[quote:1f02b5cc7e="yelapa"]I do not ask this to challenge you, but why do you have a spoiler? The function of a spoiler is to provide enough downforce to overcome a loose (oversteer) condition at high speeds. The kinds of spoilers that we see on street vehicles are not large enough to achieve this effeect because they are way too small. So they are mainly for appearance. [/quote:1f02b5cc7e]
You're right, many cars use it for appearnce, but don't underestimate the amount of downforce a spoiler generates. Even if it only produces 75-100 pounds at 65 mph, that's enough to significantly change the handling. Since many cars have the profile of wings, the rear end tends to lift up and the spoiler will really help.
On a vehicle with a boxy rear end, like the E (and most minivans), when you're travelling at speed, there's a partial vacuum behind the car, since the air flowing off the roof doesn't drop down right away. Instead, it drops down a few feet behind the back edge of the car, where it then gets pulled towards the rear of the car, sweeping up dirt and (in rain) the rooster-tail plumes from the rear tires. If you put the car in a wind tunnel, you'd see it as a swirling vortex sitting behind the car, with the bottom of the vortex sweeping towards the rear hatch. (Looking at the car from the driver's side, this vortex will be spinning clockwise.) This is why the rear of most minivans get so dirty.
The spoiler changes the airflow, moving the swirling vortex back further behind the car, thus keeping the rear cleaner.