The Lowdown on Tubing – About Steel Tubing

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 blickling 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  Tensile strength is a measure of how much force per unit of area, or the stress, it takes to break the tube;
Yield strength is the stress needed to permanently deform the tube a specific amount;

Percent elongation, a measure of the tube’s ductility, is the amount the tube stretches relative to a portion of its original length.

Frames are made from either seamed or seamless tubing. Generally, inexpensive discount store bikes are made from seamed tubing because it’s cheap to make. Most seamed tubes are made of low-strength plain carbon steel; these tubes must be thick walled to compensate for their low strength, but they are adequate for a bicycle that features a bargain-basement price rather than quality. A seamed tube begins life as a flat strip of sheet steel which is rolled into the shape of a cylinder and then welded along its length. The steel always suffers a drop in strength in the vicinity of the welded seam that only additional processing can correct. True Temper has perfected this additional processing through heat treating. This seam is easy to spot and is the best way to identify this type of tubing in other brands. Figure I shows how seamed tubes are made, and the resultant seam.

Most bicycles will be made from seamless tubes. In fact, all tubesets listed in Table 4 are seamless. Seamless tubes can be more expensive than seamed tubes because they are made from better quality steel and they are manufactured by costlier methods. Seamless tubes are made either from solid bars of steel or else fabricated like seamed tubes with additional processing. A seamless tube made from bar stock starts out red hot (between 1700-1800 degrees F). It is first pierced and drawn over a pointed steel bar called a mandrel. The tube is kept hot as it’s next drawn between a series of dies until its dimensions are close to that of the finished tube. The tube is then softened by heat treating, cooled, cleaned to remove surface oxides, and then cold drawn to its final dimension.

Figure 2 illustrates the drawing process. Cold drawing also involves placing different sized mandrels inside the tube and drawing it between dies. But cold drawing is done at a temperature below 1300 degrees F. As the final drawing operations take place, the tube is given a series of low temperature heat treatments to refine its microstructure. Lastly, the tube is cleaned and polished. The second way to make a seamless tube is a hybrid of the seamed and seamless methods. Sheet metal is rolled and welded into a seamed tube, and then cold drawn to flatten out the seam. A final heat treatment refines the entire tube’s microstructure so that it’s impossible to distinguish from genuine seamless tubing. Tubing made from strip costs less than tubing made from bar stock because the machinery, mandrels, and dies needed to hot-pierce and draw solid bars of steel are very expensive. Several good quality tubesets are made from strip, including the Magny-X and Magny-V tubesets from Ishiwata. The seamed and heat treated True Temper tubes are every bit as good as the seamless tubes from Europe; True Temper makes its tubes from strip simply to be price competitive. Most high-quality seamless tubes go through one final manufacturing step that has evolved just for bicycles. They are internally “butted”- made thicker at the ends than in the middle- while their outside diameters remain constant. Butting is done for two reasons: it puts metal where it is needed the most-at the joints where stresses are highest-and it saves weight. Butting is accomplished by placing a mandrel shaped like the desired inside dimensions into the tube. Both tube and mandrel are then drawn through a series of dies so that the inside of the tube molds to the shape of the mandrel. Once butting is completed, the mandrel must be removed. According to Tl Reynolds Limited, the standard industry practice is to put the tube into a machine called a reeler. This device spins the tube between inclined rollers which increase the tube’s diameter just enough to allow removal of the mandrel. The high-quality butted tubeset of a few years ago had only double butted, single butted, and straight gauge tubes. But some tubing manufacturers now feel that a frame needs different amounts of reinforcement in different areas. So to optimize frame design, tubing now comes in five different butting arrangements: none, single, double, triple, and quadruple. A tube without a butt has constant wall thickness; it is called a straight gauge tube. A single butted tube is thicker at one end; double butted tubes are thicker at both ends (each end of the same thickness); triple butted tubes have ends of unequal thickness; and quadruple butted tubes have ends and midsections of varying thickness. Figure 3 shows each of these butting configurations.

