I'm guessing they they are sides of a diffuser. Also keeps air from sucking around from the side of the bumper to the underside. The black color helps them be less noticeable. If they were body color, they would make the rear of the car look fat in side view. Plus they would get rock dings all over them.
Airflow under and beside your car doesn't have the same velocity -- so it keeps them separate (somewhat) so that they won't spill into each other. Keep the fast flow from speeding up slower flow (wasted energy).
It also helps to separate the flow in the middle of your your underside from the sides of your underside. For the same reason -- flow going directly down the center will be a little faster than what's on the sides
Time is the best teacher. Unfortunately it kills all its students.
As it happens, I was just out doing an errand on the beater bike and saw a parked Prius. The insides of the fences extend straight back from the inside edges of the rear tires. The outsides, as you can tell in the caribou pic are in fact tapered/boat-tailed aft of the tire at rougly a 10 degree angle. The 2 planes meet at a point at the rear edge of the bumper.
So, it generally sounds like these are multi-function devices.
Thanks, I liked that Caribou one too. It just fell in my lap.
Interesting stuff. I wonder if the distance from the tire tread to the front face of these is critical. Like on the Prius they work because they are close to the tire, but on another vehicle with a larger space, they would just add more drag. Maybe.
I really need to do something with the underside of my car. Even if it doesn't measureably improve my mileage, it would probably make it quieter, which would be nice. And in the wheel wells, too, to try to smooth things out in there, and eliminate some of the dead space. So many projects, so little Coroplast! When is the next election, so I can scavenge some? :-)
Actually the low pressure area is the rear window and the bottom is just dead air so the fence prevents bottom side air from being sucked up with road dirt to the top of the car. Think of the car as slicing through the air the bottom of the car is cutting through the stationary air with the 4 tires scooping the air in front with the air dam and making air flow OVER the top and around the sides but not under the car where it has no place to go. Ever wonder how far out to the sides that air flows around your car? Did you ever notice as a car comes up on your left side passing you how it pushes you around when it is 6 to 10 feet away on the highway? Don't think of it in terms of a wind tunnel with air passing over the car as that is not the real model of air flow of a car traveling over the ground as far as the bottom of the car is concerned.
Bill pics are really great. When I first crawl under the Prius after we bought it I thought that it could use a lot of help on the under trays. It is surprising un- areo looking but I guess Toyota did their home work. I Would love to but a belly pan on there and see what effect it would have. It seems like it would have to improve things.
I've looked under my dad's Prius and there's a lot of room for aero improvements with that car. Just for starters; SMOOTH underbody paneling, a boattail (built onto and lifting with the hatch), grill block and redirect of the radiator exhaust out through the front wheel wells rather than under the car, front and rear wheel well skirts, and smooth hub caps. Of course, I beat his MPG by about 25 to 30mpg on a regular basis.