The bell-shaped metal drum or hat (arrow) gives this brake system its name and a wear surface. Old-school 4x4s have drum brakes on all four corners. The simplest way to improve the function of a drum brake system is to include larger brakes. Old Jeeps like the one pictured here had 9-inch drum brakes from the factory. Enterprising off-roaders figured out that some passenger cars (like in this case, a ’49 Mercury) of the era had 11-inch brakes that could be fit to early front and rear Jeep axles and provided a lot more brake performance.