BROMPTON CHAINRING CONVERSION TO 39T AND LIST OF USEFUL UPGRADES
My Brompton is a steel M6R in the discontinued claret with the BWR hub and -12% reduced gearing, so it comes with the 44t front chainring. It has the latest spider crank arm. The handlebars are the old M type which I think changed to a shallower version with a higher stem form 2017. Gear shifter again are the old style ("Mickey mouse"!)
I have been very satisfied with the gearing so far but during the last couple of years my daily commune has incorporated a lot mor hills and also a slightly wounded right knee which doesn't bother me when I cycle, but wanted to make the steeping even lighter if possible.
So I dug deep in the internet and I decided to go with the easiest option to reduce the front chainring to 39t. So I ordered the parts needed. these were:
I also replaced the chain buying a new one from my friends at https://www.capproblema.com/ in Barcelona and a new set of rear sprockets. These if I recall are 16 & 13t respectively. the reason I did this is because I wanted to keep the chain and the existing sprockets- which didn't have so many milage on them as I had replaced them due to the natural wear a few months ago - in case I wanted to go back to the previous configuration. When you replace the chain, it is best to do the same also for the rear sprockets.
The length of the chain with the -12% gearing and 44t chainring is 98 links (including the quick link). In order to accommodate the new smaller chainring the chain has to be reduced to 96 links (including the quick link). You can still run the 98 links chain if you cant modify the length but in this case when you fold the rear wheel the chain will be to slack and it will be touching it self and wont; be easy to adjust the pedals as you usually do when folding the rear wheel in order to then fold the front.
So 96 links is the optimal length with the new set up. In the following link you can see the development of the gears on all types of Bromptons: http://xldev.co.uk/bgc.html
With the old setup the development in gear inches was: