In some car washes, the presoak application is followed by an empty space or idle zone. Wheel cleaning equipment, such as sill brushes or high-pressure wheel blasters, may be placed in the idle zone. A sill brush (also known as a wheel brush or tire brush) consists of an 8-foot-long brush assembly that is pushed against the car's wheels and door sill area. Brushes typically use flagged bristle, as dirt is usually most heavily concentrated on the lower parts of the car. The material on a sill brush may have alternating lengths or use material that is intentionally mounted off-center to allow wheel surfaces of various depths to be cleaned. Sill brushes rely on the rotation of a customer's car's wheels in order to achieve complete wheel contact. Similar to the CTAs, wheel brushes often only activate when the customer buys a wheel cleaning upgrade. Some car washes use wheel-rim disc brushes in addition to or in place of sill brushes. These assemblies extend out towards the wheel and follow it at the same speed as the conveyor while rotating at high speeds to clean the wheels. At the end of a car wash's presoak idle zone is often a high-pressure arch that directs water at a vehicle's surface.