Brake shoes are a critical component in drum brake systems, playing a key role in the process of slowing down or stopping a vehicle. The primary purpose of brake shoes is to create friction against the inner surface of the brake drum, converting kinetic energy into heat and facilitating the braking process. Here are the main purposes and functions of brake shoes:

1. **Friction Generation:** The fundamental purpose of brake shoes is to generate friction when the brakes are applied. In a drum brake system, the brake shoes are mounted within the brake drum and make contact with its inner surface. When hydraulic pressure is applied to the wheel cylinder, it forces the brake shoes to press against the drum, creating friction.

2. **Conversion of Kinetic Energy:** As the brake shoes press against the rotating brake drum, the kinetic energy of the moving vehicle is converted into heat through the process of friction. This conversion of energy is essential for slowing down the rotation of the drum and, consequently, the vehicle.

3. **Hydraulic Force Application:** Brake shoes are part of the linkage between the hydraulic brake system and the mechanical components of the drum brake. When the brake pedal is pressed, hydraulic pressure is transmitted to the wheel cylinder, causing it to push the brake shoes against the drum. This hydraulic force application initiates the braking action.

4. **Enclosed Environment:** Brake shoes are housed within the brake drum, providing an enclosed environment that protects them from external elements such as dirt, moisture, and debris. This enclosure helps maintain the effectiveness of the braking system by preventing contaminants from interfering with the contact between the shoes and the drum.

5. **Return to Rest Position:** Brake shoes are designed with return springs that retract them away from the drum when the brake pedal is released. This retraction ensures that the brake shoes do not continuously rub against the drum, reducing wear and minimizing the risk of overheating.

6. **Parking Brake Function:** In some drum brake systems, the brake shoes serve a dual purpose by incorporating a parking brake mechanism. When engaged, the parking brake either mechanically expands the brake shoes against the drum or uses a separate set of brake shoes to secure the vehicle when parked.

7. **Cost-Effective Design:** Drum brakes, including brake shoes, are often more cost-effective to manufacture than disc brake systems. This cost-effectiveness makes drum brakes a common choice, especially in the rear wheels of many vehicles.

While disc brakes are more common in the front wheels of modern vehicles, drum brakes, with their brake shoes, continue to be used in the rear wheels of many cars, trucks, and certain types of motorcycles. They provide a reliable and cost-effective braking solution for various applications.