Lugged steel frame construction

Lugged steel frame construction is a method of building bicycle frames using steel tubing mated with socket-like sleeves, called lugs. For most of the bicycle's history, steel has been the primary material for bicycle frames, with lugged construction the primary assembling method. Steel continues in use by builders of high-quality steel frames, though its dominance as a frame material has waned since the mid-1990s displaced largely by aluminum and carbon fiber; lugging has been displaced by TIG welding.

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 positioning structure called a jig to hold the tubes in place and maintain their precise alignment and frame geometry. Four lugs which may be used to construct a typical diamond frame include:

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.

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