Steel is the classic bicycle frame material. It's strong, inexpensive and well suited to modification through alloys and other chemical treatments. When choosing a frame, the various steel types offer an array of choices. Understanding the basic types and their characteristics will help you make an informed decision about your next bike frame.
High-tensile steel, or hi-ten as it is often abbreviated, is the most inexpensive type of steel used in bike frames. It contains few alloys, which are additional metals that improve its strength-to-weight ratio, and is the weakest of all the types of steel used in bike frames. Hi-ten also is the heaviest steel used in bike frames. You will typically find hi-ten steel on older road and mountain bikes as well as low-priced cruiser and children's bikes.
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Cro-moly is an form of steel that is alloyed with chromium and molybdenum. The addition of these two metals make cro-moly steel lighter and stronger than regular steel. This allows bike manufacturers to make thinner frame tubes and reduce weight, while preserving quality. Cro-moly is used in a variety of bikes and is the second most expensive steel frame material. Most high-production bikes over a few hundred dollars use cro-moly steel in their frames.
There are many steel bike frame types beyond simple cro-moly and high-tensile. Independent frame manufacturers are constantly trying new alloys and heat treatments to alter the characteristics of the steel. These alternative steel treatments may favor increased strength, weight reduction or stiffness. Also, many bike manufacturers combine materials in one frame. The most common combination uses high-tensile and cro-moly steel. Cro-moly is used in the frame while high-tensile is used in the fork and seat posts. This offers the strength of cro-moly in the main frame, but cuts cost by using more affordable hi-ten in other parts of the bike where strength and weight are less important.
The way a steel frame is assembled is as important as the steel used. Many frames are welded together, which is simple, effective and saves weight. If frames are not welded together, they are joined by lugs. Lugs are sockets that the frame's tubes slide into. Lugs are stronger and offer increased aesthetics, but they add weight. You can also consider butted steel. Butted frames feature tubes that are thinner at the middle and thicker near the connections. Butted frames are meant to add strength where the frame needs it most, while adding minimal weight.