I always wanted to know why some frame makers use one or another technique to put bike tubing together.
Well, as I have found, no individual technique is necessarily better than another. It all depends on the needs.
The frame shown here is brazed. The American Welding Society (AWS ), defines Fillet Brazing as a group of joining processes that produce coalescence of materials by heating them to the brazing temperature and by using a filler metal (solder) having a liquidus above 840°F (450°C), and below the solidus of the base metals.
Lugged: A lug is a socket that forms the junction between two or more frame tubes. Traditional bicycle construction uses steel tubes and lugs, joined together by brazing or silver soldering so that the space between the tube and the lug fills up with molten brass or silver alloy. Some aluminum or carbon fiber bicycles also use lugs, with glue instead of the brass or silver.
Bi-lam originates from post-war Britain and was pioneered by Claude Butler. It started primarily due to the lack of materials after the war. Originally the “lugs” would start out as a flat sheet of metal with a design cut out of it. After the frame was fillet brazed, the sheet would be wrapped around the tube and brazed in as well. It the gives the impression of a lug.
Brazing vs. Welding
It offers one big plus - strength. Properly made, the welded joint is at least as strong as the metals joined. But there are minuses to consider. The joints are made at high temperatures, high enough to melt both base metals and filler metal. High temperatures can cause problems, such as possible distortion and warping of the base metals or stresses around the weld area. These dangers are minimal when the metals being joined are thick. But they may become problems when the base metals are thin sections. High temperatures are expensive as well since heat is energy, and energy costs money. The more heat you need to make the joint, the more the joint will cost to produce. Now consider the automated process.
What happens when you join not one assembly, but hundreds or thousands of assemblies. Welding, by its nature, presents problems in automation. We know that a resistance weld joint made at a single point is relatively easy to automate. But once the point becomes a line - a linear joint - the line has to be traced. It's possible to automate this tracing operation, moving the joint line, for example, past a heating station and feeding filler wire automatically from big spools. But this is a complex and exacting setup, warranted only when you have large production runs of identical parts.
Of course, welding techniques continually improve. You can weld on a production basis by electron beam, capacitor discharge, friction and other methods. But these sophisticated processes usually call for specialized and expensive equipment and complex, time consuming setups. They're seldom practical for shorter production runs, changes in assembly configuration or - in short - typical day-to-day metal joining requirements.
Welding vs. Brazing considerations
1. Size of assembly?
2. Thickness of base metal sections?
3. Spot or line joint?
4. Metals being joined?
5. Final assembly quantity needed?