Agree, but if the point is to get the excellent tensile, yield, and fatigue strength of 4130 (or whichever is used) post-weld heat treat gets that for you. Is brazing used because it's stronger than a non heat-treated weld? I'd suspect that welding would be stronger. The joints are fillet-brazed, correct?

Brompton frames are made with a mixture of fillet brazed joints and brazed lugs. In terms of strength, fillet brazing and TIG both produce a joint that is stronger than the parent tubes, when done correctly. There are arguments made that brazing uses less heat and thus causes less damage to the tubes. TIG uses more heat, but that heat is concentrated in a smaller area and generally results in a smaller heat affected zone. I'm not a metallurgist, so I don't know exactly what happens to the metal. I have built frames using both methods and none have failed yet, even the early ones where I know some of the joints were overheated.(those all belong to me)

Kinetics in Scotland does this conversion but builds new rear triangles.

https://www.kinetics-online.co.uk/fo...n-alfine-kits/

He does other conversions as well.

There was also that Russian guy doing titanium rear triangles (sorry don't have the link) and he may have been doing modified versions but I can't be sure of this.

Yes, thank you. Kinetics sells a triangle + Alfine 8 wheel for £725/€800/$980, quite a bit more than brazing new dropouts + building my own wheel.

https://www.kinetics-online.co.uk/fo...n-alfine-kits/

Vostok seems to have folded (so to speak).

Eerder Metaal in the Netherlands is another venue, but most likely as expensive as Kinetics.

dsaul, thanks - very interesting. Appreciate your sharing directly relevant experience.
I do recall back in the day hearing that silver-soldered lugs were mega strong.