Critically Comparing Lugged And Fillet Brazed Frames

This post compares lugged and fillet brazed frames.


Before comparing the two types of frames it’s necessary to present the technological processes behind their production.

Fillet Brazed Frames

As the name suggests, fillet brazed frames are made of tubes connected via a metal-joining method known as brazing.

via: /a torch melting a brass rod over a 90-degree angle joint/

Brazing is done as follows:

  1. The two pieces are placed at the desired angle.

2. The area is covered in flux and heated via a powerful torch that can produce extreme heat quickly.

The purpose of the flux is to remove oxides from the used materials, prevent oxidation during the heating process and ensure a consistent flow of the brazing filler metal over the contact area.

3. A brazing rod made of brass is melted at the connection points.

Note: Brazing is different from welding. During brazing, the base material is never melted. It’s only heated to a temperature that would melt the filler rod which has a lower melting temperature.

The melting temperature of brass is 1810-1880°F/990-1025°C whereas that of steel is 2500-2800°F/1371-1540°C. (source)

Brass rods used for brazing

One can look at brazing as “gluing” two pieces of metal by melting a metal rod between them. The melted rod is the “glue”.

One of brazing’s main benefits is that the affected ends are never heated to melting temperatures that could temper the integrity of the tubes. For that reason, people see brazing as “kinder” to the frame.

Lugged Frames

Bottom Bracket Lug

Lugged frames rely on brazing too. The difference is the use of steel lugs (image above) which act as sleeves connecting the bicycle tubes.

Each tube is first inserted into the lug. Then, the frame-builder heats the area and melts a filler rod (usually made of silver) into the joint.

Thanks to the flux, the melted silver flows underneath the lug and creates a strong bond between the lug and the tube.

The Advantages of Fillet Brazed Frames

  • No Lugs = Freedom

The absence of lugs on fillet brazed frames provides the following advantages:

  1. The builder doesn’t have to fabricate or buy a set of lugs.

2. To change the geometry of a lugged frame, the builder needs a new set of lugs. When the frame is brazed, however, this requirement does not exist. The builder can easily change the geometry of the frame by altering the angles at which the tubes connect.

Subsequently, it’s possible to produce a variety of frames faster and in a more economic fashion.

  • Lighter

Brazed frames have the potential to be lighter (ultimately it depends on the model) because:

  1. The frame does not have lugs adding weight.

2. The tubes of the frame can be butted more aggressively because the attachment area is smaller.

Frame butting is the process of removing material from the least stressed parts of a tube while adding material to the sections facing the greatest torque. The outcome is a light and yet strong frame.

The tube ends of lugged frames tend to be thicker over a longer section. This is necessary because the lugs cover a greater area of the tube. That area has to be heated during the brazing process.

If it’s too thin, the torch may burn through it or temper its structure and thus compromise its integrity. The extra material adds a minuscule amount of weight.

Meanwhile, fillet brazed frames have a smaller connecting area and can be butted a bit more aggressively.

  • Smoother Joints

The joints of lugged frames are elegant, but they’re also “in your face”. Meanwhile, the joints of high-quality brazed frames are incredibly smooth and seamless. Some people like that look.

  • Tubing of All Sizes

When using lugs for the construction of a frame, the builder is limited to stock tube sizes. When relying on brazing or TIG welding, the builder can use tubes of all dimensions.

  • Easier To Clean

The smooth joints of a fillet brazed frame store less dirt and are therefore easier to clean. In different, lugged joints have many edges that trap contaminations.

The Downsides of Fillet Brazed Frames

  • Labor Intensive

It may seem counterintuitive, but a fillet brazed frame requires more work due to the extra mitering, brazing, filing, and polishing. Consequently, a custom frame-builder might charge more to fabricate a fillet brazed frame.

Lugged frames do not require the same finishing effort because the lugs greatly reduce the amount of work needed to acquire a stylish joint. Hence why some people consider lugs are a shortcut.

The Advantages of Lugged Frames

  • Unique Look

In a world of TIG-welded frames, a lugged frame stands out. Brazed frames are also rare these days, but the visual difference between a color-painted brazed and welded frame is too inconsequential to the untrained eye.

A lugged frame, on the other hand, stands out even when the audience isn’t educated on the subject. The effect is greater when the lugs and the frame are painted masterfully.

In short, lugs offer an elegance that brazed and welded frames cannot replicate.

  • Additional Structural Strength

The lugs act as an external shield to the joint while simultaneously adding extra support. Thus, when all parameters are equal, a lugged frame is a stronger frame.

That said, a quality frame exploited as intended should provide plenty of strength regardless of the metal-joining process used to connect the tubes.

  • Higher Resale Value

Lugged frames are seen as an exclusive, rare product and keep their price better on the second-hand market.

  • Faster Finish Process

A fillet brazed frame has to be filed and polished with various tools (e.g., a belt sander) to acquire a smooth look. Lugged frames require less sanding to become shiny.

The Disadvantages of Lugged Frames

  • Restrictions

From a functional standpoint, the main downsides of a lugged frame are the restrictions that the lugs impose on the frame builder.

The lugs “decide” the angle at which the tubes attach as well as their thickness. If you want a frame with a custom design, you need custom lugs. Those are expensive to buy or make.

As a result, frame-builders are restricted to generic lugs that come with many limitations.

  • Extra Work

In many cases, lugs need some finishing work to become adequate for installation.

  • Weight

The lugs add weight to the frame. (It’s not a lot but may matter to people who want to have the lightest possible bicycle.)

  • More Labor Intensive Painting

The paint job of a lugged bike is very important. If not done well, the elegance of the lugs will be lost. Consequently, the painter has to invest more effort into planning and executing.

In different, brazed frames are easier to paint because the joints are smooth and without lugs and edges to worry about.

FAQ: Do fillet brazed and lugged frames offer a different “ride feel”?

The short answer is no.

The method used to connect the tubes of a bike frame has a small impact on how the frame feels. The main goal of the joining process is to create a strong frame rather than a frame that feels a certain way.

The ride feel a frame is dependent primarily on the following properties:

  1. Frame material
  2. Tube thickness
  3. Geometry
  4. Design
  5. Execution.

Truth be told, most people can’t feel the difference between brazed, lugged, or welded frames.

Summary: What You Need to Know

  1. Brazed and lugged frames rely on a metal-joining process known as brazing.

2. Brazed frames are connected by melting a brass rod over the joints.

3. Lugged frames are connected by putting the tubes into a lug/sleeve and then spreading melted silver between the lug and the tube.

4. If all parameters are equal, a lugged joint is stronger thanks to the extra support coming from the lug.

The main pros of brazed frames are:

  • The builder isn’t limited by the available lugs and can produce frames with highly customized geometry while also using tubes of different sizes.
  • The joints are incredibly smooth.

The main cons of brazed frames are:

  • The frame may appear generic to the untrained eye.
  • The joints require extra finishing work.

The main pros of lugged frames are:

  • Unique appearance
  • Strength
  • Shorter finishing process

The main cons of lugged frames are:

  • Limitations imposed by the lugs. (The builder needs new lugs to produce a frame with a different geometry).
  • The painting process takes more time and effort.

In the vast majority of cases, the choice boils down to personal preference.

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