Bicycle Frame - Frame Materials - Aluminum Alloys

Aluminum Alloys

Aluminum alloys have a lower density and lower strength compared with steel alloys, however, possess a better strength-to-weight ratio, giving them notable weight advantages over steel. Early aluminum structures have shown to be more vulnerable to fatigue, either due to ineffective alloys, or imperfect welding technique being used. This contrasts with some steel and titanium alloys, which have clear fatigue limits and are easier to weld or braze together. However, some of these disadvantages have since been partly negated, with more skilled labor capable of producing better quality welds, automation, and the greater accessibility of the same modern aluminum alloys as used in commercial airliners' structures, assuring strength and reliability comparable to steel frames. Aluminum's attractive strength to weight ratio as compared to steel, and certain mechanical properties, assure it a place among the favored frame-building materials (for example, a very strong rider, who does lots of hill-climbing, may prefer the stiffness of aluminum). Some disadvantages are that an aluminum frame does not have the same "feel" to an experienced cyclist as a steel frame, excessive ride harshness in lower quality frames, and decreased ease of reparability.

Popular alloys for bicycle frames are 6061 aluminum and 7005 aluminum.

The most popular type of construction today uses aluminum alloy tubes that are connected together by Tungsten Inert Gas (TIG) welding. Welded aluminum bicycle frames started to appear in the marketplace only after this type of welding became economical in the 1970s.

Aluminum has a different optimal wall thickness to tubing diameter than steel. It is at its strongest at around 200:1 (diameter:wall thickness), whereas steel is a small fraction of that. However, at this ratio, the wall thickness would be comparable to that of a beverage can, far too fragile against impacts. Thus, aluminum bicycle tubing is a compromise, offering a wall thickness to diameter ratio that is not of utmost efficiency, but gives us oversized tubing of more reasonable aerodynamically acceptable proportions and good resistance to impact. This results in a frame that is significantly stiffer than steel. While many riders claim that steel frames give a smoother ride than aluminum because aluminum frames are designed to be stiffer, that claim is of questionable validity: the bicycle frame itself is extremely stiff vertically because it is made of triangles, the sides of which do not change in length under stress. Conversely, this very argument calls the claim of aluminum frames having greater vertical stiffness into question. On the other hand, lateral and twisting (torsional) stiffness improves acceleration and handling in some circumstances.

Aluminum frames are generally recognized as having a lower weight than steel, although this is not always the case. An inexpensive aluminum frame may be heavier than an expensive steel frame. Butted aluminum tubes—where the wall thickness of the middle sections are made to be thinner than the end sections—are used by some manufacturers for weight savings. Non-round tubes are used for a variety of reasons, including stiffness, aerodynamics, and marketing. Various shapes focus on one or another of these goals, and seldom accomplish all.

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