Bicycles can be made from a variety of materials, including carbon fiber, titanium, steel, and aluminum. There is no one perfect material for a bike frame, and each material has its own pros and cons. Steel frames have been used for over a century and offer excellent ride quality, durability, and affordability. High-quality steel frames integrate great design, superior assembly, and better alloys in the tubing. A popular quality steel for bicycle frames is American SAE 4130 steel, better known as "chrome molybdenum," and referred to as "chromoly" or "chrome-moly". Aluminum frames are relatively cheap to manufacture and are lightweight, making them perfect for bike frames. Carbon fiber frames are the most commonly used bike frame material for higher-end mountain and road bikes, including virtually every bike being raced at the professional level. Carbon fiber is a composite of carbon sheets that are bonded together in a mold using resin. The primary advantage of the material is that at a given stiffness, carbon fiber is significantly lighter than aluminum, steel, or titanium. Titanium is a relatively rare and expensive bike frame material thats labor-intensive to work with, meaning these bikes are typically quite pricey. Titanium frames offer excellent ride quality and durability.