# Another BB/crankset question

Discussion in 'Mountain Bikes' started by Bruce Edge, Apr 16, 2003.

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1. ### Bruce Edge Guest

Bear with me, I'll get up to speed eventually. BB length, 108mm, 113mm, or?

How do you choose? Is is based on the crankset, rear hub width, personal preference, flip a coin?

-Bruce

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2. ### Nelson Binch Guest

"bruce edge" <[email protected]> wrote in message news[email protected]...
| Bear with me, I'll get up to speed eventually. BB length, 108mm, 113mm, or?
|
| How do you choose? Is is based on the crankset, rear hub width, personal preference, flip a coin?
|
| -Bruce
|

The rule of thumb depends on the diameter of the seat tube. 1 1/8 generally uses the shorter one, 1
3/8 the longer one. 1 1/4 can go either way. Sometimes small idiosyncrasies in frame geometry may
throw this off, but it works most of the time.

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3. ### Small Black Dog Guest

> Bear with me, I'll get up to speed eventually. BB length, 108mm, 113mm, or?
>
> How do you choose? Is is based on the crankset, rear hub width, personal preference, flip a coin?
>
> -Bruce

A bit of googling turned up this info:

"Spindle length is determined by the type of cranks you have. High profile cranks are straight and
require a longer spindle while lower profile cranks require shorter spindle lengths. This is
important so that the backside of your cranks don't hit the chainstay and so that the "chainline is
correct". Chainline being that the middle chainring is close to being in line with the middle of the
cogset. If this chainline is too far off, it will cause surpass shifts and other chain suck like
things. Most crank manufacturers can tell you what spindle width to use. "

So there you go.

Small Black Dog, the best stoned bike tech.

4. ### Chris Phillipo Guest

In article <[email protected]>, [email protected] says...
> Bear with me, I'll get up to speed eventually. BB length, 108mm, 113mm, or?
>
> How do you choose? Is is based on the crankset, rear hub width, personal preference, flip a coin?
>
> -Bruce
>
>

It is determined by your current chainline (assuming your current one is the proper one). If you
measure from the center of your seat post to the center of the middle ring and it's say 47.5mm,
well you want to have that same measurement with your new BB and/or crankset. Most manufacturers
tell you what BB you need with what crankset to archive a certain chainline. You can also alter the
chainline according to the gears you use most. You might want to got a millimeter or two in out out
to get the chain to follow a straighter path in your top or bottom cog on the cassette. You can't
alter it too much or the front derailleur won't work. If you run a single ring up front, you can
pretty much do what you want but ideally you would want it to fall right in line with the middle
cog on the rear cassette.
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_________________________
Chris Phillipo - Cape Breton, Nova Scotia http://www.ramsays-online.com