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how do you figure overall BB width for straight chainline?

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how do you figure overall BB width for straight chainline?

Old 11-03-07, 12:49 AM
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import600
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how do you figure overall BB width for straight chainline?

How do you figure out this number? Its a sekai 2400 frame, with a 68mm shell, and the BB installed is 127mm. This is obviously too wide for my SS cranks. I'm just curious if there is a method as to figuring what size BB diameter I need. Thanks.
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Old 11-03-07, 12:50 PM
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Originally Posted by import600 View Post
How do you figure out this number? Its a sekai 2400 frame, with a 68mm shell, and the BB installed is 127mm. This is obviously too wide for my SS cranks. I'm just curious if there is a method as to figuring what size BB diameter I need. Thanks.
Nobody can answer that question without knowing the make/model of the crankset you plan to use.

See: http://sheldonbrown.com/chainline and http://sheldonbrown.com/bbsize

The BB diameter is a non issue. All 68 mm bottom brackets other than French use the normal 1.37" diameter.

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Old 11-03-07, 01:00 PM
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You can install the crank you have, measure your chainline at the crank. Then you can compare this to the chainline at your hub. Once you know the difference between what you have and what you need you can a new BB of the correct length.
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Old 11-03-07, 04:08 PM
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When faced with this situation on the installation of Da Vinci cranks, I went to a Phil Wood BB, because the cups don't have a stop and you can adjust the chainline, before loctiting them in. There is about 5mm of play. You do have to have a spindle length that is fairly close. BTW, Phils BB's are really nice. bk
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Old 11-03-07, 04:28 PM
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Related question:

If I want to shorten the chainline at the bottom bracket end by 3 mm, do I need a BB with a spindle length that is 6 mm shorter? Thanks for any pointers.
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Old 11-03-07, 04:34 PM
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No (or more accurately, not necessarily). There are three dimensions that specify an axle:



B is fixed for a given shell width (52 for a 68mm shell), A you don't really care, and D is simply the sum of the other three. C is what you care about. D (the commonly cited 'length') is insufficient because A is not necessarily equal to C.

The only real way to calculate is to know your existing C, mount the new crank, and measure to determine what the new C needs to be. Then try to find an axle with that dimension.

In your case you need C to be 3mm shorter, whatever the length turns out to be. Asymmetrical axles are often specified by length and offset - the difference between A and C (offset is positive if C>A; axles are reversible). You can derive C from the offset and length: C=(length-52+offset)/2
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Old 11-03-07, 04:57 PM
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Thanks for the info, appreciate it! The thing is, I'm looking at bottom-brackets (the Shimano UN54, to be specific) and they only specify the D-number.

Here's my situation. The chainline at the bottom bracket is currently 46mm and I want to shorten it by 3mm. My current bottom bracket is as follows: D=124mm, A=35, C=37. So the C number should be 3mm shorter--the question is: What lenght Shimano UN54 should I get? If anyone has a clue, I'd appreciate it!
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Old 11-03-07, 05:19 PM
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I think I figured it out. My old BB is equivalent to a 126 symmetrical BB, so I need to look for a 120mm symmetrical one.
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Old 11-03-07, 05:29 PM
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Originally Posted by dizzy101 View Post
I think I figured it out. My old BB is equivalent to a 126 symmetrical BB, so I need to look for a 120mm symmetrical one.
Correct! Note the non drive crank will also be 3 MM closer to the BB.
IF your ols BB was the non integrated type, you could just "flip" the spindle and get real close. It would however move the left pedal out more.
I just did this "trick" on a Mountain Triple on which I only use the larger 2 rings 99+% of the time.
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Old 11-03-07, 06:15 PM
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Originally Posted by bkaapcke View Post
When faced with this situation on the installation of Da Vinci cranks, I went to a Phil Wood BB, because the cups don't have a stop and you can adjust the chainline, before loctiting them in. There is about 5mm of play. You do have to have a spindle length that is fairly close. BTW, Phils BB's are really nice. bk
You can do the same thing with a $20 Miche Primato.

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