Final answer:
Duo-servo drum brakes are known as self-energizing drum brakes, leveraging the rotation of the wheels to increase braking efficiency without additional driver effort.
Step-by-step explanation:
Duo-servo drum brakes are also known as self-energizing drum brakes. They are called this because they use the rotation of the wheels to help augment the braking force applied by the driver. This is achieved through a design where one brake shoe is given a slight rotation in the same direction as the drum when the brakes are applied. This rotation pulls the second shoe into the drum with more force, increasing the braking effectiveness without requiring more effort from the driver. Essentially, the friction between the brake shoes and the drum creates a self-energizing effect, hence the name.
These brakes are commonly found in older vehicles and are especially useful in larger, heavier vehicles like trucks. They were an important advancement in automotive technology when they were introduced because they can provide more braking force than non-servo drum brakes with the same amount of human input.