commander_taco

How does the lugs concept work? How does a a piece of silver (or whatever) hold the two steel tubes together? Welds are easy to rationalize since welding essentially melts and fuses the tubes. In case of lugs the steel tubes do not melt, and possibly not as strong as welds when pulling forces are applied. Is this correct?

Retro Grouch 

Well, it's kind of like soldering copper water pipes together. After carefully preping the tubes and pinning them together with the lugs, you heat up the whole assembly with a torch. When the temperature is right, you use molten silver or (more commonly) brass solder to fuse the joint together. I suspect the reason why you don't see more lugged frames being built today is because it's more labor intensive than TIG welding.

I don't know which is actually stronger and I'd argue that it doesn't matter anyway. Both joining methods have been proven to be strong enough.

Scooby Snax

I've heard that lugged are stronger frames, the tig process weakens the material around the joint.
A lugged frame is easier to repair, you can remove the damaged tube and slip a new one in and braze it all back up.

roadbuzz

Another reason lugs have fallen out of favor is that they add weight. Not a lot, but measurable, thus they have been vanquished from mass-market bikes. A one point some welded bikes had internal lugs (lugs on the inside of the tubes). I don't know if this is common practice or if it's done at all on current bikes.

531Aussie

I've been boring everybody with this web page for the last couple of months; it's a great read:

https://www.henryjames.com/faq.html

Henry James talks a lot about lugs, but you've gotta keep in mind he's very pro steel and very, very and pro lugs

froze

The main reason lugs fell out of favor WAS NOT WEIGHT! It was because of the expense involved, they had to be hand assembled. Plus they now use more aluminum because it's way cheaper than steel, titanium or carbon fiber. These savings were important because in the 80's your LBS and bike manufactures were struggling to make a profit, thanks to the above applications they are once again profitable. With a welded bike a robot with the frame in a jig attacks the soon to be bike zapping it together in about 30 seconds and maybe less. Most custom frame builders agree on two things: 1) lug frames are stronger then a welded frame; 2) lug frames are a bit heavier by about a 1/4 of a pound. Even the older AL frames that were bonded used a lug, and that lug system is stronger then the welds they do on AL frames today; in fact airplane frames are bonded together.

Torelli found a happy medium by lugging the bottom bracket and rear stay but welding the rest. Torelli found that lugs were stronger but also realized they were heavier so they applied the lug where the greatest amount of stress would be. Quote from Torelli: "Lugs. Torelli believes in using lost-wax (investment-cast) parts at all the bicycle's joints. This means that all Torelli frames, with the exception of the Corsa Strada, have cast bottom bracket shells, lugs and fork crowns. The Corsa Strada still uses cast fork crown and seat lugs, but price considerations require that we use bulge-formed head lugs and bottom bracket shells. Investment-cast parts are much more expensive, and it's one of the ways in which Torelli has sought to make a superior bike in ways that do not necessarily show. The cast parts have three basic advantages:

1. They are made with great precision. The fit between the tube and the lugs is exactly right to get the strongest possible bond between the brass, the tube and the lugs.

2. They are stronger. The bottom bracket shell can be made lighter and still withstand the enormous forces of sprinting or climbing. Complex shapes can be cast in such a way as to exactly reinforce the frame where strength is needed.

3. They look much better. Their clean, precise look compliments a framebuilder's finish work.
From: https://www.torelli.com/home.html?htt...torelli.html&1

see: https://www.henryjames.com/faq.html
https://www.rivbike.com/html/101_lugs.html
https://www.richardsachs.com/articles/rsachscrown.html
https://www.richardsachs.com/articles/rsachslugs.html
https://www.richardsachs.com/articles/rsachslug1.html

commander_taco

I still have one unaswered question: How come lugs are stronger? How can a bond between steel and bronze be stronger than the bond between steel and steel? It just does not work that way owing to the metallurgical differences (also note that steel and bronze do not react). For the record, solder is weaker than a weld. Lugs may be strong enough for bicycle application (altough anything is debatable in the absence of hard data), but I like to hear an explanation as to *why* lugs are stronger than welds. It is easy to make an assertion without proof...I guess you see my point.
thanx.

stevetone

I have no proof. I think that you are probably right -- welds probably are stronger (as long as the structural integrity of the tubes being welded together are not compromised). However, the question is "Is it (a properly brazed and lugged joint) strong enough for the task being asked of it?" I think history has proved that to be a resounding YES!

Retro Grouch 

Heating steel enough to weld it weakens the metal. A welded joint will normally fail right at the edge of the weld where the metal has been heated by the welding process yet there is no reinforcing fillet.

