There was a time when all road bike frames — indeed, all bicycle frames — were lugged.
This method of construction, which can be traced back to the earliest steel frames, is an easy one to grasp: just match each purpose-built socket to the tubes, and the whole frame can be assembled like a child’s construction kit.
Brazing is used to bond each tube in its socket, and provided there is an accurate fit between the two, it produces a very sturdy and durable joint.
From the outset, it was a sound strategy, so it persisted for many decades until the industry discovered TIG-welding in the 1970s. TIG-welding was not only quicker than brazing lugs, it gave framebuilders more freedom to vary frame angles.
Nevertheless, some framebuilders persisted in using lugs throughout the ‘80s and ‘90s. This was not a matter of stubborn traditionalism, though. “Technically speaking, a lug provides the gentlest way to join one thin-wall tube to another thin-walled tube,” explained Darrell McCulloch of Llewellyn Custom Bicycles, who has been building lugged frames for the best part of four decades. “It distributes the stress across from one to the other. You can use thinner walls, and smaller diameters, because you have a lug to distribute load from one tube to the other.
“The other thing that’s very nice about lugs is that they allow the builder to impart some of their style and character and put their workmanship on display through elaborations and embellishments to the lugs. It adds a distinct flavour to the frame that’s important for product differentiation.
A TIG-welded frame made locally looks like a TIG-welded frame made in America, or Germany, or China. Yes, there can be variations in the quality of the welding, but there’s a lack of flavour there.
“There’s a lot more labour involved with lugs, though. More time is required for the fit-up, and sometimes there are limitations on tubing sizes. I’ve worked ways around that with developing my own lugs for the bigger tubes, sloping top tubes, and so on, but it still takes longer to produce a frame.”
There is more to the function of a lug than simply anchoring one tube to another. As mentioned above, a lug also helps spread the forces that are applied to the frame over a larger area so as to improve the durability of the joint. “When you have a sharp transition of stress, like a joint where two tubes meet, the way you reduce that stress is you use more material, or, bigger diameters,” McCulloch explained. “A lug is like a sleeve that goes over the joint, and that spreads the load over a larger area and away from the actual sharp transition of the two tubes meeting.
“When they first brought out TIG-welded frames in the ’80s, they were failing left, right and centre. They had tubes tearing out, bottom brackets tearing away at the seat tube, because they suddenly went from lugged construction to using the same tubing for TIG-welded construction. The loading of the stress of the joint was lousy, and it failed. So they increased the tube diameters, which made for a longer weld-line, or stress-joint line, to reduce stress on the joint.”
As beneficial as a lug can be, it can also create problems. “You can have a bad lug design with points in the wrong spot that actually creates what’s called a stress-riser, which is a concentration of stress and loading in a very small area,” McCulloch said. “Hence why you see on the underside of the down tube at the head tube, that point is always rounded. It’s a very stressed area and framebuilders learnt long ago to get rid of the point or you will fracture the down tube.
The other major stress that McCulloch worries about is heat and distortion. “The other nice thing about lugs is that they allow low temperature brazing. Brazing can be done with a brass-based filler to bond the tubes to the lug, however most craftsmen builders use a silver-based filler. It’s more expensive because it has a silver content of 55% or more, but it requires less heat than a brass filler.
“The idea is to melt the filler so it can run between the close gap between the tube and the lug by capillary action. The skill of the builder is using the torch to heat the joint to allow that flow of silver to go from one end of the lug to the other and fill the whole joint. You get nowhere near the temperatures used for welding, which actually melts the steel, and that can degrade its grain structure. Modern steel is much better at handling welding, granted, but silver-brazing is still the most gentle way because you avoid those really high temperatures.”
The evolution of lug shapes
The earliest frame lugs were derived from pipefittings, and so, for many years, the sockets were simply squared-off and lacked any kind of decoration. In time, small flourishes started to appear as framebuilders endeavored to distinguish themselves and prove their skill.
