Disclosure: currently don't own a Brompton but am considering, and researching, them.
Veteran cyclist here with wide experience (MTB, road, touring, commuting, messenger) and many bikes over the years: Al, Ti and Steel (lugged and Tig welded).
Researching the pros and cons between various folders, Brompton stands out as an outlier: it continues to hand braze their CroMo frames. Nowadays this is a process largely relegated to artisanal/custom builders.
Various grades of CroMo, the most common 4130 I believe, engineered for TIG welding, have been on the market for decades now obviating the historical reason for fillet and lugged brazing on bicycle frames. The only theoretical advantage I can see for insisting on brazing is that damaged individual frame tubes can be replaced by an experienced brazer. Practically, the cost of such a repair is prohibitive.
Modern Asian bike manufacturers couple contemporary steel/aluminium alloys with robotic TIG/MIG welders to realize some significant efficiencies and labour savings.
It's a mystery to me why Brompton hasn't adopted modern frame manufacturing methods.
by lingueenee
4 Comments
A British manufacturer competing with one based in a lower cost of production country trying to compete by matching their processes will loose every time. That they are hand built in London is part of the brand.
It’s England and artisanal is a thing. They also employ a lot of local labour and offer welding [*apprenticeships*](https://search.brave.com/search?q=apprenticeships&spellcheck=0&source=alteredQuery) which is good for the local community. They are a unique product not facing hard times so there is no need to get all fancy with your welding robots.
Brazing uses a filler metal (brass in our case), which is not the same as the base material and has a lower melting point. The component parts are not melted as with welding. This subjects the frame parts to less heat and reduces distortion, creating less stress and no altering of the physical properties of the materials. Brazing also uses less energy as the amount of heat needed is lower than welding. All this allows us to use thinner steel tubing and components, which would not be possible if we welded the steel parts because the thin material would be prone to extreme distortion or even burning.
This means that each Brompton can be made as light and durable as possible; without needing to add material to the ends of the tubes we end up with lower weight steel frame parts, meaning strong, light bikes.
Source:
https://us.brompton.com/support/knowledge/why-are-brompton-frame-parts-brazed
Brompton has an article on this:
https://www.brompton.com/support/knowledge/why-are-brompton-frame-parts-brazed