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Old 09-15-10, 03:30 PM   #1
berserker19
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BB width and front shifting

Hi all,
I have a Giant TCX cyclocross bike with shimano 105 components, all except a bontrager ISIS drive cross crankset. The bottom bracket I have is a 113mm, and I am having a problem where I have my front derailleur set all the way to allow it to extend to the furthest limit, but I still need to shove the shifter very hard in order to make the shift to the outer ring (where it makes a sluggish shift and sometimes rubs on the cage). I noticed that there is also a commonly available 108mm bottom bracket... would putting in a shorter BB help with my shifting issues? Or would the 5mm less not make a difference?

thanks
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Old 09-15-10, 03:56 PM   #2
Bezalel
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First you have to determine what chainline the 105 derailleur is designed for (43.5mm). Then you need to check the Bontrager documentation to determine what size spindle will give you that chainline.
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Old 09-15-10, 04:15 PM   #3
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I'm not familiar with your crankset, is it a double or triple. A chainline of 43.5 mm would be correct for a double. I think the Sheldon Brown website has a paragraph on chainlines and how to measure them.
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Old 09-15-10, 04:34 PM   #4
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The crankset is a standard road double fitted with cyclocross rings. I measured the way from the sheldon brown article and came up with about 47.5-48mm, but according to him the correct chainline should be 43.5 like you said... I also noticed that when in the large ring, the chain only appears to be straight when in the outermost three rear cogs.
Should I assume from this that my chainline is "wrong" and that I need the shorter BB? If this is the case could it be the root cause of my front shifting isssues?
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Old 09-15-10, 04:51 PM   #5
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If you've measured the chainline accurately, and if the bottom bracket and crankset are installed correctly and torqued to specs, your bottom bracket is too long.
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Old 09-15-10, 04:52 PM   #6
Bezalel
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Sheldon says that the Bontrager Double uses a 108mm spindle http://www.sheldonbrown.com/bbsize.html#bontrager.
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Old 09-15-10, 04:58 PM   #7
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Ok, looks like we found the problem.
Thanks all!

I've had a ruler in my toolbox for years and never knew why until now
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