Brompton 2 to 3 Speed Conversion

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  • I know that there are some Bromptonites here, so I thought I'd share some progress on a little project to run 3 cogs on a standard Brompton hub.

    The essence is to replace the 2 current sprockets with Shimano 10-speed ones. It's letting me run 12,13,14 x 54 which is fantastic for commuting.

    I'll upload some images to view inline if anyone is interested, but in the meantime instructions are here.

    If anyone tries it please let me know how you get on - I'm interested to see if it works on other bikes, and in particular the BWR hub.


  • 12,13,14 x 54 which is fantastic for commuting

    Am I missing something? That seems MASSIVE.

  • Am I missing something?

    I have huge calves.

    And 16" wheels.

  • Ha ha. Fair enough then.

  • I have huge calves.

    And 16" wheels.


    With 16" wheels that's 52 to 60 inches, so is perfect for commuting

  • ^this

    With 16" wheels that's 52 to 60 inches, so is perfect for commuting

    I think more like 62-72 - it's a long flat commute.

  • One of the product designers at Brompton did this to his 4-5 years ago, I think he used 9sp shimano cassette 12/13/14 rings and adapted his 2sp shifter.

    Was really nice to have a little more gearing for marginally more weight.

  • One of the product designers at Brompton did this to his 4-5 years ago, I think he used 9sp shimano cassette 12/13/14 rings and adapted his 2sp shifter.

    Interesting, do you have a link so I can compare notes?

    I did find a site in Spanish on the wayback machine with some details of such a conversion - Chrome's auto-translation helped a little...

  • Thinking of also doing this as I'm not a massive fan of hub gears. Since this thread is four years old, does anyone have any further comments to add?


  • Hub gear or cassette? I'm still waiting to buy my bike (hiring at the moment still).

  • Thanks. Will let you know if I'm after one once I get my bike.

  • OP here. I'm still running the conversion and it's still fine.

    I would update the PDF, but I can't find the original document

    The major change that I've made since the original is to replace the hacky garden wire circlip with the original Brompton one - you need to file back the inner face of the smallest cog to get it to fit, but I did have problems with the wire coming out and leaving rattling cogs. I wish I could just find a thin-enough circlip of the right diameter - if you do, please let me know.

    The shifting is a little temperamental. When the cables get sticky or the jockey wheels clogged I need to throw the shifter from 1st to 3rd rather than trying to go from 2nd, but the convenience of 3 close ratios for my flat commute outweigh the negatives for me.

  • Cheers Duncan!

  • What did you use for your shifter?
    Thinking of doing this to a BWR hub to give 9 gears!!

  • I bodged the stock Brompton shifter - see the PDF

  • Just got a SS Brompton and will be attempting this!

    Will report back when I get the bike and bits, how exciting... :D

  • 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.

  • Totally agree, there should be more interest in this as the manufactured Titanium alternative is about 200 quid!

    Being part of the not-so-handy community, would I need a new shifter for the 3 speed? Only asking as I thought the 2 speed shifter may do the job for shifting up and 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.

  • Super, thanks! I'll try that and report back. It'll be a good couple of weeks as I'm waiting for a new wheelset to arrive.

  • @user87446 Is the reason for using a front shifter because there is a lower cable pull ratio, ie. it would be more accurate?

  • 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.

  • This is interesting! I'm seeking a similar conversion, but want to achieve a wider range. Do you think a triple with 11/15/19T would work (always assuming I can source the relevant cogs)? And would there be any problem with a titanium rear triangle?

    I'd really appreciate advice here; I be been experimenting with a double chain ring but can't get a derailleur to work so shifting has to be manual (not a problem because I use the larger ring normally, the smaller in hilly areas.

    And I've successfully fitted a 12/19T combo and wonder about 11/19, but the jump is rather large and an intermediate 15 would be perfect.

  • 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.

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Brompton 2 to 3 Speed Conversion

Posted by Avatar for dmcg @dmcg