My Brompton is now a grownup getting promoted to 12 speeds with an addition of second chain ring.
I accomplished the upgrade upon combining tidbits of advice from forum posters, @
Winfried, @
fietsbob, @
berlinonaut and others, in these threads
https://www.bikeforums.net/folding-b...ce-advice.html
https://www.bikeforums.net/folding-b...eur-clamp.html
and from other postings on the web. The goal was to have 50/34 rings without much sacrificing the benefits of Brompton: chainguard, compact folding size, low weight and ease of use. The final weight cost was: 486-414+120+103+45+29=369g. Given that some grip length got removed, it is presumably about 350g. I could lower this a bit, but then sacrificing performance. I post details below given that others might benefit from my experience just as I benefited from that of others. I will be still tweaking my setup but there are not going to be there any dramatic improvements – it is already a finely working setup.
Left: Brompton w/2 rings, right: w/1 ring
For maintaing the folding size, the right-side folding pedal is essential.
34T implied 110BCD for the rings. A lightweight effective chainguard implied the search for a 50T ring meant to work with a chainguard. I chose Shimano Sora R3000-CG. I replaced the bolts affixing the guard to the ring with alu ones for bottle cages.
Left: Original drive crankset, right: new crankset
The 5-arm crank needed to be one where the crank fell in-between rather than over one of the chainring arms. I chose Sunrace FCR86. Unfortunately you cannot get arms separately, only a set with steel rings. As 34T ring I bought a Vuelta. My first try was Shimano Tiagra but it comes in steel while it is listed as alu. If I did it again, I might go with FSA Pro as made from harder alu than Vuelta.
As derailleur I took an old Shimano XTR FD-M900 bottom pull 34.9 clamp one. I carefully enlarged the clamp to 40mm. This is not trivial and will not work with every clamp. The issue is of the mounting bolt being able to reach across the gap in the clamp. You will find no clamp on cable stop for the Brompton 40mm seatpost and I soldered one on a hose clamp. The latter is likely to become my standard for cable stops.
My shifter is Sram X.9 friction Gripshift. I tried a couple of lighter ones, Sunrace TS-M10 and an older Sram. Sunrace turned out to be a disappointment in that it barely held the derailleur in the larger ring position, jumping off if slightly touched. The older Sram yielded uncomfortable grip.
Deformed left fender stay.
The Sunrace cranks sit farther out on the bottom bracket spindle than Brompton. At this point I did not change the non-drive crank and will look into shorter bottom brackets.
