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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 "FSA Tempo Adventure 46/30" 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.

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  • FYI those FSA cranks are significantly heavier than a external BB setup. If you can afford an external BB crank that can take 46/30, that would be a worthwhile investment.
    – MaplePanda
    Apr 1, 2021 at 17:22
  • The formula won't work. Consider that a BSA BB shell is 68mm wide. Hence, external cups have to be at least that wide. Even if you take 68 + (68*2), that's 200mm, which is quite a bit longer than any square taper spindle that I'm aware of - 124mm would be on the longer side of what I've heard about.
    – Weiwen Ng
    Apr 1, 2021 at 20:22

2 Answers 2

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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.

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First of all, that crank is one of the most problematic bike parts I've ever touched. They bend and fold their weirdo made-up-BCD chainrings with amazing regularity. I've had to deal with them a lot. They're bad.

To answer your question: most cranks have technical specs available to tell you the recommended spindle length as well as the chainline. 4600 implies you're presumably putting it on a non-disc road bike and would ideally have a narrower front chainline providing your frame gives you the clearance. So first you're going to measure or look up your rear chainline, then you're going to confirm however you want that the frame will give you the clearance you need in front if you set up the 46/30 for optimal chainline. Then, you're going to subtract whatever the chainline you want is from the published chainline, divide by half, and get a spindle that much shorter. (Or longer, if you were widening the chainline).

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