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  1. #1
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    Brompton Gearing - Try something new?

    Hi folks,

    I'm looking to lower the gearing on my Brompton 6-speed. It's the current 50t setup now.

    I would like to get significantly lower gears, as I basically never use the top two gears, and the first gear isn't all that low.

    I'm considering something in the 40t range, and I would like to have a better (metal) chain guard. The Shimano Alfine 39t hollow tech crankset + bottom bracket looks like it may be an option. Any feedback on the ease of installation of these lower gearing options?

    I'm hesitant to spend a large sum on a smaller version of the Stronglight one-piece crankset if it's of lower quality (and similar cost) to a different third party option.

    Thanks!

  2. #2
    coprolite fietsbob's Avatar
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    Stick a 110 double crankset on may be able to put it on the same square taper BB or.. fit one of the JIS UNxx types in ,

    Greasy finger up shift, kick it down with your heel ..



    probably dont want to drop another $600 on a Swiss Mountain drive It drops a 54-15 to 17 " in reduction gear low as it acts like a 21.6t then ..

    a 50 20 comes out even numbers ....

    The overdrive speed drive crank.. fitted with a 34 , acts like a 54t in overdrive .

  3. #3
    Senior Member Ed in Toronto's Avatar
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    If you want to save some cash. Leave the bike as it is and peddle harder

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    My 6 speed BWR has the 44 tooth crankset:
    As I no longer live where I can commute by bike, I rarely feel the need for more speed.
    A 39 tooth setup would be a good fit for touring.

  5. #5
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    I recently did the opposite to my 3-speed Brompton from 50T to 54T(+8%), done by the shop since it's so cheap. The idea would be the same if you want to lower it. If you have the 2013/14 Brompton, all you need to do is get the chainring, remove few chain links and that should be it. For earlier Bromptons, you'll have to change the whole crankset as I understand it, which will cost more.

    If you want to stick with Brompton Chainring which comes with the chainring guard, the 44T version will give you -12% lower gearing, your 5th gear will go from 6.49M -> 5.71M and your 6th gear will go from 7.98M to 7.03M.

  6. #6
    coprolite fietsbob's Avatar
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    or the S-RF5(N) S-A Hub [also 28 hole], add the 2 cog set on its 9 spline driver and have a 10 speed rear wheel ..

  7. #7
    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by exarkuhn15 View Post
    Hi folks,

    I'm looking to lower the gearing on my Brompton 6-speed. It's the current 50t setup now.

    I would like to get significantly lower gears, as I basically never use the top two gears, and the first gear isn't all that low.

    I'm considering something in the 40t range, and I would like to have a better (metal) chain guard. The Shimano Alfine 39t hollow tech crankset + bottom bracket looks like it may be an option. Any feedback on the ease of installation of these lower gearing options?

    I'm hesitant to spend a large sum on a smaller version of the Stronglight one-piece crankset if it's of lower quality (and similar cost) to a different third party option.

    Thanks!
    Well ... if you change to an alternative square taper crank, there is a decent chance you would have to change the bottom bracket to get the appropriate chainline.

    The external bearing BBs and two piece BBs are easier to install, IMO.

  8. #8
    coprolite fietsbob's Avatar
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    The external bearing BBs and two piece BBs are easier to install, IMO.
    If you own the installation Tools ..

    you could get a 130 (or 110 'compact'double) BCD crank(ala Shimano) and put 1 chainring on the inside position,

    and a chainguard disc from Cyclocross race bike build parts and fit it on the outside.

  9. #9
    LET'S ROLL 1nterceptor's Avatar
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    I swapped the 44T on my 2013 to a 39T; lost the chain guard
    though.Which is fine by me; just about to go cliplless anyhow:

    https://www.youtube.com/watch?v=5AT6...6zPoymgKaIoDLA

  10. #10
    Senior Member tk1971's Avatar
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    Another vote here to try out the 39T in the front, assuming you have a newer Brompton with the user replaceable chain ring.

    Maybe someone more knowledgeable can chime in regarding the old vs. new cranksets / bottom brackets. I was under the impression that one can simply buy a new RH spider arm for $23 and replace the one-piece older RH crank (w/ chain ring). All one would need is to get their choice of chain ring in 130 bcd for a total of about $50. Replacing the non-drive side crank to match would be optional.

    Here's the spider arm:
    Crank RH only "Spider" (for detachable ring)

    Here's a 39T:
    Amazon.com : Shimano Ultegra 9-Speed Double Chainring (Silver, 39 Tooth) : Bike Chainrings And Accessories : Sports & Outdoors
    FAIR is a four letter "F" word.

