BB lengths and why I may be bad at maths

Trying to figure a BB length to get the 42mm chainline on new ride. Current 107mm BB gives 48mm. But to get 42mm you’ve gt to take 6mm off each side, so 12mm in all. Which is a 95mm BB. WTF. Am I going wrong someplace?

Crank manufacturer says I can get 45mm with a 103mm BB, not 42. Maybe I’ve got to use a different crankset instead.


Lemme guess, Sugino RD2 (RD = road double)

Best bet for a 42mm chainline on the outer is a different crank.

Other option is to put the ring on the inner if you’re trying to spend less money. You could put a chain guard on the outside to improve aesthetics, cheap from BBG bashguard.

Duuuuuude. I have no idea what all those numbers mean.


CRS190 - $0.00 : :::Welcome to Ducas-Bike:::

from S&A.

Looks like different crank time.

fuck maths… i just use whatever i have, and then run the chainring on the inner, or the outer to make up for it. hasn’t failed yet.

Yeah. But I cant mount this ring on the inner. No tabs.

Blerg. The RD2 is also a 45mm chainline on a 103mm BB.

That crank is confused, apparently it has 3 BCDs “BCD: 110, 130, 144 mm”.

maybe you have your Tapers mixed up??

Have asked. Ducas website doesnt say JIS or ISO. TBH I dont know which one is which. Pretty sure my BB’s are all JIS (Shimano, VP). Pretty much already reached conclusion that crank will not work for 42mm, so will parts box it, or sell it to buy right one.

Sweet reply from Ducas International about JIS/ISO and getting a 42mm chainline:


Thanks for your e-mail.
May I know more about you?


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Benson Wang

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