I know this is an old thread but it is not "too old" and I like the idea of keeping the info in a few threads and not spread out too much.
What I am working on at the moment:
I wanted more gears and more low gears on my Brompton. Since it started its life as a 5 speed (two wire) I started off buying a newer 5 speed hub for a single sprocket thinking I`ll never do 5x2 at the back.
Then I realised I wanted to do that after all, fixed the rear frame problem and bought this piece to convert the hub to take two (or more) 9 spline cogs: "Sturmey Archer BWR Driver Assembly- ZHSA800" is written on the plastic bag it came in.
That thingy is now installed. I forgot to buy the cogs so I butchered a 8 speed casette I had (actually from my NWT who is also getting an upgrade) and so far I have installed a 12t and a 16t cog. Bike is not yet ridable but playing around with a gear calculator and looking at the front chain rings I have available (and also thinking about cog and chain wear) I started to think about cog sises at the back.
What I wanted to ask: Has anybody successfully used bigger cogs at the back with the stock Brompton der/tensioner? What combinations can it take? Searching for answers I found this old thread but I am not sure if there is a different chain tensioner involved in the 13-16-19 setup.
Anybody out there has experience with this?