KyoKusanagi said:
Spoilers are supposed to increase downforce, improve aerodynamics and reduce drag at high speeds. They don't directly improve traction in the way that a better quality tyre would - it puts pressure on the area directly underneath the spoiler and pushes the tyre towards the ground therefore creating a greater surface area of rubber touching the ground at any time.
Spoilers that are effective at very low speeds often generate excessive drag at high speeds, and spoilers that work well at high speeds are often ineffective while moving slowly. So half the ones rolling around on the streets do absolutely nothing at all even slowing the cars down in some case.
and the more tire touching the ground the more traction you have hence nascar tires have no tread in them.
Spoilers aren't ment for low speeds, they are ment for high speeds. And it doesn't put more pressure underneath the spoiler, the pressure underneath the spoiler never changes...ever. Unless you change in altitude. Moving air has a lesser push (lesser atmospheric pressure) than still air. The spoiler pushes the air up over the spoiler causing a pressure difference between the top and bottom of the spoiler. So the pressure underneath the spoiler pushes down on the trunk of the car, thus giving it more traction. Take a car with no spoiler, and the same car with a spoiler and test them out, you'll be able to turn sharper with out spinning out. Granted the turn won't be a 45 degree turn but there's a signifigant difference.
I personally wouldn't want a spoiler on my car unless it had a wind dam. Maybe side skirts, but those aren't as important to me because they aren't very necissary.
So in short spoiler = more aerodynamic. So when you put a spoiler on an SUV or minivan, it doesn't do ****, because the air is more likely to go under it anyway. Either that, or there is nothing to push down on even if the air did flow over it. In fact, if my logic is right, it would worsen the aerodynamics of the car because the pressure under the spoiler would be worse creating lift. And that's not good.
And one more thing: Spoilers (depending on the shape) work at certain speeds. There needs to be a certain ratio of pressure change in order for the downforce to occur. Weight of the car also has a lot to do with it. So again, spoilers won't work at low speeds.