2 Speed to 3 speed external cogs upgrade in detail (MiniMODs X3 Unibody Cog Set or X2+1)
X3 Unibody Cog set (11-14-17t) on Joseph Kuosac (was 2 speed wheelset)
In addition to the narrower 11 speed chain upgrade. Standard three speed Brompton shifter (latest type) inline cable adjuster is needed and tensioner washer to stop plastic derailleur and three speed cassette interference. Also 0.5mm free hub spacer to get correct fit on JK wheel hub. vidoe Credit : Alex Warwick Cycles
This setup will work for stock Brompton 2 speed or 6 speed (3x2 BWR hub made by Sturmey Archer). But since every hub has a little bit manufacturing variaance on free hub body, so you might need to match with different snap ring circlip (we have 1.2mm, 1.5mm, 1.8mm and 2.2mm options) + free hub spacer washer.
What's the benifit?
Let's say your Brommie is with standard setup (50T chainring + 12/16t rear cogs). You found this gear range is not enough on either high gear or low gear end. By simply changing the cog set, you will essentially widen the gear range on both ends.
What if you just need a much lower gear?
Combine with 11t cog now, you can shrink the front chainring to 46T (from original 50t) and still keep the same high speed gear.
50T/12t = 69 --- 50T/16t = 51.7 stock setup will become
46T/11t = 69.2 --- 46T/17t = 44.8 ..
source : Brompton Gear Calculater
Oh, and you will also get the advantage of efficient drive by unibody structure and an extra middle gear ===> 46T/14t = 54.4
X3 Unibody Cog SET is probably at the limit that you can get without intrusive modification on rear frame and it is most stock parts compatible. i.e. You can revert back to original if don't feel like it. Not possible if you have to file or clamp the frame end in order to squeeze in bigger cog (e.g. 18t for titanium rear frame).
MiniMODs also have X2 + 1 cog set. With 11t/16t Unibody and one extra larger cog for your choice (18t,19t,21t ...).
Or with 4 speed free hub body (ETA end 2020), we can achieve 4 speed upgrade with less effort and cost. This will be another story.