The bottom bracket on my new Brompton is this one (I believe): http://www.sjscycles.co.uk/brompton-bb-cartridge-jis-for-spider-crankset-119mm-prod31207/

It is a JIS BB with 119 mm spindle length according to the description. The bike comes with 130 bcd chain ring/right crank, the crank length is 170 mm. I now want to use a shorter crank, 165 mm or shorter if available. So what cranks out there that I can use? The current crank is a single chain ring. Ideally, I want to just swap the cranks and use the current chain ring.

  • 1
    You probably won't see a lot of crank arms sold individually without the chain ring unless you get into the very high end. So there isn't much savings to be had from buying just the crank arms. As far as selection goes, look for stuff made for fixies or single speeds. There's a lot of stuff available with 165 mm cranks because shorter cranks are needed on fixies to prevent the pedals from hitting the ground as you go around corners.
    – Kibbee
    Apr 5, 2016 at 15:50

1 Answer 1


It seems to be fairly straightforward, I found a number of links around the web to people who have done it. Any crankset can take a single chainring, just make sure you mount it on the correct side of the spider when you do the swap. Try to match the existing chainline.

You may need to change the bottom bracket to maintain chainline and crank-frame clearance, but none of those links mentioned it as a problem.

FWIW I'm 180cm tall and run 150mm cranks on my commuter because they were the shorted I could get without paying silly money. They were the same price as cheap standard cranks. Unfortunately they're also made of low-quality aluminium and have bent somewhat in use. If you're strong or heavy I suspect the same may happen to you, but a Brompton is sufficiently flimsy that it may not be an issue (you might not be able to apply enough force without breaking the bike). IMO the best solution is to wait until it happens and buy more expensive cranks then, rather than paying up front when you might not need to.

  • You don't seem to consider it's a Brompton I am talking about, I am not sure swapping the BB and crankset will or will not affect the folding. And the thread regarding Brompton upgrade is for a old version, The new version uses a different BB (the one mentioned in my question. Apr 5, 2016 at 10:22
  • I thought I had considered that. But it sounds as though you need to take it to the dealer you bought it from and get them to do the swap.
    – Móż
    Apr 5, 2016 at 21:10

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