OK, so I finally took the plunge and added a manual change small front chainring to the Brompton. Here’s a write up of how what I did and parts used. This will work on a 6-speed 2007 Brompton. I can’t vouch for other models, but with a bit of common sense it won’t be too hard to work out the possibilities for other models. The numbers after the parts are the SJS Cycles part numbers.
1 x Stronglight 28t, 86mm bcd chainring (3022)
5 x 2mm chainring spacers (17702S2.0)
5 x 1.2mm chainring spacers (17702S1.2)
5 x double chainring bolts (10652)
1 x 127mm bottom bracket (10859) (this is longer than the original, but needed because the one fitted at the factory isn’t long enough, and the extra chainring will rub against the chain stay unless the whole crank is moved out an extra few mm)
Wrench, crank puller, bottom bracket tool, allen keys
Remove the cranks and the bottom bracket. Install the new bottom bracket, bolt on the new chainring, and put the cranks back on. Done, and very simple. Just remember to check the bottom bracket and crank bolts after a few miles of cycling, the crank bolts especially are difficult to get tight from a fresh install, and they can work themselves loose. Now you’ll have a Brompton with a good set of standard gears, changeable with gear levers, and an extra chainring for those really tough climbs, but changeable manually. Once on the smaller chainring, all the other 6 gears work as normal as well.