How To Braze A Broken Old Schwinn Mountain Bike's $150 Frame Back Together

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Photo: Raphael Orlove

A few months back, I got lucky. My 1980s mountain bike, a 1987 Schwinn Cimarron I got for $150, snapped in half. The luck was twofold: First, I was just putting along a little trail and not flying over a jump, and second, I would get to see how an old steel bike gets brazed back together.

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What Is Brazing? 

There are lots of ways to join tubing together to build steel bikes, but they boil down to two basic techniques. You can weld them, or you can braze them. Again, there are lots of different ways to do each, but there’s a pretty basic technical distinction between the two.

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Welding means joining two like pieces of metal together by bringing them to their melting point and introducing a like filler material to join them. In practice: You have two steel rods, you heat them up until the steel is at its melting point, and you join them by pooling a melting steel rod along the joint. A good welder will make this look beautiful, like a stack of dimes laying across the metal.

Brazing means joining two like pieces of metal together with a third, different kind of metal. The two initial pieces of metal are not brought to their melting point; they are heated to a lower temperature just hot enough to bring the filler metal to its lower melting point. In practice: You have two steel rods, you heat them up just enough so that they melt a little rod of 50 percent silver. The silver melts and is drawn into the space between the steel rods by capillary action. And this is how (or at least half of how) my horrible old Cimarron got patched together.

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“People always say that brazing is like gluing metal together,” Johnny Coast tells me. “That always bothered me.” He’s a really kind man, who made the switch from chopping and channeling hot rods with his dad to bikes by way of starting a fix-it-yourself bike library for neighborhood kids back in Colorado. He came to New York as part of the squatter scene and went legit, going to school to be a licensed framebuilder. His little shop in Brooklyn still has a backyard full of DIY projects for kids, as well as tall bikes for Bike Kill, which he helped found. It’s rare to hear him complain about anything, other than his wood stove going out.

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Coast explains that what’s going on during the brazing process is much more than gluing. Steel, he explains, is a crystal. Well, it has a crystal structure, or a crystalline form, or however you want to explain it. That structure changes as steel heats up. When he is brazing a bike together, Coast says wiggling his fingers, he is heating up the steel so that the little molecules in it space out. They almost relax. In comes his filler material, be it silver or brass, actually mingling into the molecular structure of the steel. When it cools, all is chemically bonded together. This is more than just a metal version of Elmer’s.

Finding Someone To Braze A Bike Is A Challenge

I bought my Cimarron because the head tube is fillet-brazed. That is, there is a smooth, seamless blending of all the steel tubes at the front of my bike, laid out in brass. The rest of the bike is lugged, a much more common kind of bike construction, and I forgave the Cimarron for it.

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What I didn’t realize is that all lugged bikes are joined using brazing. Tubes are joined at these metal sleeves — lugs — and melted metal is drawn into them using capillary action. Brazing!

This is why it was a bit of a challenge finding someone to repair my bike when the seat stays snapped off their lug at the seat post. There are lots of people out there who can weld a bike together. There are far fewer who can braze, and most of them are busy building complete custom frames for top-end buyers. I reached out to a couple framebuilders and few even knew anyone who brazed bikes anymore. The first I managed to get a hold of, Coast, was in between orders and he graciously let me sneak into his schedule.

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How A Bike Gets Prepped For Brazing

There is a lot of technical work in brazing, but the process doesn’t actually take all that long. What does take a long time is prepping a bike for getting brazed. It must be cleaned. It must be aligned. In the case of my bike, it also had to get stripped so that it could be held in place to keep it from warping while it got heated up for brazing.

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I was amazed at how much time it takes to prep a bike for this kind of work. This was actually my second visit to Coast’s shop. The first one just entailed brazing together new seat tubes for the bike. This time they needed to be measure precisely, then cut, then slotted so that they would perfectly fit into my frame. Coast even filed out grooves on the lug where my old seat tubes had snapped, giving the new ones a bit more surface area to cling to.

