Understeeting as a rally technique

Understeer is a term for a car handling condition during cornering in which the circular path of the vehicle's motion is of a greater diameter than the circle indicated by the direction its wheels are pointed. In other words it means that understeering happens when the front of the car does not have as much grip as the driver wants, and the car has a tendency to plough straight on at a corner.

Physics

Under all high speed (greater than approximately 10mph

(16 km/h) for a typical automobile) cornering conditions a wheeled vehicle with pneumatic tires develops a greater lateral (i.e., sideslip) velocity

than is indicated by the direction in which the wheels are pointed. The difference between the circle the wheels are currently tracing and the direction in which they are pointed is the slip angle. Or simply saying, slip angle is the angle between the tyres actual path and its natural path. If the slip angle of the front and rear wheels is equal, the car is in a neutral steering state. If the slip angle of the front wheels exceeds that of the rear, the vehicle is said to be understeering. If the slip angle of the rear wheels exceeds that of the front, the vehicle is said to be rally oversteering.

Design

Any vehicle may understeer or oversteer at different times based on road conditions, speed, and available traction. The design of a vehicle, however, will tend to produce a particular "terminal" condition when the vehicle is pushed to and past its limits of adhesion. "Terminal understeer" refers to a vehicle which, as a function of its design, tends to understeer when cornering loads exceed tire traction.

Terminal handling conditions are a function of vehicle length and front/rear weight distribution (and thus the vehicle's polar moment of inertia) and front/rear tire traction (further modified by the relative roll stiffness of the front and rear, which affects the outward weight transfer during cornering). A front-heavy vehicle with low rear roll stiffness (from soft springing and/or undersized or nonexistent rear anti-roll bars) will have a tendency to terminal understeer: its front tires, being more heavily loaded even in the static condition, will reach the limits of their adhesion before the rear tires, and thus will develop larger slip angles. Front-wheel-drive cars are also prone to understeer because not only are they usually front-heavy, transmitting power through the front wheels also reduces their ultimate grip.

Although understeer and oversteer can each cause a loss of control, as explained earlier, many automakers design their vehicles for terminal understeer in the belief that it is easier for the average driver to control than terminal oversteer. Unlike terminal oversteer, which often requires several steering corrections, understeer can often be reduced simply by reducing speed.