Steel is, by far, the most common frame material. Steel makes a frame that is strong, rigid, and light enough to suit most riders. Perhaps most importantly, it is easy to work with. Steel is easy to machine, and it can be joined by methods learned in shop class with equipment that is affordable. However, not just any steel is used; there are only a few varieties which are suitable. But you wouldn't get this impression by looking at all the tube decals on bikes at the local bike shop. Nearly every tubing manufacturer has its own code or generic names of common bicycle steels.

These codes are a numbering system devised by the American Iron and Steel Institute (AISI) consisting of four numbers to identify steels. These numbers are broken into two pairs. The first pair indicates the principal alloying element(s). For example, Tange Champion No. 2 is an AISI 4130 steel with the 4 indicating it's a chromium-molybdenum steel and the I indicating it contains a total of about one percent chromium and molybdenum. The second pair gives the average carbon content in hundredths of one percent. So the "30" in 4130 means that it contains an average of 0.30 percent carbon. With this chemistry, a steel is given the catchy phrase "Cr-Mo," or "Chrome Moly." The first two AISI steels listed in Table I are called plain carbon steels because they contain only carbon and manganese as intentional alloying elements. Steels that contain a total of about one to 4.5 percent chromium, molybdenum, nickel, and other elements (in addition to carbon and manganese), are termed "alloy steels." Both 31XX and 41XX are alloy steels. Table 2 lists the specific elements that are added to steels. The main function of these alloying elements is to increase strength. In addition, they control the strengthening process when steel is heat treated. A stronger steel can withstand a higher level of stress so less of it is needed in a frame. This means that the wall thickness of the tubes can be decreased, which results not only in a lighter frame, but also enhanced ride comfort because the frame damage to occur as cracking or buckling which can be easily spotted before catastrophic failure occurs.

So to ensure adequate ductility, the carbon content is kept below 0.4 percent. Table 3 lists the mechanical properties of the steels in Table 2 before and after brazing at the manufacturers' recommended brazing temperature. But there is one problem with this data-it is not always reliable. I have tested different brands of bicycle tubing over the years, and found that the data is usually exaggerated, so take it with a grain of salt. It's clear, however, that the strength of tubing both before and after brazing varies widely, depending on the type of steel. Confusing the situation further, test methods are not always realistic. However, don't be too concerned about small discrepancies; the tubes are strong enough in normal use. I've also determined that it is not easy to pinpoint where in a tube this strength reduction occurs; its location depends on how hot the tube got when it was welded or brazed. Knowing the magnitude and location of the strength reduction is important, because if it drops too far in the wrong part of the tube, then the frame may not hold up to the normal forces of cycling. For those readers unfamiliar with the strength terms given in Table 3, here is a quick summary: continue