The whole point of brazed lugged joints is that it's necessary to heat the metal less. Silver is better than brass because it melts at a lower temperature so less heating of the parent tubeing is required.

sorebutt

When you use lugs the 2 surfaces of the steel are bonded with brass or silver. it the case of a welded frame the two points of the steel become one via a bead of steel.

the major difference between the two is the size of the area where the 2 pieces of the joint.

In the case of a lug, the tube and the lug have a large area where they overlap, so stresses on the joint are spread through this area.

In the case of a weld, the the width of the joint, and the small overlap of the 2 tubes (basically 2 thin edges) make it more susceptible to stress fractures. Also, the act of welding itself with its high heat melts part of the tubes and changes the molecular structure of the steel (some time can be fixed with heat treatment).


so strength-per square inch of a weld is probably much stronger then a brazed lug, but in a bike application the lugs are stringer..

However, when you use lugs, especially for a custom bike you will be limited by the geometry of the lugs. Lugs are made in different angles and it gives you enough flexibility to build almost any bike. But if you are anal about the geometry you can use welds to achieve any angle you want just by cutting the tubes accordingly.

that said, I have ordered a custom bike made with lug. It is more expensive, maybe a few grams heavier, and maybe half a second off the "perfect" geometry, but IT LOOKS SO PRETTY..

fujibike

Lugs are beautiful

commander_taco

Is this still true in terms of modern steel? I remember reading somewhere that Reynolds 85x (and the like) tubing become stronger after a weld, since they do not perform the final cycle of heat treatment essentially leaving the welding process do the final heating cycle. If the steel is not heat treated at all it is possible that the resulting grain structure is not as uniform as the one that was formed due to forging or drawing or whatever process is used to roll the tube. However the tubes are usually butted at the welding joing percisely for this purpose, to add more strength. Is it not?

The bending stress is maximum at the end point of a cantilever. If a tube fails due to cyclic stress, then chances are it fails at the point where it is attached to other tube. A welded joint also has more metal (due to welding bead and butting) just like a lug. Unless the welder left stress raisers (sharp notches) it would be intuitive to reason that a lug will separate earlier in this application since the lug joint is not as strong as a welded one inch per inch. If anything a welded joint cannot be weaker!

Relevent in the past when a lot of technology was not there, but I doubt if it is still true due to welding specific heat treatment and butting.

commander_taco

Without raw data it is hard to argue if the increase in metal overlap of a lug offsets the strenght advantage of a weld coupled with increased metal at the welded joint due to butting, for bending type cyclic load. Only experiments can tell the truth.

Secondly, it stands to reason that when *pulling* forces (as opposed to bending) are applied a weld could very well be stronger than a lug. In case of pulling the only thing holding a lug is the joint between steel and bronze which is getting sheared. In case of a weld the forces are tensile (not shearing) and hence the joint is much stronger.

commander_taco

Off topic.

commander_taco

That is not the question I am asking! (not to be rude or anyting )
To address the topic you touched upon, is it enough to depend on the conventional wisdom? What is the definition of 'is it strong enough?'. I have a Raleigh that is atleast 30 years old (and still alright) that is passed on from my father. I bet there are plenty of instances where lugged bicycles have survived several decades. If I were to speak from experience, my openions are not much different from yours. However this is irrelevent to the question of whether lugs are stronger than welds for the bicycle application. I was interested in the comparison of two joining processes and nothing more.

goatmeal

Speaking of lugs,

https://www.velostuf.com/chris_kvale_cycles.htm

Hand filed, really how can you deny a beauty and artisan qualities of such attention to detail.

Retro Grouch 

What IS your question?

Initially it sounded to me like you didn't understand the brazeing process. Now it sounds to me like you're asking will a brazed joint be as strong as a welded joint.

If you want to compare only the joining method, then you'd have to hold the tubeing material and everything else constant. I know for sure that some Reynolds tubesets are qualified only for silver brazing only so clearly those would be stronger than if they were tig welded.

Fortunately, that's not how the world works. People who build quality lugged frames choose materials that work best using the low temperature brazeing process. People who build high quality tig welded frames choose their materials with tig welding in mind. While all of this is going on, the metallurgy guys are constantly looking for ways to minimize the disadvantages of using lower cost manufacturing methods (like tig welding) so the answer to your question is always going to be a moving target.

Have you considered the possibility that this whole argument might even be passee'? Composites look to me to be the cutting edge in material technology.

Revtor

what about fillet brazing?

~Steve
(ducking : )

shecky

True. But not many bikes made with it. Most welded steel frames I've come across are chromoly. Or plain ole' "high tensile" steel. I don't know how many welded steel frames are heat treated. Anecdotally, on generally abused bikes, I've noticed frame failures tend to develop around welds. I've not known many lugged frames to get similar abuse.

