Lugged steel frame construction

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[3][unreliable source?]

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.

[6] Lugs used today on higher performance bicycles are investment cast (aka lost wax process).

[citation needed] Lugs are usually stamped with a letter or symbol code identifying the maker, and possibly the angle.

However, recent advances in metallurgy have created steel tubing that is not adversely affected (or may even be improved) by high temperature welding.

This has allowed both TIG and MIG welding to displace lugged steel construction,[8][unreliable source?]

Better frames are nearly always made of some variant of chromoly steel alloy and include forged or investment-cast dropouts.

The utility bicycles manufactured in large quantities in China and India, (such as the Flying Pigeon) and exported to countries in the region, are made from lugged steel.

Prugnat type 62 D lugs.
Clockwise from top left: seat lug, upper head lug, lower head lug.
This seat lug joins the seat tube, top tube, and seat stays of a steel touring bicycle frame. It also has an opening in which to insert the seat post, and a clamp to hold the seat post securely in place.
Simple, pointed head lugs on this bike boom road bicycle
Elaborate hand-cut and filed head lugs which have been outlined in a contrasting colour (luglining) on a Hetchins frame
A stainless steel investment cast seat lug, with hand filed cutout, which has been polished to a mirror finish. These lugs necessitate construction with silver rather than brass filler.