Commercial Driver’s License (CDL) Practice Exam 2025 – All-in-One Guide to Master Your CDL Test!

The S-Cam drum brake is most commonly found on commercial vehicles due to its durability and high braking performance. This braking system consists of a drum that rotates with the wheel and brake shoes that expand against the inside of the drum when the brakes are applied. The S-Cam mechanism converts the force from the brake chamber into a rotational movement, causing the brake shoes to press outward and create friction against the drum.

This design allows for effective stopping power, especially under heavy loads, which is crucial for commercial vehicles that often operate with substantial weight. Additionally, S-Cam drum brakes are generally easier to maintain and repair in the long run compared to some other braking systems, making them a popular choice in the industry.

The other types of brakes mentioned, while they are used in various applications, are less common in heavy-duty commercial vehicles. For instance, disc brakes are found in some commercial vehicles, but they have not been adopted as widely as drum brakes due to higher costs and more complex maintenance requirements. Hydraulic brakes are more typical for passenger vehicles, and air-over-hydraulic brakes are also used but are less prevalent than the robust S-Cam drum brake system.