Duo-servo brake shoes are classified as ________.

Duo-servo brake shoes are classified as primary and secondary due to their specific roles within the brake system. In this type of braking mechanism, the primary shoe is the one that is engaged first when the brakes are applied, which helps reinforce the braking force. The secondary shoe then follows, enhancing the overall stopping power by effectively utilizing the design of the duo-servo system, where the leading shoe pulls the trailing shoe into the drum for greater friction and efficiency.

This classification is important in understanding the dynamics of braking performance and how the shoe arrangement affects the stopping capability of a vehicle. The primary shoe typically contacts the drum first and provides most of the stopping force, while the secondary shoe offers additional support and stability, making it a critical aspect of brake design in drum brake systems.