In the earliest days of the motorcycle frame, welding was not used to assemble the various sections. Most manufacturers used the same methods employed in bicycle manufacture. This was the lug method of construction. The lug from one piece such as the neck was inserted into the frame and silver soldered in place. This was followed by brazing the pieces together and finally by welding motorcycle frame pieces together as they became more powerful than mere bicycles with engines.
The Need to Reduce Weight Brought Several Changes
There were other reasons why welding the motorcycle frame became the most common form of assembly such as new technologies and metals. The one main reason for this major change in the way they were built was the need to reduce weight. The old methods resulted in a structure which was incredibly heavy. By switching to welded joints, manufacturers no longer needed the lugs (stubs or pegs made to be inserted into the tubular sections). This led to significant weight reduction; allow bikes to be faster and more fuel efficient.
Welded Joints Allow for More Accuracy
When the lug and brazing method was in use, frames were generally considered to be relatively accurately assembled. However, this accuracy came at the expense of excessive weigh. As motorcycles became more powerful, the need for a better method of assembly developed. At first manufacturers switched over to using carbon steel and holding jigs to weld the framework together. This was followed by the use of ultra-lightweight materials such as aluminum and magnesium. Currently there are three different methods of welding in use.
Arc Welding or Shielded Metal Welding
Shielded metal arc welding is the first form of electric welding developed for commercial use. It involves the use of electricity to generate the heat necessary to melt the metals to be joined. The heat is also used to melt the filler rod being used. When the rod is placed close to the metals to be joined, an arc is formed which generates the required heat. The rod is wrapped in a chemical flux. When the arc is formed, the heat generated vaporizes the flux, which in turn provides a shield around the weld. This prevents air from getting into the area and compromising the weld.
Metal Inert gas or MIG Welding
This form of welding still uses electricity to generate the required heat. However the metal filler is supplied from a spool of wire inside the welding machine. The wire is feed through a tube, into the handle and into the weld. The arc is formed where the wire comes into contact with the metals to be joined. The integrity of the weld is protected by carbon dioxide, which is an inert gas. This form of welding is very fast and produces an extremely durable weld.
Tungsten Inert Gas or TIG Welding
The latest form of motorcycle frame welding is TIG or tungsten inert gas. TIG welding involves the use of a tungsten filler rod, AC or DC current and an inert gas. The choice of gasses is of no particular importance as it is used to keep air out of the joint. Because TIG uses highly localized, intense heat, it is perfect for use with metals where distortion can be a critical factor. These include aluminum and magnesium, to metals which have been gaining in prominence for many purposes in motorcycles as they are extremely lightweight and durable.
The methods of welding motorcycle frames have changed dramatically over the years. Manufacturers have learned how to use lighter metals to build their motorcycles. Along with this came the need for even lighter, but stronger welds. However, even today you will find a number of mass produced bikes which still use mild steel tubing which is shielded arc welded together. Each of the different types of welding has its advantages and is used in different areas of the various bikes on the road.