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The bike frame itself needed finagling. Coast had to hang it over a perfectly smooth surface and check that it was perfectly straight. It wasn’t and getting it there required some work. At first it would appear straight in one dimension, slightly askew in another. Coast would tease the frame a hair, bending the steel with a wheel. A twist, not enough. Another, not enough. Back and forth, bending the frame millimeter by millimeter until it was perfect, perfect enough for it to be joined back again.

Flux: Why The Bike Gets Its Teeth Brushed Before Brazing

The process of brazing itself is hard to even gauge from an observer’s perspective. Coast lined up all of the tubing, pressed and filed to be as closely aligned as possible. Then he got out a toothbrush and started rubbing everything down with what looked, yeah, just like toothpaste.

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That is flux, a kind of goop that acts both as a thermometer and an air-proof seal. The seal part is easily explained in this article from framebuilder Alex Wetmore, under the header of “Basic Brazing Information For Bicycle Builders.” Not the most romantic title, but it’s a great explainer:

At the temperatures involved in brazing, most metals oxidize very rapidly. This would interfere with the melted silver or brass wetting the steel and flowing smoothly into the joint. To prevent this, chemicals called fluxes that dissolve oxidized metals and protect the surface from further oxidation are added to the process. Most commonly, these fluxes are placed in and around the joint as pastes, though they can also be caked around the filler rod or added to the torch flame itself.

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So the flux keeps out anything that would dirty up or oxidize the brazing environment, but it does also serve as a bit of a temperature gauge. It starts of looking like toothpaste, but it cooks when heated and turns glassy.

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Too hot and the flux burns. Coast watches the flux to know when his metal is just the right temperature for the brazing he wants to do.

How The Brazing Itself Gets Done

It goes fast! Once the green flames have heated the steel to the right temperature, Coast gracefully introduces his filler rod and the heat does the rest. Again, the melting filler — in this case silver for the top lug and brass down at the dropouts — is following capillary action. It follows the heat. “It’s defying gravity a bit, actually,” Coast says while melting silver draws its way up along the hairline gap between the new seat tube and the lugged frame itself.

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Photo: Raphael Orlove
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As mentioned, Coast uses silver at the top lug and brass down at the dropouts. According to Coast, when they’re heated up, silver flows “like water.” Brass flows “like peanut butter.” The silver does a good job filling the small space between the seat tube and its lug. The brass does a good job filling up the whole sleeve of the seat tube slotting over the sturdy, solid dropout. The dropout has to handle the job of holding the rear wheel, and the rear derailleur, and the hub and cassette, all while you are shifting and riding and bouncing over rocks. It is a sturdy chunk of metal, and Coast gets it glowing red-orange-yellow for everything to braze together.

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Brazing complete, the bike goes into a phosphorus bath to melt off the flux. I stand too close, trying to take as many pics as I can, then remembering that I shouldn’t be breathing in phosphorus fumes. I step back, laughing, happy that wearing a mask all the time does periodically come in handy.

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This process has taken place over two days of work, and the bike is still not done. There is a brake bridge that still needs to go into place, as well as brake bosses and rack eyelets. It’s incredible how much work goes into what seems at first like a relatively straightforward repair. Every development of the bike industry makes sense in this context. Everything that is done to switch from hand-brazing to welding to robot welding machines saves time and cost. It also goes to show how much work goes into hand-building a bike. What started out as a $150 Schwinn rescued out of a dusty shed is quickly becoming something more. Soon, I’ll just have to put the bike together. And paint it, too, I suppose, though it’ll be hard to cover up the coolest this bike will ever look.

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Illustration for article titled How To Braze A Broken Old Schwinn Mountain Bike's $150 Frame Back Together
Photo: Raphael Orlove

Raphael Orlove is features editor for Jalopnik.

DISCUSSION

I know they usually don’t, but can you tig weld it, or would that just be a bad idea?  I just don’t know much about bicycle frame construction.