Various gearing modifications have been attempted for the Brompton folding bicycle over the past decade to boost the ratios and have these bikes fulfill a broader function. Although the 6 speed (2x3) factory version has been successfully used for distance loaded touring, it barely squeaks by in overall gear ratios for a true "touring bike".
Rohloff 14 speed hub conversions are very expensive, very heavy (about a pound heavier) and they add a great deal of width to the original folding size. A typical Rohloff conversion costs close to $3500 USD and that is not including the original cost of the Brompton bike.
The Shimano 8 & 11 speed hubs also use wider drop-outs of 135mm having the same problems that the Rohloff conversions do but without the solid reliability of the German made hub.
For some years now a few Brompton custom shops have been adding an additional sprocket (triple cog kit) to extend a 6-speed Brompton's gear range from 302% to 368%. This gives 9 distinct gears and takes care of the gearing problem, however, the conversion is tricky often needing to grind metal from the frame to make room for the additional parts as well as swapping out the Brompton 6 speed chain for a standard 10 or 11 speed chain to fit between the 3 narrower cogs. This conversion costs around $500 USD.
There is an 8 speed Sturmey-Archer Brompton-width hub available which will effectively boost a single speed Brompton to an eight speed but it changes the chainline quite a bit and adds some weight to the build.
All of these methods have their pro & cons. My ideal modification is to keep the standard 2 speed "derailleur" chain pusher system and overall drop-out width of 111mm but add additional gears within the internally geared hub. Sturmey-Archer manufactures a 5 speed hub (S-RF5 W) with 28 Holes, 111mm O.L.D. & 148mm axle which fits two gear sprockets. No modifications need to be made to the Brompton frame and only the shifter needs to be swapped out for a 5 speed Sturmey-Archer thumb shifter. This kit essentially gives you a (2x5) 10 speed freewheel with a gear spread of 362%. That's 27.0 - 97.0 gear inches as compared to the factory 6 speed Brompton's 28" - 85" gear inches using a 44 tooth (-12%) front chainring. This seems to be the perfect balance of usable gears without compromising weight or having to permanently modify the Brompton frame and parts. The fold is exactly the same width as an unmodified Brompton and all original parts are interchangable with the exception of the shifter. Plus, this modification is reasonably priced at around $150 USD if you re-purpose your existing spokes and rim.
- Stock Brompton 6 speed (2x3) - 28" - 85" gear inches (44t chainring) Cost N/A
- Rohloff 14 speed - 19” to 102” gear inches (44t chainring) Cost $3500.00 USD
- Brompton 10 speed (2x5) - 27" - 97" gear inches (44t chainring) Cost $150.00 USD
- Brompton 9 speed (3x3) - 23" - 92" gear inches (44t chainring) Cost $500.00 USD
- Sturmey-Archer 8 speed - 29" - 95" gear inches (33t chainring) Cost $575.00 USD
For the value, ease of installation & gear spread, The Sturmey-Archer S-RF5W (10 speed) conversion is the clear winner.