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    Thanks, all!

    I thought about getting the new spider crank, but I think you need to change the bottom bracket, too? Bottom brackets are totally outside my wheelhouse, which is why I was hoping someone could just identify what works on a Brompton. I didn't really want to pay the higher cost of the Brompton one (like the older one-piece in lower gearing) if I could get something nicer for the same money. My ignorance on the topic is the limiting factor!

    I have the pre-2013 version, so the one-piece crank/ring. I would prefer it if whatever I swapped to had nice, low gearing, and a metal chain guard that doesn't break like the plastic one does.

  12. #12
    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by exarkuhn15 View Post
    I have the pre-2013 version, so the one-piece crank/ring. I would prefer it if whatever I swapped to had nice, low gearing, and a metal chain guard that doesn't break like the plastic one does.
    I believe that it's a straight swap if you get another Brompton one-piece crank/ring. If you go with an alternative crank -- say shimano -- then you might have to deal with chainline issues and such.

  13. #13
    LET'S ROLL 1nterceptor's Avatar
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    Swapped a 50t one piece crank/chain ring on a 2012 for 44T(comes w/ a guard from B).
    I think there is an alloy chain wheel guard available in Asia in different colors.

    https://www.youtube.com/watch?v=hYo8...IoDLA&index=17

  14. #14
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    Quote Originally Posted by 1nterceptor View Post
    I swapped the 44T on my 2013 to a 39T; lost the chain guard
    though.Which is fine by me
    Did this Brompton come with the post-2013 spider crank, or did you replace a one-piece for the spider, too? If the latter, did you need a new bottom bracket?

  15. #15
    coprolite fietsbob's Avatar
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    Number 9 is a picture/vid of the current Brompton spider crank .. post 013..

  16. #16
    LET'S ROLL 1nterceptor's Avatar
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    Quote Originally Posted by exarkuhn15 View Post
    Did this Brompton come with the post-2013 spider crank, or did you replace a one-piece for the spider, too? If the latter, did you need a new bottom bracket?
    That bike(my current B) is a 2013 and has a spider crank. I got the impression that a post 2013 crank would just swap with a 2012 and below using the same
    bottom bracket(spindle). This would give you more choices for chainrings/wheels; Brompton 44T, Shimano 39T, etc. After market alloy guard is only good for
    standard B'ton ring sizes; 44T, 50T, 54T:

    Brompton Pimping

  17. #17
    Senior Member tk1971's Avatar
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    Pieces of the puzzle:

    Found a part listing for the older Brompton bottom bracket:

    BB cartridge ISO (for "Fixed" crankset)

    Replacement bottom bracket shell with ISO taper for Brompton folding bicycles.
    • 68mm shell
    • 122mm spindle
    • English thread
    • For use with Brompton's "fixed" crank used on bikes shipped prior to April 2013.




    After some googling, I found the newer bottom bracket:

    [T4] Bottom bracket cartridges | Brompton BicycleBB cartridge 119mm JIS (for 'Spider' crankset)

    Part code: QBBCART-119-JIS Part number: 9044016




    Looks like you'll need to replace the whole thing (bottom bracket, drive crank spider and the non-drive crank arm). Good news is that they appear to be very reasonable (price-wise).
    FAIR is a four letter "F" word.

  18. #18
    coprolite fietsbob's Avatar
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    A 90's Mk2 would have a loose ball BB , the more recent, Mk4 ones use a cartridge BB..
    my fixed chainring set pre 013, has the cartridge BB ..

    threaded parts are Plastic, so wont rust or corrode, bearings and axle are of course steel.
    Last edited by fietsbob; 05-19-14 at 01:14 PM.

  19. #19
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    Quote Originally Posted by tk1971 View Post
    Pieces of the puzzle...
    Very helpful!

    It looks like I could go with the new spider crank, which is a bit pricey by the time you add on the crank, the chainring, and the new bottom bracket, and another option I've seen is to buy a non-Brompton spec Stronglight crank that is exactly the same as the old Brompton one, but without the chain guard. If you buy that Stronglight one - the S55, it's way cheaper, interestingly. I wonder if there is a way to get a chain guard to fit on that if I were to buy the 40t version...

  20. #20
    coprolite fietsbob's Avatar
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    BB shell will take a BSC standard BB. so doubles will work too .. chainring inside chainguard disc outside...

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