Up thru most of the 80s if you bought a bike book, you were warned that a butt welded frame was junk. You needed to buy a lugged frame brazed or silver soldered at low temperature. The snobs basically said cyclist ride a lugged framed bikes, and the unwashed rode junk.

Does anyone have a scientific engineering reason why butt welding doesnt weaken tubes now?

Because welding techniques have improved immensely, newer tubing alloys and shapes are better able to take the welding heat and brazed lugged assembly has gotten too expensive.

I may be wrong, but...

I seem to recall a study done in the mid/late 80s that showed, I think, improved strength from joints or material around joints (the heat affected zone, or HAZ) heated at the higher temperatures required for welding, but for less time, compared to the longer time spent at lower tempertures to ensure brazing material filled all the joints or built up a good fillet. I think Keith Bontrager was involved with, or conducted, this study.

The tubes used prior to the availability of air-hardening steels weakened significantly when exposed to the heat of welding. The only way to compensate was to make the tube thicker, therefore heavier. The new classes of steels actually get stronger with the heat, so the tubes can be quite thin. My welded Waterford R-33 has a downtube that is .7MM at the welds. Amazing. And the frame itself, made of S-3 steel weighs 2.9 lbs. Also, with the advent of laser tube mitering, tubes can be cut to fit precisely together, so that a very small weld bead is required.

Air hardening tubes are certainly a common thing in exotic bikes, but plain 4130 and most other common less expensive steel alloys are not air hardening, and welding does, in fact, weaken the tubes on these. And TIG welding has still been the preferred method of joining these for 20+ years.

Here are some comments made online by Keith Bontrager:

https://search.bikelist.org/getmsg.as....9911.0067.eml

Basically, the pertinent info presented here is that welding does weaken tubes, but since the heat is applied and removed quickly, the HAZ is small and easier to deal with. Brazing requires either heating a larger area (generally with lugs) or heating for a longer time (as with fillet brazing).

#1, TIG , Tungsten Inert Gas.. welding is not Butt welding .

Where the spark from the tip of the torch with the tungsten point in the center,
there a fillet of wire laid down around the miter. with the heat joining all 3.

A combination of all the reasons above, including mainly materials and techniques.

The issue with welding in the past is that, with the steels in use, a weld created a weakened area just beyond the joint, making the joint radically stronger than it's surrounding area and therefore prone to cracking there. The welds held up fine, and the tube would crack adjacent.

We still see that from time to time, but not all that often because the current materials and techniques manage it much better. It should be noted that during the same period that bike buyers were warned against welded bikes, motorcycles, airframes, auto roll bars, and all sorts of critical strictures were being welded very successfully.

There's also a bit of confusion in that many of the "welded" bikes of the period in question, weren't in fact welded, but had brazed butt joints without benefit of filets or lugs. These so-called frames were very common at the low end of the spectrum, and while usually OK, prone to failure.

Current mat

But the maximum temperature is much lower for brazing than it is with welding.

Max temp is only part of the story, and higher isn't always worse. Different alloys respond differently to heating, and much more importantly cooling. Often the rate of cooling is the most important consideration.

Welding techniques and joinery got better. So did tubing.

Did the same books from the 80s also pooh-pooh butt brazed joints, i.e. fillet brazed "butt" joints? Didn't think so...

Plus it might be that the books you are reading weren't based on the most awesome science and technical knowledge... They might have just been bringing their preconceived notions of aesthetics into play instead of sound technological know-how.

Tungsten Inert Gas welding is a welding process.

Butt welding describes a type of joint and can be welded by any of several different processes, including TIG.

Yes, at the weld bead itself but the distance the heat transfers is so much less that the adjacent, thinner tubing actually sees much less heat.

Expanding on FBinNY's posting above, so-called "fillet brazed" joints made without lugs were used on both ends of the quality spectrum. Some very high quality custom frames were made using that technique as well as low quality cheap ones.

I don't think that had anything to do with the actual welds, and everything to do with bike snobbery and retro-grouchery.

I think the only mass produced bikes with actual butt welds were the Electro-forged Chicago Schwinns. Not considered a top line bicycle, but I think history has proven the strength of the welds. Otherwise, the frames during that time (the 80s) were most likely TIG welded. Tig welding has a long history in aircraft structures. It is a proven joining technique.

You're mixing terms with different meanings.

Butt weld refers to the type of the joint, namely 2 members meeting end to end, or at an angle with no overlap or reinforcement save the weld itself.

TiG welding refers to the welding process itself, which uses an inert gas to protect the weld area from harmful exposure to the surrounding air.

