Not all bike frames are the same. Road and mountain frames differ considerably according to their intended use, but even road- and mountain-specific frames vary in materials, geometry and function. There are no set rules about what makes one frame material or design better than another, so long as they both meet some basic requirements. The function of all frames is based on the physics of riding a bicycle.
The most essential part of a bike frame's design is its geometry, or the measurements of the different tubes that make up the frame. The geometry determines overall stiffness, aerodynamics, rider position and ride comfort, and also affects handling and responsiveness. Generally, bike frames are designed to be laterally stiff to make power transfer from rider to wheels as efficient as possible, while allowing the bike to flex vertically to absorb stress from rough roads or trails.
Aluminum alloys are popular in mountain and mid-grade road frames, although many manufacturers also offer professional-quality road frames made from aluminum. This light, stiff metal allows for excellent power transfer with minimal materials, making it easy to produce a lightweight but efficient frame. The drawback to aluminum is its ability to absorb shock, making an aluminum frame somewhat uncomfortable to ride over rough terrain or for long periods of time.
Carbon Fiber Frames
Carbon fiber is the preferred frame material for most professional riders and bike manufacturers. The fiber can be woven to create a frame that is laterally stiff, but that flexes to absorb road vibration, making frames made from this material quite efficient and comfortable to ride. It is also extremely lightweight, and frames made from carbon can be designed with advanced geometry that would not be possible with aluminum tubing. Carbon fiber is expensive, however, and the quality of the fiber can significantly affect frame performance.