A car's steering and suspension geometry is defined by three basic settings, with all angles expressed in degrees; the steering axis is an imaginary line through the suspension strut's axis, extended to contact the ground. Camber refers to the angle between each roadwheel and a vertical line through its center and tyre contact patch, viewed from the front or rear; positive camber indicates the roadwheels tilt outwards at the top, while negative camber indicates an inward tilt. Adjusting camber angle involves loosening the front suspension subframe mounting bolts and shifting it slightly, which also affects the castor angle, and can be checked with a camber checking gauge. Castor is the angle between the steering axis and a vertical line through each roadwheel's center and tyre contact patch, viewed from the side; positive castor means the steering axis tilts forward of the vertical, while negative castor tilts it backward. Adjustments to castor are limited and primarily for reference; significant deviations from specified values require professional inspection due to potential wear or damage. Toe is the difference between lines drawn through the roadwheel centers and the car's center-line, with toe-in indicating inward pointing roadwheels and toe-out indicating outward splaying. The front wheel toe setting is adjusted by screwing the track rod(s) in or out of the outer balljoint(s), while the rear wheel toe setting is not adjustable and requires professional inspection if significantly different from specified values. Given the specialized equipment and skill needed for checking wheel alignment, it is advisable to leave these tasks to an Audi dealer or similar expert, although many tyre-fitting centers now have advanced checking equipment.