high the whirling draft created behind it sucks road crud up also, something like wing-tip vortices on airplanes.
There may be a sweet spot, and it also depends where it is hanging in relation to the back of the coach. All too complex. Bye-bye!