Been following the news on the recent failure of a headtube on a Brompton G-Line, and that weld strikes me as looking a lot like a TIG weld, the way those beads are stacked on top of each other. That said, I’m also aware that Brompton hand brazes all of their frames.
My question for those familiar with metal fabrication, is, “is that really a brazed joint, or did Brompton opt for TIG weld due to the heavier loads that the bike might have been expected to deal with?”
I don’t recall fillet brazed joints looking like that, granted those would likely be filed down to a smoother, cleaner joint?
Pic of the headtube failure for reference, where the joint can be see on the right of the crack line.
Don’t they do a Ti frame as well? Not fillet brazing a Ti frameset. Possibly a Ti? Fillet brazing is usually ground and filed down to create that smooth join that you usually see. They could just be skipping that step, it’s mostly aesthetic.
Yup, for the bikes running the smaller, 16-inch wheels. The G-lines run the larger 20” wheels, specifically, the 451mm variant (as opposed to the 406mm) ones. Yeah, you’re likely right, that that’s fillet brazing right there, just not smoothened. Strikes me as a little odd, given that you’re one step away from a nicer looking joint, especially given the price of these bikes.
All common brazing alloys are based on copper. The highest silver content of any common brazing alloy is 56% and that alloy won’t form a fillet that large. There are filleting alloys with high silver content but none above 50% AFAIK: highest I can think of currently available is 38%.
In any case even 56% is still distinctly golden, especially when compared directly to steel. The fillets in the pic below were done with the (no longer available) CycleDesigns Fillet Pro, a 45% alloy, the bridge was done with 56% (probably Harris). Tubes and shell are stainless (17-4 PH) .
Reynolds 931. Don’t bother enquiring: they made it OEM only some years ago. I found Reynolds an utter PITA to deal with, my advice is to stick with Columbus.
Thanks for the reply. Not a builder but a mechanical engineer. I have used 17-4 for many designs/applications but didn’t realise it had been used for bicycle tubing.
Shame to hear that Reynolds are a PITA; my first custom steel bike was Reynolds 531SL (1986 and I only weighed about 60kg). I couldn’t quite afford the 753 option from my local builder, 531SL was a good alternative.
Thanks as always, @Lyrebird_Cycles_Mark_Kelly! Appreciate the insights as well - though I’m nowhere familiar with the nuances of welding and frame fabrication.
I’m curious though - why would fabricators choose to fillet weld a joint, without subsequently sanding down that joint to form a smooth finish? If it was an aesthetic choice, wouldn’t TIG welding be a more straightforward approach instead? I’m aware that brazing places significantly less stress on the tubes, and consequently, the joint, but ignoring the sanding step seems like a missed opportunity for a more refined looking frame. Would it be a fabrication choice driven by the economics of the exercise?
Separately, was that bike fabricated by you? Was always under the impression that you only worked on timber-framed bikes.
Thanks!
*NB: Renamed the thread to better reflect the current discussion.
To the first question: Everyone I can think of files down their fillets: it’s pretty easy to do and there’s little danger of undercutting the tube since the fillet alloy is much softer than steel and the file feels and sounds different on the two materials.
I thought that looked like a weld from the get go but also thought it was worth suggesting methods to make that definitive.
To the second: that’s an old pic. This was the first bike I built for my wife and it was an experiment, substituting renewable fibres for carbon to make a frame with the lowest footprint I could. This meant all the tubes had to be a bit larger diameter to maintain stiffness but I couldn’t make this work for the chainstays so I made them in SS.
That’s a dreadful run on sentence but I hope you get what I mean.
Just for balance, I am nothing more than a hobbyist but get great service from.Reynolds directly in the UK. I might spend £1,000/year with them and always helpful.