“Lug cutting is an old process that the English excelled at,” McCulloch explained. “If you go back to the Second World War, everyone was using the same frame parts and tubing, so they started elaborating the lugs to show their workmanship and impress their customers. A lot of builders had their own little style and that’s how they differentiated themselves from each other.”
While some framebuilders, like Hetchins, created highly ornate lugs, most were content with small flourishes, such as letters and emblems. The stamped and sand cast lugs that predominated at the time required hours of preparation before they could be used to build a frame, so there was always a limit on the amount of time, and motivation, that was available for decorating lugs.
There is a certain amount of romance surrounding lugs, due in part, perhaps, to all of the history and tradition associated with them. Much of that disappears in the workshop because it’s unglamorous work that leaves the hands filthy and raw.
At the very least, every lug must be carefully cleaned, inside and out, before it can be brazed, and the tubes must fit each socket properly. “You make sure the lug is clean, you make sure that the dust is off, and you make sure the shorelines are right,” McCulloch said. “You can’t proceed if the tube is a tight fit — that’s going to give you a problem when brazing because the flow won’t happen properly. There are points where you don’t make compromises, but you know, perfection is impossible. You have tolerances that you work to, and that’s as good as your tooling, your materials, and what you’re prepared to invest in.”
McCulloch relies on a mixture of tools for this work, some powered, and some unpowered. “You have got to hold the lug first, so you have a thing called the lug vice which is like an expanding-type tube. You’ve got to have a vice with various diameters to hold different lugs, then you can hold that device in some wooden blocks in a vice so you can then shift the lug around to work on it at different angles.
Every lug, polished or not, and decorated or not, ends up in the same place, namely a framebuilders jig for brazing. This is where the lugs are manipulated to suit the tubes and the final frame angles. However, before the lugs can be fitted, the tubes must be mitered so that they sit cleanly against one another.
“It’s important the frame goes together in the fixture with the tubes all being held at the right places with no stress in the lugs. You’re not actually forcing the tube, and the jig isn’t constraining or pushing the tube into place. It should all just fall into place. Nestle. There can’t be a tight fit with the lug, because the braze won’t flow. Capillary action has to take it through there.
Non Traditional Kinds of Lugging:
Bridgestone / Kabuki
The Kabuki line used some unusual construction techniques, specifically, a system of placing the frame tubes into a special mold and forming cast aluminum “lugs” in place around the ends of the tubes. The most notable of this line was the “Submariner” which used un-painted stainless steel tubing, and was marketed in seacoast areas for its rust-resistance. Because the cast aluminum lugs were not flexible like steel lugs, these bikes didn’t use a conventional seat-post binder. Instead, they used a seat post with an expander wedge like that of a handlebar stem…you had to remove the saddle from the seatpost to adjust the height, then re-install the saddle. These lugs are noticeable for their very large diameter relative to the tubing.
Alan / Vitus / Raleigh Technium / Nishiki frames used an epoxy to bond their frames together instead of brazing.
Alan used a ‘glued and screwed’ process to both mechanically and chemically mate their tubes to lugs.
Vitus used only epoxy
The Technium process was originally created to allow joining of dissimilar materials (Aluminum Tubes with Cro-Mo lugs / Aluminum Lugs with Reynolds 531 or 753 tubes / Aluminum Lugs with Titanium Tubes / Aluminum Lugs with Carbon Fiber Tubes) and allow designers more flexibility . However, it also economized the manufacturing process, eliminating, the high labor rates of brazers. As steel tubing gets thinner, the temperature has to be more closely controlled. That requires a more skilled and expensive brazer. With 753, you had to certified by Reynolds. Technium allowed frames to be assembled by relatively unskilled labor. The aluminum lugs further reduced costs by eliminating the need to miter tubes. Using Technium on high end steel was no brainer for Raleigh. The cost savings were sufficient that Raleigh could both undercut their competitors and increase their profit margins.
A peril inherent in riding these bonded frames is that, even when new, it was not unknown for the tubes to separate from the lugs.
Currently, large volume bicycle manufacturers utilize TIG welding to create their frames and only Custom Bicycle shops create lugged frames. One more reason to appreciate Vintage Bicycles.