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You need to use the left 2-speed shifter, and modify it as indicated by the original author of this a post. I however have had better luck using any type of thumb lever friction front derailleur shifter. You really don’t need to index shift three cogs, plus friction shifting allows you to “trim” or adjust the shift so the chain rides on the center of the cog quietly.

I definitely had to remove some material from the circlip, mostly from the O.D.- if I could remember correctly. I don't think it matters how it is installed. My sprockets wiggle just a wee bit, but this does not affect the shifting. I am continually amazed how well this set up works. I want to hear about your success with this modification when it is completed:)

post #12 was from the original author to this article, not me.

I used a Dremel type of high speed hand held grinder to remove some material from the small cog. I sort-of chamfered it to allow the thickness of the original Brompton circlip to be installed, but not before I ground some material from the O.D. and a bit of the thickness of the circlip. I do not think grinding this clip compromises its strength. It is made of hardened spring steel, and its only purpose is to take up the space in the groove in the cassette body to keep the sprockets in place. Fitting the circlip is a trial and error application, but if you go (grind) slowly it will eventually fit and work fine.

I only shimmed the plastic chain tension mechanism out about 1 mm using two pieces cut from a plastic spiral notebook cover. No other modifications were necessary. You will have to adjust the two set screws on the chain pusher too. As far as removing material from the chain pusher wings: that may help a small bit, but this part is not very thick to begin with, so filing may weaken it. I have been riding with this set up regularly for the past few months, and it performs flawless.

When you stack three cogs on the Brompton cassette you need the narrow width of the 10 speed cogs (sprockets) and chain for everything to fit and shift properly. The existing Brompton chain tensioner and pivoting shifter mechanism will work with no modifications with a 10 speed drive train. Ten speed stuff is readily available; International Racing Design (IRD) carries a nice 10 speed cassette in their 12-23 combination that have the necessary sprockets to experiment with.

You were using a slightly narrower chain (9 speed) to shift on a 6/7/8 speed cog spacing, so I see why you didn't have crisp shifting.

Glad your interested, this is a neat conversion. As far as I can tell looking at a 11 tooth cog, there might be a problem getting it to work. There may not be enough room for the snap ring to be mounted. I had to open up the 12 t cog with a dremel type tool, and on the 11 t there probably isn't enough metal to grind away without getting into the teeth. But you could try designing a different way for the cogs to stay on the cassette body. Your 12-15-19 would work well as far as shifting. I have a smaller 42 on the front along with a 52. I very seldom use the 42, so it's a manual shift. I don't think it is worth to try and fit a front derailleur, plus the added weight, fold issues and busy like clutter. If you're currently using a 19 t with the titanium triangle you shouldn't have a problem fitting the triple. Actually the 19 will fit closer to the spokes (eliminated plastic spoke disk), so there is more clearance from the stay. Good luck, let me know how it works out. My 12-15-17 works flawless and is easy and quiet to shift.

The reason I used a front derailleur shifter is because it is made to go on the left side of the handlebar. So the exit cable follows the same route as the original 2 speed Brompton shifter. Also, the shifter I used is an older Suntour, and it is not indexed- meaning it is a friction shifter. Any friction shifter will work, cable pull doesn't matter with this kind of set up. One point I failed to mention in my earlier explanation is: if you use a non Brompton shifter, you will have to install a cable stop on the spring end near the changer located on the right chainstay. You can find these on eBay if you look hard, or make one like I did. It is basically a 10 mm bolt (that replaces the Brompton one) with a hole drilled through to accept the shifter cable and a set screw to lock the cable down.

You can use the existing 2 speed shifter by following the instructions from the author of this post: dmcg. The middle gear (cog) is going to be indexed into the small indent that you cut into the Brompton shifter.
I had better luck with an older Suntour friction thumb shifter. One that would be used to shift the front derailleur (left side shifter). The friction mode is easy to hit the middle gear even on the closely spaced 10 speed cogs. Any friction shifter for a front derailleur will work.

Thanks so much for this! I had success using 12, 15, 17 cogs. I chamfered the edge of the 12t so the snap ring would have clearance. I also ground the original Brompton snap ring a bit. I believe the snap ring has plenty of holding spring power even after removing some material. I used a 10 speed chain and cogs. I don't know why there is not more interest in this modification, it's fairly easy and cheap. Most Brompton owners from what I read on bike forums are not very handy, so maybe that is why.