Table 4 lists the wall thicknesses and weights of many popular brands of tubing. I’ve also included approximate guidelines for the type of cycling each tubeset is best suited and the limitations on rider weight. The tube thickness given in Table 4 corresponds (from left to right) to the dimensions of the tubes shown in Figure 3. Bicycle manufacturers take pride in displaying tubing manufacturers’ decals on their bicycles that say the frame tubes are butted. But sometimes the decals aren’t entirely accurate. Take, for example, the decal found on a frame built with an Ishiwata EX tubeset. It says, “Guaranteed built with EX Cr Mo Triple Butted Tubes.” This statement implies that all the tubes are butted, and triple butted at that. In reality, only the top and down tubes are triple butted. The seat tube and steering tube are single butted, and the fork blades, chainstays, seatstays, and head tube are straight gauge. This mix of butted tubes is standard for most high quality tubesets, but check Table 4 for variations.

“Taper Gauge” is another buzz-word bantered about in conversations about frame tubing. This term, used by Tl Reynolds to describe the cross sectional dimensions of their fork blades, implies that the wall thickness of the blades tapers. But it doesn’t; the wall thickness of a Reynolds fork blade is constant. Rather, the tubes are tapered prior to becoming fork blades. The tapering is done to assure that the wall thickness of the fork blade is constant after one end is reduced in diameter. Otherwise, a tube of constant wall thickness would be made into a blade with a wall thicker at the narrow end. So to make fork blades with a constant wall thickness, you must start out with a tube which has a gradual decrease in wall thickness over the length of the tube. Hence, a “taper gauge” tube. Some fork blades listed in Table 4 appear to be single butted, but those numbers represent their dimensions prior to tapering.

As supplied to a bicycle manufacturer, they are straight gauge. Bicycle frame tubes are joined by welding, brazing, or braze welding (also known as fillet brazing). Most BMX framesets are welded, as are several brands of off-road clunkers. But most good road bicycle frames are joined by brazing. And most tandem frames are joined by the braze welding technique. Brazing bonds tubes together by heating the joint to a suitable temperature, then introducing a non-ferrous filler metal. The filler metal, usually copper- or silver-based, must melt at a temperature above 840 degrees F, but below the melting point of the base metal (i.e., the tubes). Molten brazing alloy is sucked into and distributed throughout the joint by forces developed by close-fitting surfaces, called capillary forces.

Braze welding is similar to both brazing and welding. Like brazing, braze welding uses filler metal to bond the tubes together, but the filler metal is built up around the joint like a weld bead rather than being distributed into it by capillary forces. Braze welding simply involves getting the joint hot enough so that the filler metal will stick to the tubes and hold them together. Unlike welding, the base metal is not melted. Each tubing manufacturer recommends a maximum brazing temperature, as shown in Table 3. They feel that higher temperatures will jeopardize the strength of the steel tube. If this and a few other recommendations are followed, the manufacturers guarantee their tubes against failure.

But many, if not most, experienced bicycle manufacturers and frame builders neglect recommended brazing temperatures. They’ve discovered that tubing failures are uncommon, even when they braze at temperatures well above the recommended limit. Another reason manufacturers exceed the recommended limits is to make production more flexible and economical. If they were to conform to the recommended temperatures, they would have to use a silver brazing alloy which contains 45 – 50 percent silver and is just too expensive and time consuming. It’s really the skill and technique employed in the frame building process that determines the integrity of the brazed joint. However, Tl Reynolds won’t distribute their ultra-thin 753 tubing to any frame manufacturer that won’t follow recommended brazing techniques. Reynolds specifies a brazing temperature of 1200 degrees F or below for this tubing. They’re concerned that higher temperatures will create a weakened area in the tube.

Figure 4 shows the results of some work comparing the location of the softened zones produced by brazing at about 1200 and 1700 degrees. Notice that when brazing at 1200 degrees, the tube is softened up to about 7 mm behind the lug, while the higher temperature softens the tube at a point about 22 mm behind the lug. In each case the softened zone is normally well within the butted section of the tube. But it’s possible that when sizing tubes, some frame builders may have to cut off a good portion of the butted section. If they then use a high brazing temperature, the softened area may form past the butted section in the tapered section or even thinner straight gauge section of the tube. This puts a weak spot in the tube in an area of the frame that may not be able to take the stress.

