Spoilers are the wings attached to the back of a car which increase the grip between the tire and the ground. You can often find spoilers on the back of performance cars. They are used to create downforce so the back wheels get pushed into the ground when the car is moving at high speeds. When the tiers are getting pushed into the ground more friction is being created thus more grip between the tires and the ground. At an angle of about 21 degrees the ideal amount of drag and downforce is being created from the air passing over the spoiler. When air passes over a spoiler at low speeds it is unnoticable. But, at an angle of 21 degrees you would start to feel a big difference at around 75 mph and up from a car with a spoiler to one that does not.