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What BB spindle length for single-speed crank ?

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What BB spindle length for single-speed crank ?

Old 07-06-19, 09:00 PM
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Brocephus
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What BB spindle length for single-speed crank ?

I'm such an idiot ! I just had the bottom bracket in my hand, as I was transferring everything to a new frame, and I completely forget to measure the damn thing !!!!
As I suspected, it's some funky off-brand I'm not familiar with, and I'd been considering installing a Shimano UN55 ($20 or so is worth the peace of mind), so I've been wondering what size it would be. It's a 68mm English thread, I just don't know what the spindle length would be for a single speed crank, and figure somebody here is gonna know.

edit: I've been searching the question, and the answer seems to be, "it depends". But I just measured it (externally), estimating where the spindles ended inside the cranks, came up with something around 4.5", which is about 114mm, and the Shimanos come in a 113 and 115mm. So i guess I can yank the cranks again and get an exact measurement, or just wing it, figuring a mm either way shouldn't make much difference!!

Last edited by Brocephus; 07-06-19 at 10:09 PM.
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Old 07-06-19, 10:47 PM
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The best answer depends on several things. Crank arm offset, rear triangle spacing and I suppose rear hub design are the most significant.

Not being an engineer I just install the widest bottom bracket I happen to have on hand that has the right spindle interface to match the crankset I want to use. Slap on your chosen crankset and see how the chainring lines up with your rear cog. Since you now know the length of your test bottom bracket and how far you need to move your chainlink, you are ready to order a bottom bracket that fits your needs.

May not be the most elegant answer, but it works.
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Old 07-07-19, 10:18 AM
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Originally Posted by Retro Grouch View Post
............

Not being an engineer I just install the widest bottom bracket I happen to have on hand that has the right spindle interface to match the crankset I want to use. Slap on your chosen crankset and see how the chainring lines up with your rear cog. Since you now know the length of your test bottom bracket and how far you need to move your chainlink, you are ready to order a bottom bracket that fits your needs.

May not be the most elegant answer, but it works.
I am an engineer; and the above is probably the most effective and safest method.
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Old 07-07-19, 10:23 AM
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You can measure it in the frame. Just pull the arms.
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