The Basics of Bike Frame Brazing and Lug Preparation

Another summer day in the palm of Michigan’s mitten begins with me blinking at the sun for all of the forty seconds it takes to get from Doug’s guesthouse to his workshop on the other end of the yard. I don’t know if I’ve ever avoided sunlight as well as I will during this class: all day in the workshop, all night at the guesthouse, one short walk in between the two. The upshot is that should I foul up a braze in some spectacular fashion, and render my frame into a smoldering lump of melted steel, congealing on the workshop’s concrete floor, at least I successfully avoided any lasting UV light damage for these three weeks.

The morning starts at the drill press where my classmate and I take turns mitering the rest of the tubes we picked out yesterday, keeping in mind that we need at least a hundred millimeters of butt on either end of the top tube and down tube, according to the passed-down rules of frame building lore. Though frame building is a science, the vital details of construction seem to be maintained through the oral tradition. I’ve yet to read an instructional about the importance of butt length, but butt talk should not be pooh-poohed.

 A butt is a strengthening technique used by the tube manufacturer where they make the ends of a tube a little thicker than the middle. My frame’s tubes are ‘double butted,’ which means the ends are twice as thick as the middle. Butt matters most around the head tube, where tube flex is most pronounced due to the normal stresses of aggressive road riding, like running joggers off the path and crashing into car doors. Kidding aside, on some of the cheaper old lugged bikes from the seventies and eighties, there will be tiny cracks in the paint just below the lug. The micro-cracks circle the tube like a ring; this is the exact spot where the butt ends and the thin tube starts. When you see this you know the builder cut the butt too short, and years of uneven torque vectors at that butt juncture cause the aged and brittle paint to tattle on the shoddy workmanship underneath.

On a new tube, the butt is invisible from the outside. By peering inside I see a ring of distortion that marks the butt transition, and using a thin ruler I note how far it is from the tube end, then mark that spot on the outside of the tube with a felt-tip pen. I cut and miter my tubes so that both ends have a fair amount of butt, though I make sure the head tube joint gets the full hundred millimeters, even if I have to shave some from the seat tube or bottom bracket joint.

  

This is a sharpened lug

Before shaping and fitting in the tubes, you have to ream the inside of the lug

I spend a big chunk of my day filing my bottom bracket. When lugs come straight from the lug factory the edges look rounded when they should be square, and sometimes there are bits of flashing or marks from the casting and manufacturing process that need to be removed for aesthetic reasons. With a hand file I can buzz the ugliness away, and also make the points of the lug pointier. This takes hours, but details like that separate a custom bike from something off-the-shelf.

In the afternoon Doug’s assistant and protégé, Herbie, came by to help with brazing practice. The intricacy of Herbie’s award-winning frame designs (under the brand “Helm Cycles”) show he’s as adept with a torch as the master himself, plus he’s an industrial tech teacher at the local high school, so he has a measured, patient approach when instructing. But before starting we clean the tube with some Emory cloth, alternating back and forth and up and down. Then we put flux on all the tubes that will be connected, with an extra globby layer on top of it all. Flux is a type of goo that prevents the heated metals from interacting with oxygen. There is no such thing as too much flux on a braze, but too little will result in a burned tube. After applying a healthy coating, I use the torch to melt the flux a little, just until it becomes hard. Flux is weird stuff, alternating from semi-liquid to solid to watery-liquid depending on exposure to heat. It also changes colors. I let the first layer cool, solidify, and turn brownish and apply another layer of white semi-liquid on the hardened foundation so that they melt at different rates, prolonging the duration of flux-life on the braze, thus giving me more time to get the braze right. With that all ready I tack one little spot with some silver braze to hold everything in position, then orient the parts the way I want them, and now I’m ready to add silver.

 

Torch flame is composed of the correct amount of oxygen and fuel -in this case propane. This flame has too much fuel. 

This flame has too much oxygen.

This flame has a good ratio of fuel and oxygen, but is too weak. You should be able to hear the flame, and it sounds like wind in your ear. 

Now we're talking. 

And here I am drying the flux.

Brazing is an art of warming up a whole area evenly, but that’s difficult with a hand-torch, which applies heat in targeted spots. So to distribute the melting power, I write an invisible “W” on the tube and lug surface, making sure to warm up everything within one quarter of the tube’s circumference at a time. Once the flux goes from opaque to clear and looks like it’s sweating, I can put a piece of silver to the lug joint and melt it. By warming the lug the silver draws down into place though capillary action. The goal is to move all the silver from the top of the joint through the bottom. You do this for all four quarters of the lug. Sometimes silver builds up in globs at the top of the lug, and I have to make another sweep of the torch to direct it through the bottom. This is the trickiest part of brazing because to do it right Doug directs me to make a little wave of molten silver that captures other globs as it runs from the flame. Go too fast and the wave will backtrack, go to slow and the wave over-heats and makes more globs. They call this, “cleaning the shorelines,” –and shorelines, Doug and Herbie tell me, are the mark of an amateur. Though structurally shorelines will not affect the frame.

 

This is the welding setup we used. On the left is the Acetylene and bottled oxygen that we almost never used. Most of the torching was done with a tank of propane from the gas station and grandma's oxygen concentrator. 

Today we only did practice brazes and brazing on the fork crown to steerer tube, because apparently those pieces are so big that you don’t have to worry about screwing it up and warping the metal.

You can see the tube is pretty thick, and so is the lug, so you can sit the torch on there forever without doing much damage. 

Here's my classmate showing good form, adding silver. 

Tomorrow we’ll work on cutting the fork arms so they’re perfectly even, bending them forward to match the desired rake and trail specifications, and brazing them to the rest of the fork. Also, I’ll do some more hand filing. Lots more hand filing.