Roberts Cycles, 89 Gloucester Road, Croydon, Surrey CR0 2DN, England
Tel: +44 (0)20 8684 3370
How are bicycle frames built?
There are many ways of constructing frames, but three are commonly used for steel tubes – welding, brazing and fillet brazing. In every case, the tubes have to be mitred (cut and sanded) to fit up against each other perfectly to ensure a strong joint.
Whether done by machine or by hand, skill and care are necessary to make the tubes fit together with virtually no gaps between them. Throughout the fitting and welding process, the frame builder will need to check and measure the length, angles and alignment of the tubes, and there are several methods of doing this, from almost freehand with set straight edges, to custom jigs, large and small.
What are lugs?
Lugs are metal sleeves that the frame tubes plug into at the joints. In the case of steel frames, they are made of steel and add strength, allowing the builder to use less heat in the process of building the frame. This means the tubes distort less and fit together more precisely, which also aids longevity.
We use only precisely cast lugs for superior performance and low weight. Lugs can be simple and workmanlike, or cut and filed into fancy patterns. Some designs are exclusive to a particular brand. We tend to try to strike the right balance between aesthetics and cost, but if you want fancy lugs and can afford them, we can do it.
What is TIG welding?
This is the modern method of choice for mass production in steel, aluminium and titanium frames. The tubes are joined together (welded) by fusing the melted ends in an atmosphere of inert gas (this stops oxygen contaminating and weakening the weld).
What is brazing and how is it different from welding?
Rather than heating the tube ends up so much that they melt, which allows a weld to be created, a lower temperature is used. At around 800°, a material known as braze (usually a type of brass – an alloy of copper and zinc) is introduced at the joint and this melts into the gaps between the finely mitred tubes and the lug. When it cools, the braze is what holds everything together.
A version of braze known as silver solder only requires heating to about 600-650°, which means the tubes are weakened less by exposure to heat. Silver solder is almost always used for small brazed-on fittings such as bottle cage bosses, cable stops and rack mounts, and may be used throughout.
Most builders use a welding torch with an oxyacetylene flame to heat the lug, tube and brazing rod, though some still use an open hearth for a less localised heat (harking back to past times and more traditional methods).
What is fillet brazing?
This is a construction method where the tubes are held together by layers of melted brass that have been built up over the joint between the tubes (as well as being drawn into the joint). Once this is finished, the joints are carefully filed to create a smooth, curvaceous joint.
PRIMER - Frame-building