For better or worse thermally fused metal joints are an integral part of motorcycle construction, and despite some serious advances in liquid adhesive technology, are likely to remain so for the near future. That's not a bad thing; people have been welding metal together since roughly 3,000 BC so most of the procedures are fairly well established. In light of that, we thought it might be interesting to take a brief look at the various processes involved. The following won't make you a welder, but it should provide some insight into what it takes to keep all those loose pieces of steel, copper and aluminum that make up our motorcycle, flying in close formation.
Soldering
Soldering is the joining of two metals, like or unlike by the use of a third metal that's entirely different from either of the two base metals. Which is a long winded way of saying that soldering fastens two metals together without melting them, by using a third molten metal to bind them together. Soldering in fact might best be described as "metal gluing."
Common solder is a mixture of tin and lead that melts at a relatively low temperature, by definition all soldering takes place at temperatures under 800 degrees F, and is normally sold in either a roll or less often these days by the stick or bar. Typically the solder will be described by a ratio like 60/40 or 50/50, the numbers referring to the amount of tin and lead respectively that make up that particular grade of solder.
Before a soldered joint can be made the joints have to be chemically cleaned with soldering flux to prevent dirt and oxidation from ruining the bond. Fluxes are available in liquid, powder, solid and paste form, or it may be embedded in the solder itself. Fluxes are further divided into two loose groups, acid, which is used for connections where corrosion won't present any particular problems, and rosin, which is a non-corrosive flux used for electrical joints.
Older motorcycles made extensive use of solder. The fuel and oil tank joints were soldered, solder was used to smooth out rough body work before the paint was applied, this is where the term lead sled comes from in hot rod circles, and fuel and oil lines had their ends soldered on. Additionally nearly all of the electrical connections were soldered in place as were the control cable nipples. Currently the use of solder is pretty much limited to securing crucial electrical connections, especially where a wire terminates at a circuit board or terminal and as a means of securing control cable nipples.
Brazing
Like solder, brazing uses a third metal to create the joint without melting the base material, however the temperatures are higher, all brazing takes place above 800 degrees F, and the filler material, typically a brass alloy, stronger. An off-shoot of brazing is known as braze welding, but since the basic process is so similar we'll forgo any distinctions between the two processes. Brazed joints are stronger than soldered ones, and in some applications are just as sturdy as a weld, and may even be preferable. This is because the lower temperatures involved in brazing create little if any warping of the base metal and doesn't affect its heat treating. Brazing is also a handy way to join certain dissimilar metals, such as copper to steel or tool steel to low carbon steel.
When men were men and motorcycle frames were flimsy, quite a few manufacturers brazed their frames together, and given the technology of the day it worked rather well. Those days are long gone, but brazing is still a very handy, and low impact way to repair a lightly stressed part, I use it a lot to attach things like mounting bracket tabs when I don't want to get the base material too hot, or to occasionally repair small cracks in something like an exhaust pipe Y connector because the filler material is strong yet easy to blend in to the base metal with a minimum of grinding, after which it can be painted or plated.
At the OEM level you no longer see much brass used to assemble motorcycles, certainly no one is brazing their frame joints these days, although I wouldn't be too surprised if some of them were silver-brazing things like fuel line fittings together.
Automated mig welder in a...
Automated mig welder in action
Welding
Welding might be defined as a process where two metals are joined together by heating them to the melting point, and then allowing them to flow together and fuse. Actually it's slightly more complicated than that, and generally involves the introduction of third metal to act as filler, but melting metal is essentially what welding is all about.
There are so many different types of welding that it'd take a book, to cover them all, and quite a few of them such as explosive welding, which uses high powered explosives to drive plates against each other fusing them together or forge welding, where two pieces of metal are brought to a white heat and then literally beat together, are a little impractical for our purposes so we'll just concentrate on the more common forms used in motorcycle construction and repair.
Oxygen/Acetylene Gas Welding
Torch welding is one of the oldest and most popular ways to join metal. When made by a skilled welder, a good gas weld is every bit as strong as anything made by a machine, and in most cases far more attractive, which is an important consideration when we're discussing motorcycles, with their many visible joints. The most common gas welding device is the Oxygen/acetylene torch. In this process oxygen and acetylene are stored separately in high pressure cylinders. Flexible hoses connect the tanks via pressure regulators to a torch equipped with adjustable flow control valves. The gas flows through and mixes in the torch, and is burned at the torch tip. The flame produced melts the base metal and the filler rod, which all combine to create the joint. Oxy-Acetylene torches are extremely versatile, they can be used to heat, cut, braze, solder and weld, and because of that they are found in every properly equipped repair shop in the world. Ironically because they are so common, and unfortunately so abused by guys that don't know how to properly use them, some armchair experts tend to look down on them, if they knew how to actually use them it'd be a different story.
The rigs do have some drawbacks. Gas welding is an art, and some people just can't get the hang of it. I've met more than one guy that could lay down a pretty credible weld using an ARC or MIG welder, yet couldn't get the hang of gas welding or brazing. This is one of the reasons why most welders of my generation were taught to use a torch before anything else. The feeling was that if you had a talent for the job, it'd show up in your torch work, if it didn't, well, you might make an OK production guy but you'd never be a real "welder." Secondly, it is a fairly slow process. That's rarely a problem if you're making a repair, or building a custom part, but it's a real issue if you're an OEM trying to churn out a couple of thousand frames a week. Lastly Oxy-Acetylene welding isn't a particularly easy or efficient way to weld aluminum, which is a drawback when it comes to some motorcycle work.
At the factory gas torches have long been replaced by other, more efficient welding methods but in the repair shop they're still one of the most popular ways to weld.