Steel & Aluminum Frames

December 12, 1997 on 2:50 pm | In Product Tech. |

There are many different frame materials available today. Despite some very sweet carbon fiber frames (see carbon fibers and sordid materials), most bikes sold in the world are steel or aluminum. You can identify steel and aluminum frames by the methods used to join the tubes.

The most commonly used method of joining steel tubing is TIG (Tungsten Inert Gas) welding. TIG welding is powered by electricity and shielded from oxygen and hydrogen by controlled flow of an inert gas, usually argon. The needle-like tungsten electrode creates a narrow heat path by concentrating an electric arc for welding steel to steel. Steel TIG welds are characterized by narrow, concave or flat welds. Unlike steel, aluminum TIG welding produces a broad, convex bead weld path.

Bonding, another joining method, is visually identifiable by a sudden square change in the external diameter of the tube. It is literally one tube inserted into another with an epoxy adhesive to bond the two tubes. This method used is used with several frame materials, but is most commonly used to bond aluminum to aluminum.

Brazing is used to join steel tubes by bonding them with molten brass. Brass melts at a lower temperature, thus maintaining the integrity of the material. A gas torch is used to melt brass around the tube junction. Visually, brazing produces a smooth flowing junction.

Decisions, Decisions… These two metals have distinct characteristics that a rider should consider in their frame choice. The benefits of aluminum are rigidity, lightweight, corrosion resistance and its capability to achieve a high-polish finish. The drawbacks of aluminum are the high cost of refining the aluminum into tubing and the tooling cost for preparing the tube junctions. The fatigue life of aluminum is shorter than steel for similar tubes. To compensate for this, aluminum is often designed in larger tube diameters.

Steel is produced by highly developed methods that efficiently extract ferrous (iron) ores to produce carbon steels. When alloyed with chromium, molybdenum and other elements, a strong, lightweight tube is drawn. Steel tubes are cheaper to produce and prepare, providing an inexpensive frame material. Rust is the greatest concern of steel bikes, making maintenance a must.

Whether using steel or aluminum, the design is made up of the combinations of materials, method of joining and geometry. Design can manipulate the characteristics of each material when used in a structure, such as a bike frame. With either aluminum or steel, buy a proven design. Either material can last a lifetime providing proper maintenance and care of the bicycle.

by Robert Turner, PhD