When most people see my frames in their raw state, they immediately touch the frames’ joints and ask, “What is that?” Based on these experiences, I would say that the vast majority of people (even the vast majority of avid cyclists) have no idea what fillet brazing is.
How is it Different from Welding?
I usually start the description of fillet brazing by describing what the inquirer already knows: welding. Welding is the joining of two pieces of metal by melting those pieces of metal plus a similar filler metal together to form one molecularly continuous piece of metal. Then I go on to describe brazing as a way to join the same two tubes together, but instead of melting them together, brazing melts a secondary filler material (bronze) between and around the two pieces of metal. The result is almost a casted joint around the two pieces of metal.
Fillet Brazing Cross-section
What about Strength?
Inevitably the inquirer asks, “So, welding is stronger because it is molecularly continuous, right?” The short answer to this question is “No,” but I always like to qualify it. When two pieces of tubing are welded together, the joint is very small, usually less than 1/8 of an inch. Fillet brazes, on the other hand, are typically larger, up to 3/4 of an inch, which spreads the force placed on the joint over a much larger area. Furthermore, the contact area for a weld is very small, too. It is a less than 1/8 of an inch weld bead joining two tubes, each with less than a millimeter or so of wall thickness. There’s not a lot to a weld. On the other hand, the 3/4 of an inch of braze covers a good portion of the tubes around the joint and has a lot more contact. Additionally, the bronze brazing material goes between and inside the tubes to form about a 1/16 to 1/8 of an inch fillet on the tube interior, as well.
From a heat perspective, when two pieces of tubing are welded together, those two pieces along with the similar filler metal all melt together. When they freeze and become solid again, there are internal stresses introduced into the joint. When two pieces of tubing are brazed together, only the bronze filler is melted, not the actual tubes, reducing the internal stresses. Moreover, the bronze filler is made of mostly copper, which is very flexible, even after melting and re-solidifying, so it is much less likely to build up internal stresses. In the end, welding produces a harder, less flexible joint, where brazing produces a softer, more flexible joint.
Expanding a bit on the flexibility issue, fillet brazed joints are typically less stiff than welded joints, but they are plenty stiff for the use of a bicycle in most any situation. In fact, for most of us, the flexibility of brazed joints on steel tubes offers some very nice benefits in ride softness.
Why Weld at All?
Welding did not come to pass because it is necessarily stronger. It came to pass because it is faster. It takes somewhere between 8 and 15 hours for a custom bike frame to be handbuilt using welding. On the other hand, it takes about 25 to 40 hours for a custom bike frame to be handbuilt using brazing. The reason for this is the time requirement for finishing the brazes, which is usually upwards of 40% of the entire build time, in my experience. From a cost and production perspective, once affordable welders came onto the bike manufacturing scene, there wasn’t much of a reason to continue with brazing. Also, the only popular bike frame material that can be brazed is steel, so brazing limits material options considerably. Conversely, welding can be used to join steel, aluminum, and titanium.
Part of the reason I fillet braze is to conserve the way that bikes were traditionally made. Cycling is so fad-driven that some conservation needs to happen so we don’t forget our roots. Just look at what has happened since aluminum came on the scene, even after welding was prevalent. Steel frames are harder and harder to come by, aluminum (a more easily worked, but inferior material for long-term durability) is prevalent on most bikes, and carbon fiber frames are beginning to make a real dent in the total market share of frames. Lately, we are just building and buying throw-away bikes. I braze steel because I want to build a bike frame that will outlive its owner!
Fillet Brazing Definitely Looks Cooler
One point I never have to argue is how fillet brazing looks. The smooth transition between tubes is something that everybody appreciates, even if they don’t know what it is. But that sweet transition is the result of many extra hours of labor filing the joints down smooth. Nice welds also look great. For welds, anyway. But they just don’t seem to compete with a good fillet braze as eye-candy.
Welding does compete on a cost perspective, though. One of the most frustrating things about fillet brazing is the high price I have to charge for a frame. This high price is not just because it looks cool, but, again, because I had to take the time to finish all the joints. Nice welded joints look nice pretty much as soon as the torch passes over the joint. No finishing required.
Because of this fact, I am taking a page from the welder’s book on my next 69er MTB size run to make my frames less expensive by offering some fillet brazed frames that are unfinished, i.e. not filed down or painted. This is an effort to make the cost to get a handmade, fillet-brazed frame quite a bit less expensive. I’ll sell the unfinished, unfiled, unpainted framesets for $800-900 instead of upwards of $2,000 for the filed and painted framesets. The unfinished frameset provides all the ride quality and durability benefits of a fillet brazed frame and fork without the high cost.
To sum it all up, I build fillet brazed (instead of welded) steel frames for four reasons:
- Stronger and more flexible, especially for mountain applications
- Will last forever
- Conserves a part of the frame building craft that could easily be lost to technology advances
- Looks absolutely beautiful