As already said, it really doesn't matter a whole lot. Lugs, welds, fillet brazing have all proven to be quite durable enough for bike frames.

Brazed joints are quite resistant to "pulling out". Which is the comparison to welding. There may or may not be more surface area bonded between lugged joints, and brazing material in such tight joints have very high resistance to shear stresses.

Seems to me the bending strength of lugged joints may be mostly a result of the strength of the actual lugs, not necessarily the joining method. The area of tube/lug overlap are probably quite strong. However, tubes can only extend so far and it seems ultimately the weak point would be the lug (if it can be considered weak at all).

I have wondered about the strength of cast lugs. I wonder if lugs are (were) sometimes made differently. Perhaps formed from sheet or tubing.

Retro Grouch 

Fillet brazing is a joining method that replaces the steel lug with a larger "fillet" of brass. The joints appear to flow seamlessly from tube to tube. I have a nivacrom tubeing tandem that is fillet brazed. It's absolutely beautiful, but it turns out to be the heaviest framset that Santana makes, in spite of the paper thin nivacrom tubes, due to the weight of the brass fillets.

shahidbashir92

In the case of bike frame lugs, the brazing is under sheer forces only - as the tubes are inserted into the lug. The joint will be as strong as the metal of which the lug is made. If you used the same brazing material in place of a weld, it would fail - because its now subject to variety of forces (pulling apart and sheer).

squirtdad

dude this is a 17 year old zombie thread. please don't wake the zombies

Andrew R Stewart 

[QUOTE=shahidbashir92;22143146]In the case of bike frame lugs, the brazing is under sheer forces only - as the tubes are inserted into the lug. The joint will be as strong as the metal of which the lug is made. If you used the same brazing material in place of a weld, it would fail - because its now subject to variety of forces (pulling apart and sheer).[/QUO

Not quite right. This statement speaks to tensile strength of fillers with little understanding of what a well joined joint consists of. A well made joint has the filler join the two tube's contact surface, the miter face. Whether it's a lug, a fillet or a weld if the two tubes don't touch (or have the filler preferred gap) the joint is compromised. Now in addition to proper tube miter set up is how much surface area of filler is needed to be "strong enough" (and any greater strength is excess, although "enough" strength is a debatable aspect). Silver (as in the common 56% filler usually referenced) is real strong with the right gap but looses that level quickly as the gap increases (pretty much scant fractions of an inch from the crotch of a tube to tube contact). So greater surface at a close gap is needed to attain enough strength. Brass/bronze has a lower ultimate tensile strength but retains that lever far further from the crotch of a joint. (The rule I learned was the root thickness of a fillet should be 3 or 4 times the tube wall thickness). Welds have the highest tensile strength but have greater base material strength changes happen right at the crotch (and where the stresses are focused).For the quoted statement to apply the lugged joint would have to have no tube to tube contact or pretty poor penetration of the filler, never getting deep into the joint enough to flow between the tube contact area.

Any type of joining can be designed to be stronger or weaker. Gaps, joint prep, filler amount, filler surface area contacting the tubes, penetration can all be jiggled to suit the fillers and the skills of the builders. Tube surface prep, purging, second heating cycles (like a fillet pass on welds) further affect stuff.

I tend to dismiss this type of discussion because well done joints of any method are fine and last a long time AND the discussion is instead one of preference and aesthetics, rarely one of actual joint engineering.

Not yet mentioned is how joints fail (well one mention of welds cracking was made). In the early/mid 1980s Bicycling mag had an article about this stuff. They had an experienced builder fab samples of three joining methods. Silvered lugs, brass fillets and steel welding. They had a lab test various aspects of the resulting structure (remember that? We are talking about riding bikes and not make believe models). Hardness of the tube at different distances from the crotch (which was claimed to be a pretty good reference for after joined strength). They applied bending forces enough to cause each joint method to fail and noted the progression of the failure, where along the tube/proximity to the crotch the plastic deformation happened. Base material annealing and HAZ was discussed. The conclusions were that all methods fail but with different manors and all can be more then strong enough.

In real life of making frames (from steel) these joining methods chosen are much more about cost, builder's skills, marketing aesthetics then theories. Like has been said each type of joining can be done really poorly or well and each can be more then enough. Still a rider's ability to stress a bike can exceed the strength of a bike they actually want to buy and ride.

BTW we haven't brought up bonded steel frames like the Technium Raleighs or the die cast junctions of the Kabukis. Andy

shelbyfv

Promising first post.

Nessism

Welding or brazing, with or without lugs, doesn't define a quality frame. It's the worksmanship that does. Lugged, fillet brazed, TIG, all are good joining methods. It's the quality of execution that matters, not the method.