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My bike currently has an old 105 crankset. It's a 105 SC, from 1989. The rest of the components are Claris R2000.

I'd like to replace the crankset with a new one with smaller rings. I thought of the Claris FC-R2000, but that would require changing the bottom bracket too.

Then I saw the Claris FC-RS200 which requires a square bottom bracket, and thus should be a simple swap (and it has a 46-34T version too, which is nice).

But I've seen that I should be careful with the BB and axle measurement, which if too long, could make the new chainrings too far to the right and outside the front derailleur's range.

Considering that my current BB shell is 68mm wide, that my current 105SC cranks are flush with it and work quite well with my FD-R2000, can anyone confirm or not that a simple swap will work as it should?

Or should I remove my current cranks to measure the axle first?

Thanks

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It is not worth worrying about keeping the bottom brackets. Various square taper cranksets require axles of different lengths anyway and bottom brackets do not have infinite life. Either buy a square taper crankset together with a square-taper BB or buy a modern Hollowtech II crankset with the appropriate bottom bracket.

68 mm is the usual width but make sure that your bottom bracket housing in the frame uses the usual English threading otherwise your BB options may be more limited.

It is not worth worrying about keeping the old bottom bracket and it is completely normal to buy a new one for a new crankset.

Measuring the axle length would only be necessary if you were really trying to reuse the old BB for the RS200 - the required length is specified by Shimano as 110 mm. For R2000, the axle is integrated in the right crank and any road Hollowtech II BB that fits your frame housing, most likely RS500, should be compatible.

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  • I was thinking of changing the BB too (it should be BSA), but thought it wouldn't be useful if I went with the RS200. If I change the BB too, I'd be tempted to go with the more modern R2000, but I'd be limited to 34-50T chainring. But finding a BB for the RS200 seems to be hell too... I don't know what to do now. I've seen an ultegra Di2 R8050 groupset for 510€ new, maybe I should buy that :D
    – elcye
    Jun 11, 2023 at 12:12
  • @elcye The Shimano BB-UN300 MM110/68 should be fine for RS200. I do not think the availability of 46/34 is really too important. It would not give you any easier gears, just limit your top gears. I would personally surely go for R2000. Jun 11, 2023 at 18:23
  • I'm not that worried about limiting top gears, since the main point of the swap is to get easier gears, and I also think about changing the cassette. I currently have 52-39T at the front and 13-26T at the back, so my max ratios are currently 4 and 3.7, which I never use. If I change the cassette and go with a 11-xxT, I'll still have a max ratio above 4 with the 46-34T, but with steep steps though... I'm charting all the possible ratios with the different chanrings and cassettes, and I really understand the appeal of 11 or 12 speeds! Finding the 110mm BSA version of the UN300 is what's hard.
    – elcye
    Jun 11, 2023 at 19:51
  • @elcye With 46-34 the steps between the gears on a given chainring will be as big as with 50-34. You will not gain anything important. You will only get bigger overlap between the big and the small chainring gears. I did a lot of riding with Claris R2000 with an 11-32 cassette and I still do with Sora on an R3000. The steps could be smaller, but it is rideable. The way to get really low gears are supercompact cranks, where the small chainring is 32 or 30. With 8 or 9 speed the choice is limited, though. Jun 11, 2023 at 20:51

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