Whether TiG welded or not, most modern welded frames are butt welded though it would probably to refer to the joints as "T" joints (though nobody I know in the bike world ever does).

Yes, but TIG welding involves filler rod (for structural use). The Schwinn electro-forged process used resistance welding with no filler. You can do TIG welding with no filler, but it is generally not used for structural welds. I have not heard of any TIG welded bikes with no filler rod used for the joints.

You are correct on the nomenclature, a butt weld is a geometry, but the electro-forged process is not the same as most other welding techniques. It is more akin to spot welding, in that it did not use filler material and it was a resistance weld.

As far as I can tell, the techniques used in mass produced metal bicycle frames were thus:

Fillet brazed
silver brazed with lugs
Tig welding
Schwinn's Electro-forged
Adhesive bonding with lugs

And I'm not sure of MIG welding, but I wouldn't be surprised if it is used in the mass production of consumer aluminum frames today.

its the big rooms with a thousand people in Asia doing skilled hand welding
for low cost per unit produced, of a million frames that make it OK..

We're on the same page, but to add to your list.

brass brazed with lugs, internal or external
brass brazed of mitered tubes with little or no built up filet save for the bit that occurs naturally due to surface tension.
furnace brazing with copper, often used for attaching steerers to crowns as a sub assembly.

Also note that while Americans and British were brazing with gas, the Italians were doing very nice work with induction heating which produced very clean joints.

I don't think lugged silver brazing was ever used on mass produced bikes, they are lugged brass brazed. Silver brazing is an expensive technique used mostly by custom builders dealing with particularly temperature sensitive tubing as uses lower temperatures than brass brazing.

1: Bicycles don't have butt welds,they have fillet welds.

2:CroMo,Tig and fillet welds,as a team, have been around since WWII,we understand them pretty well by now.

3:CroMo work hardens as you use it,so the HAZ zone actually improves in strenght as you use it.

4:The only difference between now and then is the internet,more crap get spread around.CroMo,Tig and butt welds made alot of things before bicycles.....like airplanes and race cars/bikes,buildings...ect.

5:Tig welding was a "black art" for many many years.Ask most machanic types about tig welding in 1960-1970, they would look at you like Nipper the RCA dog! Hell,most of them STILL think it's magic.

6: Really,the only difference between Tig and OA, is one has a flame and one has a lightening bolt to melt the metal,short of the sheilding,and that can be cured with flux.

7:Before 1940 they has CroMo and they used to OA it together to make airplanes,it's a wonder they didn't fall from the sky......They welded aluminum with OA also.....how did they do it without TIG.....I think all of the smart people died.....

Most (all?) of the connections on an electro-forged Schwinn actually have an internal sleeve and are not butt-welded.

Yea there was a spigot in the headtube, the main tubes went over.
those were punched out and butt welded themselves .. edge to edge.

it was the BSO of the era , that cranked up the power
and the end of the tube was hot enough to
stick in a spot welding like situation, no filler needed.

Until about 1980, all Trek's frames were silver brazed.

When I first saw the title I thought he was talking about welding a butt and I thought that would never be OK.

Uhmmmm......."prone to failure"?.......I don't know if I would make the same generalization about lugless brazed frames as Peugeot made thousands of lugless brazed frames frames in the 80's using their own internal brazing method to construct their lugless frames and those bike frames are known through the years to be tough and reliable and have excellent ride characteristics, despite being entry level frames/bikes, and are now quite popular with new C&V cyclists.

Chombi
.

I was under the impression that little in terms of welding technology has changed much, with exception of robotics being very consistent in quality and repeatability. That said, I was told by one maker that for their mass production steel frames, they were now getting cheaper tubing that was "super-butted" and pre-formed for their frames and this allowed them to butt weld simply because now the tubing at the joint was thicker. And the use of robots to do the welding made the work much faster, cheaper, and more precise than could be for average humans. So this reduced the HAZ issues that might migrate out of the super-butted region.

For aluminum, I think the extrusion technology has really been the key and we see curved frames and not straight tubes even with customized joint designs for frames because robots can TIG weld extremely well. I think that's what makes the biggest difference. In the old days of working in the machine shop building pressure vessels, I learned to TIG weld and stick weld and braze. And it wasn't easy to lay down a fine bead consistently, and every time I had to stop and itch something and took the inert gas off the bead and restart, it weakened the joint especially with aluminum. I think the real difference here is in robotics and automation of the process that has enable butt welds to achieve strength. They were always strong if done right, only humans were flawed and only a few could do it right consistently. Machines have changed that.