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Replacing BB,Crankset, freewheel Compatibility Sizing help.

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Replacing BB,Crankset, freewheel Compatibility Sizing help.

Old 03-21-15, 03:39 AM
  #1  
ninvin21
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Replacing BB,Crankset, freewheel Compatibility Sizing help.

Hello,Thank you for your help in advance I have two questions.

I replaced my freewheel from a 14-24 to a 15-25 Is that going to effect anything?

My Old BB is 68mm X 120mm with a chainline of 47.5mm I believe.
Can I replace to A 68 X 113 BB as long as the crankset puts it at a chainline of 47.5 or must I use a 68 X 120 setup?

Most Crankset Don't tell you what BB to use to put me at 47.5
I'm have a hard time understanding this.

Thank you
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Old 03-21-15, 07:08 AM
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Bill Kapaun
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Those are somewhat strange numbers for cog counts.
The 24 >25T largest cog would mean the chain needs to theoretically 1/4" longer. Since the chain can only be sized in 1" increments, it most likely will work as is. Just make sure you have "some" slack on the current Large ring:24T cog.

The reason you have different length spindles is to set the proper chain line.
A 120 will place the chain farther away from the center line of the bike than a 113. IT'S THE MATH!
Some spindles have an off set on one end, so if flipped end for end, you can get a shorter chain line.
That's about the only way a 120 & 113 "might" result in the same chain line.
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Old 03-21-15, 12:41 PM
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ninvin21
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So Let me get this straight. The BB length does not matter as long as the chainline comes out to 47.5 correct?
(I have seen cranks that recommend a 222.5mm to bring me to 47.5.)
(I have seen cranks that recommend a 113mm to bring me to a 47.5.)
(I have seen cranks that recommend a 118mm to bring me to a 47.5)

Thank you So very Much I'm trying to order a Crankset and BB today.
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Old 03-21-15, 01:21 PM
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fietsbob
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Throw the dice .. Buy several length and see what Happens.

a Crank set profile is a designed way it uses the BB spindle

Old, straight right angle types at one end , and super low profile at the other end of the spectrum.
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Old 03-21-15, 02:50 PM
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You buy the BB to fit the crank, just as you buy a tire to fit your car. Not the other way around.

Don't obsess about being a mm off on your chain line.
If you have multiple cogs on back, it's only going to be correct (maybe even not then) on 1 cog.
Think more of having a "straight" chain line where the crank & cog are lined up exactly, rather than some absolute number.
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