Brompton 2 to 3 mod?

[Fastpedaller]

I have a 2007 superlight which (until last year) had covered zero miles, and is probably at about 300 now, so barely used. It's a 2-speed, and I'm looking at changing it to 3 (derailleur) gears. My understanding is the crankset will need to be replaced for 3 reasons 1) The current chainring is one-piece with the cranks 2) I'll need a narrower chainring in order to fit sprockets suitable for 11 speed. 3) The bb is ISO square taper, I'm ok with fitting a new JIS cartridge.
I'm happy to get a new chainset from Spa (and would also choose 160mm cranks) and maybe go for a double and fit a front changer (although unless there is a large chainring jump I appreciate it may result in 5 (or even 4) distinct ratios. I understand I can alter the 2 speed derailleur changer to give an additional central position. I have 13 and 16 sprockets (originally 12-16) but really want the middle gear, so would probably go 12/14/16 or 13/15/17 if there is enough clearance. Does anyone know if the Spa chainrings are ok for 11 speed chain, or can highlight anything else I've missed?

 

[wafter]

Stuff on chain width here, with relevance to chainring selection.. Basically in single chainring config you can mix and may 5-8sp chains / rings, and 8-12sp chains / rings. The latter's teeth are apparently around 0.2mm / 10% narrower, so depending on tolerances you might be able to mix beyond the above limits..

Highly likely that the Spa chainrings are intended for the 9-12sp standard..

 

[Schwinnsta]

I converted my B from 2 to 3 speed with kit from BikeGang. I went with 12, 14 and 19 cogs. I changed the chainring to 10 speed along with the chain. I shift it with a SunRace M90 3 speed shifter.

 

[Fastpedaller]

Stuff on chain width here, with relevance to chainring selection.. Basically in single chainring config you can mix and may 5-8sp chains / rings, and 8-12sp chains / rings. The latter's teeth area apparently around 0.2mm / 10% narrower, so depending on tolerances you might be able to mix beyond the above limits..

Highly likely that the Spa chainrings are intended for the 9-12sp standard..

My current chainring (which is the earlier non-removable) is definitely too wide. Thanks for the info - I'd certainly have to check with Spa before ordering. Thanks also to Schwinnsta - A friction shifter will certainly be suitable. If I decide to modify the existing 2-speed it will be a difficult decision to actually attack it with a file.

 

[Gunk]

One minor issue with fitting a non Brompton crankset or crank arms is that you lose the tab on the drive side arm that stops the folding pedal hitting the frame. Obviously not a problem if you use removable pedals.

 

[Fastpedaller]

One minor issue with fitting a non Brompton crankset or crank arms is that you lose the tab on the drive side arm that stops the folding pedal hitting the frame. Obviously not a problem if you use removable pedals.

Mine didn't have anything to stop the folding pedal hitting the frame - one reason I sold the pedals, the other being I wanted to use SPD (so Shimano pd520's are on it unless I can find some quick release SPD pedals! I guess the earlier cranks (2007 in my case) didn't have the tab. Interesting to note the detail improvements over the years.

 

[Gunk]

Mine didn't have anything to stop the folding pedal hitting the frame - one reason I sold the pedals, the other being I wanted to use SPD (so Shimano pd520's are on it unless I can find some quick release SPD pedals! I guess the earlier cranks (2007 in my case) didn't have the tab. Interesting to note the detail improvements over the years.

My 2011 has the one piece crankset but with the tab