Conforming to Reynolds’ temperature restrictions requires brazing 753 with low-temperature, high-priced silver brazing alloys. But some frame builders prefer to use these alloys on all frames they build, even when they don’t have to. There are several reasons why they choose to do so, although there is less validity in some than others: -Lower temperatures cause less distortion of the tubes, so less post-brazing frame alignment is required, along with the tubeset’s integrity remains intact. It’s easier to braze with silver brazing alloys. This is true; silver brazing alloys flow better into a joint and there are fewer problems during brazing and with post-brazing cleanup. -Silver brazed joints are stronger. This is not true; joint strength depends on factors other than just the type of filler metal. The main factor is where the tubes soften, which is temperature dependent. Silver brazing places the soft spot closer to the joint. -Frame repairs are easier to make on frames that have been silver brazed.

This is true; less heat is needed to remove damaged tubes. Low-temperature silver brazing is a sales feature. No doubt about this. Many hand-built frames are regarded as jewelry by their owners. To say that the frame is silver-brazed adds to the mystique. It is also easy to strike fear into a customer with talk of the dire effects of heating steel tubes to orange-hot when brazing with brass filler. But Figure 4 clearly shows that higher temperatures only push the softened zone farther back from the joint and actually detract less from the yield strength of the tube ahead of the softened zone. This is not a problem if the right tubeset is selected. Most frame tubing is not nearly as finicky as Reynolds 753. In fact, the only reason that 753 needs special care is that it is so thin, not because it is any special sort of steel.

A frame built from Tange Prestige tubing will give a great ride to a sub-150-pound rider, but the same frame in the hands of a 200 pounder will be too flexible and may actually fail in use. Heavy riders need heavy gauge tubing. Since all the steels are very similar to each other, it is hard to pick favorites. Yet people who have ridden a number of bicycles made from different brands of tubing often claim that one brand of tubing is more rigid than another. This is not true; rigidity of steel tubing is a function of its outside diameter, wall thickness, and length. And since the outside diameter of tubing is fairly standard, a frame’s rigidity will depend only on the thickness of the tubing and frame geometry. Thicker tubes resist bending simply because there’s more metal there; short tubes bend less because forces act over shorter distances. So for equal lengths, gauges, tapers, and frame geometry, a frame made of Columbus SL tubing will be as rigid and ride identically to the same frame made from Ishiwata 022.

Even though many new types of tubing have been introduced in recent years, the steels used to make them are nearly the same as those used for the last 40 years. I think that is surprising, considering how much more we know about steels today. What have changed over the years are the types of heat treatment and the manufacturing processes used to impart impressive before-brazing strengths. Reynolds 753 and Tange Prestige tubesets are examples of tubes whose chemical compositions are the same as sister tubesets (Reynolds 531 and Tange Champion, respectively), but have different microstructures because of different heat treatments. Their increased strength allows them to have very thin walls. However, you may notice that Tables 3 and 4 list other tubesets with walls as thin or thinner than these (Columbus Record and Ishiwata 015, for example), which aren’t any stronger than thicker grades in the same quality product line. These tubes have proven themselves to be reliable performers (within the limits of their intended use), so the justification for producing exotic tubing like Reynolds 753 and Tange Prestige out of standard alloys with current manufacturing techniques can be called into question.

All this information about tubesets, and your new knowledge about their virtues will help you understand why I am working with True Temper and Henry James. The consistency of manufacturing that this company has for their product will give consistent results in the final product. Also, if I cannot receive the proper materials that I have ordered, then time constraints can ruin a project. Hence the reason I have chosen Hank as my supplier. His attention to this concern helps the experience be more pleasurable. I want you to know that just about any tubeset (steel) is available and if there are certain needs that you have just let me know!