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. (Table 2 shows that 753 is the same alloy as 531; 753 just has a different heat treatment.) Columbus Record tubing, for example, is just as hard to work with because its walls are also very thin. All tubing listed in Table 3 is plenty strong if the tubing gauges are sized correctly for the intended rider. This is why I included the tubeset weights and maximum rider weights in Table 4.
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!
Saturday, September 22, 2007