Originally Posted by
Brompton_X
The Brompton OEM main frame is really not that heavy. If you want, you can still build a 2-speed Brompton below 8kg.
The biggest problem with those 3rd party main frame is you might experience some compatibility issue when assembling your bike. Some of them are easy to be fixed but some of them are not. If you really want to buy one, just remember to assemble your bike as soon as you receive it. If you find any problem, just return it. Buy from ebay, not Alibaba so if there is any problem, eBay can still protect you and help you to get your money back.
Steel main frame + post set is roughly 1kg heaver than full titanium set, and two sets cost almost the same. Titanium is lighter, easier to maintain and personally looks better.
And I know >20 people who has built full titanium Bromptons and they seem to be mostly fine, if built by skillful mechanic with proper tools.
My frame has very slightly oval headtube and the BB shell threads were bit too tight (couldn't hand tighten the bottom bracket). Using tools solved both issues just fine.
A brand new UK frame needs BB tapping and headtube / seatpost collar / rear hinge reaming anyway.