Frame material has a major influence on many of a bike’s qualities such as strength/longevity, weight, ride quality and price. The primary materials for bike frames are steel, aluminum, titanium and carbon fiber.
Steel used to be about the only material for bike frames and it’s still an excellent choice. Chromoly (Chrome Molybdenum) steel is very strong and relatively light as steel goes. When it is shaped to remove excess weight, chromoly can result in a fairly light frame that will last for many years. Chromoly is also “responsive” meaning that it can flex without losing its shape or strength.
Aluminum alloy is a very common material for bike frames because it is relatively inexpensive, light, strong and stiff. While aluminum bike frames are not as durable as steel frames, their much lighter weight has made them very popular.
Titanium is a fantastic material for bike frames because it has a very high strength-to-weight ratio. Like steel, it also flexes well while maintaining its shape. Unfortunately, it is also very expensive. Therefore, you will generally not find Titanium frames on cheap bikes.
Carbon fiber is essentially a laminate of fibers bound together with glue. Modern carbon fiber bike frames are generally very strong and lightweight. Unfortunately, carbon fiber requires labor-intensive manufacturing and is therefore relatively expensive. It also doesn’t flex well without cracking so carbon fiber bike frames are often over-built to be particularly strong.