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How do i choose the right size bottom bracket for my new crank?
#1
Hello

My 1-speed crank need replacing and currently I have a 122mm bottom bracket with it. Ive seen a cheapie 1-speed crank on Ebay for around £15. Would the same 122mm bottom bracket definitely fit this new crank? Or is it the case that all cranks are made differently and might need a shorter or longer bottom bracket?

Thank you
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#2
(01-04-2014, 08:44 PM)painkiller Wrote:  "Or is it the case that all cranks are made differently and might need a shorter or longer bottom bracket? "
correct statement, need more info on the one you want. check manufacture for recommended Bottom bracket. This should be close for most applications but in some cases one may opt for a slightly different size to tune in a straighter chainline. Some Bottom brackets can be adjusted one way or the other and cost quite a bit more than standard models

Thank you for your reply. So what are the factors that affect how to choose the correct bottom bracket length? I am assuming the BB length would vary depending on whether it is a 1, 2 or 3-speed crank?

If I was using a 122mm bottom bracket with a single speed crank, and I changed my rear single speed freewheel to a 7 speed, would that result in an adequate chainline and force me to change my bottom bracket length?

By the way, it is just a cheap unbranded crank on Ebay that im looking to buy so wouldn't really know the manufacturer. I will send the seller a message and ask him for the recommended bottom bracket to use.

Thankyou
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#3
(01-04-2014, 09:47 PM)ibie1983 Wrote:  1. So what are the factors that affect how to choose the correct bottom bracket length? I am assuming the BB length would vary depending on whether it is a 1, 2 or 3-speed crank?

2. If I was using a 122mm bottom bracket with a single speed crank, and I changed my rear single speed freewheel to a 7 speed, would that result in an adequate chainline and force me to change my bottom bracket length?

1. The factors are not only the number of chainwheels but also the particular design of each crank. More recent cranks are designed to work with shorter BB spindles, for example.

2. Question does not make sense - chainline is a measurement, it is not adequate/inadequate, and changing to a 7 speed generally requires an entirely different wheel and possibly wider dropout spacing, both of which alter the required chainline.

Read the posted link until you understand the relationship between spindle and crank and front/rear chainline, and do find out if the seller can give any info on spindle required. If he can mount the crank on a spindle of known specs and tell you how far from centerline the chainwheel is that would allow one to figure what is needed for proper operation - given an understanding of how to do the calculations.
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