Michigan CDL - Air Brakes Practice Exam

Which type of brake is most commonly used as foundation brakes?

1. Hydraulic brakes

2. S-cam drum brakes

3. Disc brakes

4. Power brakes

The correct answer is: S-cam drum brakes

S-cam drum brakes are the most commonly used foundation brakes in commercial vehicles, particularly in heavy trucks and buses. This type of braking system operates through the use of a cam mechanism that pushes the brake shoes against the inner surface of a drum when the brakes are applied. The design of S-cam drum brakes allows for effective heat dissipation during operation, which is crucial for maintaining braking performance, especially under heavy loads. Additionally, the simplicity of the mechanical components makes it easier to maintain and repair, which is a significant advantage in commercial transport applications where vehicle downtime can be costly. While hydraulic brakes, disc brakes, and power brakes are also used in various applications, they do not have the same level of prevalence in heavy commercial vehicles as S-cam drum brakes. Hydraulic brakes are often found in lighter vehicles, disc brakes may be used in some newer heavy trucks but are less common than drum brakes for heavy-duty applications, and power brakes refer to assisted braking systems that can enhance the operation of the brake types but are not a standalone foundation brake type. Thus, S-cam drum brakes remain the standard choice for foundation brakes across most commercial vehicle configurations.