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Background: Sugino RD2 single-speed cranks call for a 103mm bottom bracket which gives a chain line of 45mm, which is what I need, but there aren't suitable square-taper BBs in this size. Alternatively, I could get a longer BB and move the chain ring to the inside of the crank spider. How much longer the BB needs to be depends on how much the chain line changes when mounting the chain ring on the inside of the crank spider.

Research: In this thread, someone had a difference of 7mm. Haven't been able to find anyone else who's made this measurement for comparison. Has anyone else measured this?

Application: If 7mm was the difference in my case, does a 117mm BB with the chain ring on the inside have the same chain line as a 103mm BB with the chain ring on the outside? (14mm longer from adding 7mm to drive side and necessarily to non-drive side)

Visual for people like me who don't know what I'm talking about

*This is for a belt-drive so the front & rear chain lines need to be pretty close, wanting zero offset, the chain ring (sprocket) is not dished, rear IGH can't be spaced, I currently don't have any of these parts at hand so I can't measure, Q-factor be darned

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    Are you sure that no 103mm BBs exist? On Google, I can find a Sugino 103mm JIS BB for US$50 or an FSA for US$18. More expensive ones exist, e.g. Phil Wood.
    – Weiwen Ng
    Feb 21, 2022 at 11:16
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    On the other hand, mounting chainring to other side of crank spider is free and has zero shipping time. I'd say go for it if it gives correct chainline.
    – ojs
    Feb 21, 2022 at 15:42
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    inside the crank arm? I think that would be better stated as "inside the crank spider". The crank arm usually refers to that part of the crankset where the pedal attaches. I've yet to see a crankset with the chainring mounted in any place other than inside the crank arm. ;-) Feb 21, 2022 at 19:40
  • @AndrewHenle Fixed. I thought "spider" was only for the detachable variety.
    – Laoshi
    Feb 22, 2022 at 2:01

2 Answers 2

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Yes you can do it that way and you are doing the math right if 7 is the number you're getting. You sacrifice (or at least change) q-factor doing this and I would encourage not to do that lightly on a fixed gear or singlespeed (or really any bike).

Sugino makes bottom brackets for at least all their cranks' recommended spindle lengths, availability concerns aside. 103 is an uncommon size but it's not truly weird by any stretch; a number of nicer track and other single-ring cranks have used it. Phil also makes 103.

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  • Good to hear the math checks out at least. Almost added a bit about already being settled on a specific BB for touring and that Dr. Phil's are out of stock, but wanted this question to have a more universal relevance about how many mm the change I should expect.
    – Laoshi
    Feb 22, 2022 at 1:59
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Yes this works but depends on the design of your spider.

I have a 1981 track frame I want to use for time trial. Rear wheel spacing is 120mm. I bought a Stronglight compact single 48t chainset. I know a 107mm JP400 spindle will let the casting go inside the cup by about 2mm. Still gives a 50mm chainline so moved the ring to the inner arm recesses of the spider. 45mm perfect! Misses the chainstay by 1mm!

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  • Hi, welcome to bicycles. This is interesting, but I'm not sure it's relevant to the question, which is about a different mix of components. If you want to post a comment you'll need to earn a bit of reputation first; you might want to take the tour to learn more.
    – DavidW
    Apr 27, 2023 at 15:48

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