For this installment of my A to Z series about materials, engineering, and making stuff, "B" was to be for "Bicycle Materials," something I've been interested in since I got into bicycling and began working on my engineering degree twenty-five years ago. (Yes, I did finish). But it's far too expansive a subject for an A to Z blog post, and these folks here and here do a better job of explaining it than I could anyway.
So instead, I'd like to narrow the topic down to what the guys in the pictures at the above links are doing, which is called "brazing." This is not to be confused with "braising," which I imagine someone is explaining at this very moment in a cooking blog somewhere.
I remember when I was a kid in the late '70s talking to another kid about bikes (BMX bikes, natch) and him talking about how his bike was great because it was made of "alloy" instead of steel and how it was also better than steel bikes because it was welded, and the steel bikes, his father told him, were "soldered."
|Rivendell Seat lug. Tubes brazed into it and seatstays onto it.|
This used to be my bike.
Saying the steel bikes were soldered implied that they were weak, structurally unsound, because soldering is a low strength proposition. As any BMX-riding electronics geek can tell you, soldering is a secure connection for wires and such that aren't under a structural load, but it's not for bunny-hops and double-jumps. But brazing is a great way to make a strong bicycle frame that gives up nothing in strength to welded aluminum.
Like soldering, brazing involves connecting two or more metal objects with molten metal, but brazing is more precise. With brazing, the parts are close-fitting and capillary action draws the molten metal into a thin gap between them (recall that when you dip a corner of a sponge into water, capillary action is what draws water up into the sponge, even against gravity). In bicycle building, the parts are a steel frame tube and connections, called lugs, or other fittings. The framebuilder liberally brushes a liquid called flux onto and all around the surfaces to be joined, which protect the metals from oxidation and other contamination. He or she then carefully applies heat to the joint with a gas torch and touches a metal wire (silver alloy in higher-quality frames) to the gap between the parts. The silver is melted and drawn into the gap. Brazing is a fairly simple process, but one that takes years to master. Good framebuilders know just how much heat to apply - too little and the gap won't be completely filled, too much and the steel will be weakened. They also know where to apply it - the silver actually flows toward the heat source, so by directing their torch, they can make sure the gap is evenly and completely filled. The close fit of the parts, the large surface area of their interface and the metal filler make an extremely strong joint; failures of brazed joints in well-made frames are beyond rare. You'll pull a tube apart before you pull it out of the socket of a properly brazed connection.
As with many crafts, the brazed steel bicycle frame has been replaced in the mass market by cheaper materials and methods that are marketed as "improvements" to buyers, especially new ones. But, as with many crafts, the professional bicycle framebuilder makes something of extremely high quality that is infused with his or her passion and can easily outlive its owner and be passed on to his or her children.