Lugged Steel Frame Construction - Method of Construction

Method of Construction

Lugged steel construction uses standard cylindrical steel tubes which are connected with lugs, external fittings made of pieces of steel (sometimes stainless steel) which fit over the ends of the tubing. Before assembly, the builder cuts the tubes to the desired length and precisely mitres their ends, providing a tight fit. The end of the tubes are inserted into the lugs and subsequently brazed with a silver or brass filler metal. The lug greatly increases the strength of the joint by distributing the molten filler metal over a larger surface area via capillary action. When brazing a bicycle frame, builders may use a small scaffolding called a jig to hold the tubes in place and maintain their precise alignment. Four lugs which may be used to construct a typical diamond frame include:

• seat lug or seat cluster joins the top tube and seat tube; the ends of the seat stays are usually brazed directly to the sides or back of the seat lug, although some newer designs of seatlugs also have sockets for the seat stays. Also has an opening for the seatpost
• bottom bracket shell joins the chain stays, seat tube, and downtube, and includes a threaded cylindrical socket for the bottom bracket
• upper head lug joins the head tube and top tube
• lower head lug joins the head tube and down tube

The two rear dropouts (which join the seat stays to the chain stays, and hold the axle of the rear wheel) may have integral lugs in some designs. The fork dropout or ends may be joined to the fork blades using a similar method as well. The fork crown, which joins the steering tube to the fork blades, may have either an external or internal socket design.

In most lugged steel frames, the lugs have a simple pointed shape. The lug's curves maximize the strength of the joint, while minimizing the possibility of stress risers, which would otherwise make the frame prone to cracking at the end of the lug.

At increased expense, frames may have lugs which are cut and filed by hand into fancy shapes, both for slight weight savings and as a display of craftsmanship. In addition many frame builders file the lugs to thin them, in order to reduce stress concentrations. These ornate or fancy lugs may be painted to accentuate their appearance.

Lugged steel frame can be repaired more easily than MIG or TIG welded steel frames; a broken tube can be removed by the application of heat to un-braze (known as sweating) its joints, enabling tube replacement.

Steel frames generally are easier to repair than aluminium or exotic materials, and for this reason steel frames are preferred by many bicycle tourists, who often ride long distances in remote areas. Lugs also reinforce the joints, often resulting in a stronger frame, this gives lugged frames in particular an advantage for touring cyclists.