Car handling and vehicle handling is a description of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. It also includes their stability when moving in a straight line. Handling and braking are the major components of a vehicle's active safety.
Two general terms are used to describe the handling of a car: understeer and oversteer. Understeer occurs when the front wheels of the car lose traction before the rear wheels. The car is difficult to turn and “pushes” toward the outside of a turn. Oversteer is the opposite condition. The rear tires lose traction before the front tires. Hence, the rear of the car is “loose.” It slides toward the outside of the turn, and the car feels like it is going to spin out.
The goal of suspension tuning is to make the car “neutral,” not oversteering or understeering. When the car is neutral, you should be able to control what it does in a turn by using the throttle. Giving a burst of throttle should make the back of the car slide toward the outside of the turn and allow the car to turn better. Getting off the throttle should make the back of the car step back in line.
When the car is neutral, it is easier to control, creates less wear on the tires, and usually allows for faster lap times. I say usually because a slightly oversteering car can also yield fast laps. However, this is difficult to achieve because the driver must react instantly to the car sliding by counter-steering. If the reaction is not quick enough, the car will slide sideways achieving a large yaw angle (the angle between the direction the car is pointing and the direction it is actually moving). This will scrub off speed and may even cause you to spin out. Therefore, an oversteering car can be fast but only with a skilled driver. On the other hand, an understeering car is limited by the grip of its front tires on the racetrack. Even a skilled driver will not be able to clock fast laps with a severely understeering car. Nonetheless, you may want to set up your car for slight understeer to make it more stable and forgiving.
Aerodynamic forces are generally proportional to the square of the air speed, therefore car aerodynamics become rapidly more important as speed increases. Like darts, aeroplanes, etc., cars can be stabilised by fins and other rear aerodynamic devices. However, in addition to this cars also use downforce or "negative lift" to improve road holding. This is prominent on many types of racing cars, but is also used on most passenger cars to some degree, if only to counteract the tendency for the car to otherwise produce positive lift.
In addition to providing increased adhesion, car aerodynamics are frequently designed to compensate for the inherent increase in oversteer as cornering speed increases. When a car corners, it must rotate about its vertical axis as well as translate its centre of mass in an arch. However, in a tight-radius (lower speed) corner the angular velocity of the car is high, while in a longer-radius (higher speed) corner the angular velocity is much lower. Therefore, the front tyres have a more difficult time overcoming the car's moment of inertia during corner entry at low speed, and much less difficulty as the cornering speed increases. So the natural tendency of any car is to understeer on entry to low-speed corners and oversteer on entry to high-speed corners.