You can find bikes today made from any number of materials. While the most
common materials are steel and aluminum. Exotic materials once only used in the
aerospace industry are finding there way into an increasing number of bicycle
frames.
Steel
Once the only material used in frame construction, it is still a good choice.
Steel frames are still available in lugged construction (mostly by small
specialty builders) and on frames that are TIG welded. My personal preference is
toward lugged steel frames, but welded frames are cheaper to produce and hold up
well.
Aluminum
Aluminum is widely used today by several large manufacturers and provides
light and stiff frames. These frames are easy to identify, because most have
tubes that are of larger diameter than their steel counterparts. There is a
fairly wide range of prices among aluminum bikes, but in general they are more
expensive than steel.
Exotics
Truly at the top end of the food chain along with custom frames. Frames today
are made from carbon fiber, carbon fiber / aluminum, titanium, or various
combinations of titanium and carbon fiber.