how to calculate square tapered BB spindle length when coming from external threaded bearings I want to move from 50/34 crank to 46/30 crank.
currently I have tiagra 4600 crank which is using external threaded bearings.
The crank which I am planning to buy &quot;FSA Tempo Adventure 46/30&quot; takes square tapered BB.
I want to know how I can buy correct spindle length carriage BB in first attempt.
One way I think might work is, 68mm + (external BB width * 2)
PS, I also have old 124MM and 122.5MM worn out BBs to test the crank on my road bike.
The spindle length is defined from the crankset, not the frame. There is no relationship between your Tiagra (external) and the future FSA (tapered). You should find with the FSA crankset the data of the required spindle length.