Crankset compatibility, old 105 SC to new Claris FC-RS200

Question

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

Answer 1

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.