Lugged steel frame construction

Lugged steel frame construction is a method of building bicycle frames using steel tubing and sockets called lugs. For most of the history of the bicycle, steel has been the material of choice for high-quality bicycle frames (though its dominance has waned in the last two decades). Likewise lugged construction has been the preferred means of assembling a steel frame, and retains this status among builders of high-quality steel frames.

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Method of construction

  Lugged steel construction uses standard cylindrical steel tubes which are connected with lugs. Lugs are external fittings made of pieces of steel (sometimes stainless steel) which fit over the ends of the tubing. Before assembly, the framebuilder cuts the tubes to the desired length and precisely mitres^[1] their ends so that they fit closely together. The end of the tubes are inserted into the lugs and are then brazed together 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^[2]. When brazing a bicycle frame, most framebuilders use a small scaffolding called a jig^[3] to hold the tubes in place and maintain their precise alignment.^[4] At least four lugs are used to construct a typical diamond frame:

• the 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
• the bottom bracket shell joins the chain stays, seat tube, and downtube, and includes a threaded cylindrical socket for the bottom bracket
• the upper head lug joins the head tube and top tube
• the 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, as seen to the left. The lug's gentle curves maximise the strength of the join, while minimising the possibility of stress risers, which would otherwise make the frame prone to cracking at the end of the lug.

Some more expensive lugged frames have lugs which are cut and filed by hand into fancy shapes, as seen to the right — both for slight weight savings and as a sign of craftsmanship. In addition many frame builders file the lugs to thin them, in order to reduce stress concentrations. These ornate or fancy lugs are often painted distinctively to accentuate their appearance. Particularly fine lug-work is highly prized by some bicycle collectors.

Unlike MIG or TIG welded steel frames, a lugged steel frame can be relatively easily repaired in remote areas due to its simple construction: a broken tube can simply be removed by the application of heat to un-braze it, and then its replacement can be brazed in place. For this reason, lugged steel frames are preferred by many bicycle tourists, who often ride long distances in remote areas. Facilities for brazing steel are widespread even in developing areas of the world, and so it is often easier to find someone who can repair a lugged steel frame than, for example, a TIG-welded aluminum frame.

Types of lug

Traditional pressed lugs are formed by pressing sheet steel over a mandrel, and then brazing or welding the seams. These lugs are of constant thickness, necessitating large amounts of file work in order to minimise stress risers.

Most lugs used today are investment cast, allowing much finer detail, tighter tolerances, and correspondingly less manipulation and file work.

A relatively recent trend has been toward the use of stainless steel for investment cast lugs, as shown in the picture on the right. These lugs do not require painting, but must be brazed using silver filler rod, which necessitates much better heat control and better fit of lug to tube.

One advantage of pressed lugs over investment cast lugs is that the angles of pressed lugs can be very slightly altered.

Lugs are usually stamped with a letter or symbol code identifying the maker, and possibly the angle. For example 'BCM' indicates Bocama of France.

History of lugged steel construction

From the late 19th century through the 1970s^[2], this method of frame construction was favored because the lower temperatures of brazing (silver brazing in particular) had less of a negative impact on the tubing strength than high temperature welding, which can seriously weaken many steel alloys. Brazing thus allowed relatively thin walled, lightweight tubes to be used without loss of strength. 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^[5], in large part because these methods lend themselves more easily to automation and reduce the cost by eliminating the lugs.

Conversely, lugged steel construction remains popular among builders of custom-fit bicycle frames, and among amateur framebuilders, since it is one of the simplest methods of constructing a bicycle frame by hand, in a small workshop.^[6]

Finding lugged steel frames today

Despite the fact that lugged steel frames are no longer mass-produced, affordable frames are still available. There is a thriving trade of used bicycles in North America, especially in large cities and college towns. Popular sources of used bicycles include:

• Craigslist classified ads
• eBay online auctions
• Thrift stores

Because of their durable construction, many lugged steel frames from the 1980s, 1970s, and earlier remain in good condition.

Japanese- and French-made frames from the bike boom are particularly common in North America. These range widely in quality: low-end frames are made of ordinary high-tensile steel using thick, heavy tubing and stamped steel dropouts. Better frames are nearly always made of some variant of chromoly steel alloy and include forged or investment-cast dropouts. Higher-end frames are often made of butted steel tubing, which is thicker at the ends and thinner in the middle, reducing weight but increasing the cost. One of the most famous and prized types of tubing used in classic steel bicycles is Reynolds 531.

References

1. ^ Mitering steel frame tubes
2. ^ ^{a b} Atlantis Frames and Bicycles from Harris Cyclery
3. ^ Henry James Universal Jig
4. ^ Metallurgy For Cyclists: Steel is Real
5. ^ How bicycles are made
6. ^ Rivendell Bicycles: Frame Materials 101