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Frame Windsor - The Hour

Hub Width - 120mm

Stock Bottom Bracket Sizing - 68 x 103 mm

Miche Bottom Bracket Sizing - 68 x 107 mm

Miche Crank Set Requirements - 68 x 107 mm bottom bracket

I'm looking to grab a pair of Miche cranks which call for a 107 mm bottom bracket (most likely the Miche primato). My bike currently has a 103 mm bottom bracket. Will the 2mm difference cause issues with my chain line?

On Sheldon Brown's site, I read that 107 mm is best for 126 and 130 mm hub spacings. I've also read that the Miche Primato has adjustable cups for tuning alignment. Would this be enough to compensate for the extra 2 mm on the drive side.

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My experience has been that adjustable cups give you 3-5mm of adjustment range -- more if you cheat a bit. And you can get adjustable cups for any cartridge that does not have an integral fixed cup. –  Daniel R Hicks Jul 30 '13 at 11:13
    
The presumption the BB divided evenly is flawed. A 107mm could change the driveline 4mm compared to a 103mm. Read Sheldon Browns BB size database –  mattnz Feb 3 at 1:13
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3 Answers 3

I replaced my stock 103mm BB with a 107mm BB and had no problems with the chainline. I could barely even tell it was different.

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With the maybe 100 mm axial travel of the chain over the cogs, when switching to the extreme gears, 4mm indeed sounds insignificant! –  Vorac Nov 27 '13 at 13:17
    
The Windsor "The Hour" frame is a "track" fixie from BikesDirect - only one gear to worry about in this case. –  Batman Dec 5 '13 at 4:46
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Crank manufacturers specify a BB length for a given chain line. Track drivetrains can vary from 42mm to 46mm (measured from the centreline of the bike). To complicate matters, Miche switched from ISO BBs to JIS BBs. You can use a JIS (eg. Shimano UN50-series) BB with an ISO crank, but for the same chainline, the reqired JIS BB would be a few mm shorter than the correct ISO BB...

If you use a BB that brings the crank inwards 2mm each side, the spider arms (where the chainring bolts on) may hit the chainstays.

I'm not familiar with the frame in question but on a fixed/singlespeed drivetrain, the tooth profiles tend to be taller and wider to prevent detailing. Running the chain out of alignment will increase wear.

tl;dr: Measure the hub, then measure the chainring posision with whatever BB you have. If the numbers don't match, change the BB.

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I agree with @Mac that you may not be able to tell the difference between 103 and 107. If your current 103mm is in good shape, why not buy the new crankset and try it on the old bottom bracket? Evaluate the chainline. If it's good, you are done. If the (front) chainring is too far inboard, buy the 107mm bottom bracket.

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