A lug is a socket that forms the junction between two or more frame tubes. Steel has historically been the material of choice for high-quality bicycle frames. Likewise lugged construction has always been the preferred means of assembling a steel frame. Traditional bicycle construction uses steel tubes and lugs, joined together by brazing so that the space between the tube and the lug fills up with molten brass.
Before assembly, our German frame builder cuts the tubes to the desired length and precisely mitres their ends, so that they fit closely together. The ends of the tubes are inserted into the lugs and are then brazed together with a brass metal filler. The lug greatly increases the strength of the joint by distributing the molten metal filler over a larger surface area via capillary action. When brazing a bicycle frame, our frame builders use small scaffolding called a jig to hold the tubes in place and maintain their precise alignment.
Four lugs are used to construct a typical Velorbis diamond frame:
1) Seat lug or seat cluster joins the top tube and seat tube.
2) Bottom shell bracket joins the chain stays, seat tube and down tube, and includes a threaded cylindrical socket for the bottom bracket.
3) Upper head lug joins the head tube and top tube.
4) Lower head lug joins the head tube and down tube.
Lugged steel frames can be repaired more easily than MIG or TIG welded steel frames. For instance, a broken tube can be removed by the application of heat to un-braze it, and then its replacement can be brazed in its place.
Steel frames are generally easier to repair than aluminium or exotic materials, and it is for this reason that steel frames are preferred by many cyclists who cycle regularly. Lugs also reinforce the joints which results in a stronger frame, thus giving lugged frames an advantage for touring cyclists.