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Question about bottom bracket length

billcycle81billcycle81 Posts: 9
edited February 2018 in Workshop
Hi all,

Not strictly road cycling, but I'm about to embark on a conversion job on my Specialized Sirrus Elite hybrid. I'm changing it from it's standard 3x9 setup to a 1x9 with a 42T Narrow-wide chainring from Amber Bikes - which is currently on it's way to me.

The bottom bracket is feeling a bit rough so I figured I might as well change this too. Would going from a triple chainset to a single chainring mean changing the length of the bottom bracket? I'll be getting a Shimano UN55 68mm as I believe that's what's installed from factory. I'm not sure what the length of the one in there is at the moment, unfortunately, but I'm guessing a shorter one is required.

Sorry if this is a daft question but it's the first time I'm attempting anything like this. Previous tinkering involves nothing more than changing brakes pads and cables etc.

List of other things I'm upgrading, for those interested in the bigger picture:

Fulcrum Racing Sport Wheelset (2nd hand)
Shimano Alivio CS-HG400 9-speed Cassette (11-32)
KMC X10 73 10 Speed Chain (apparently works better with the NW chainring)
Shimano Brake Cables Inners and Outers
Jagwire Gear Cable inner and Outer
Amber Bikes 104BCD 42T Narrow Wide Chainring
BBB Tristop V Brake Pads

Should hopefully shed some weight and make my commute more fun.

What length BB for a 1x setup?


  • cooldadcooldad Posts: 32,599
    Nope, you want the middle ring, where you'll fit the single ring, in the same place, assuming it all works properly now.
    I don't do smileys.

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  • Ah ok, makes sense. So if I measure the current BB then just order the same size, all will be grand?
  • cooldadcooldad Posts: 32,599
    Should be.

    Measurements should be marked on the BB shell, but you'll have to take it out to see.
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

    London Calling on Facebook

  • I kinda wanted to get everything ordered and ready before taking the bike apart, tbh. But probably best to get it right.

    Cheers for the help!
  • MatthewfalleMatthewfalle Posts: 17,380
    Tell thetruth, if the chainset is coming off anyway it’s literally a 5 minutes job to take the bb out.

    I’d suggest giving it a good soak in penetrating fluid or a while beforehand and just make sure you have the correct removal tool.

    Loads of videos on YouTube about techniques if it starts getting a it pesky,
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.
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