What is the rim defined as?

The rim is defined as the outer metal hoop of a bicycle wheel. This component plays a crucial role in providing the structure and support necessary for the wheel to function properly. It is where the tire is mounted, and its design and material contribute to the strength and durability of the wheel. The rim must be rigid enough to withstand various forces and impacts while also enabling the tire to be securely attached.

The other options refer to different components or tools related to bicycles. The inner structure of a tire is not the rim but rather the tire's casing and other internal elements. The center hub of the wheel is where the axle connects and allows the wheel to rotate but is distinct from the rim. A tire pressure gauge is a tool used to measure the inflation level of the tire, which is unrelated to the structure of the wheel itself. Understanding these distinctions helps to clarify the role and significance of the rim in bicycle mechanics.