Components 101: Rim
The wheel rim is a hoop of extruded metal or a carbon fibre structure around which the tyre is fitted and the spokes attached. Many are designed for use with rim brakes, providing a smooth braking surface. Some modern wheels consist of a carbon fibre hoop with a bonded aluminium rim which provides a good braking surface (often lacking with carbon fibre).
There are a wide range of cross sections available, each designed for a particular performance goal including; aerodynamics, mass and inertia, stiffness, durability, tubeless tyre compatibility, brake compatibility, and cost considerations.