Final answer:
Both shoes in a duo servo drum brake system are the same size; however, they have different lining lengths with the primary shoe toward the front having a shorter lining and the secondary shoe toward the rear having a longer lining.
Step-by-step explanation:
The student has asked which of the shoes on a duo servo drum brake is smaller than the other. In a duo servo drum brake system, both shoes are typically the same size. It is a misnomer that one shoe is smaller than the other. Duo servo drum brakes have a primary and a secondary shoe, which are distinguished by their lining length and position rather than size. The primary shoe is located towards the front of the vehicle and typically has a shorter lining, while the secondary shoe is positioned towards the rear and usually has a longer lining, providing more braking force because of the servo action. This servo action allows the rearward shoe to apply more force, as the rotation of the drum assists in pressing the secondary shoe outward, enhancing the braking effect.