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Bottom bracket width question(6 posts)

Bottom bracket width questionNug
Mar 23, 2003 1:53 PM
I'm thinking of upgrading my hybrid to a Tiagra triple crankset. Recc. bb width is 113, but I'm assuming that's given a 68 bb shell width. Mine is 73, so at the risk of a stupid question, I should go with a 73x118 to maintain the proper chainline, right? TIA
re: Bottom bracket width questionNessism
Mar 23, 2003 5:46 PM
I think you still need to use the same spindle width as before. Just because the bb shell is wider, you don't need to splay the crank arms wider apart. Do they make a 73x113?

Wrong analysisKerry
Mar 23, 2003 5:58 PM
The distance from the center of the BB axle to the right end determines your chain line. So you really need the 113 length but made to go with a 73mm BB width. If you followed your logic, you'd move the chain line 2.5 mm to the right (half of the 5 mm additional axle length). Picture the axle hanging in space to give the right chain ring position relative to the rear cogs - you can widen or narrow the frame width (as in 68mm to 70 mm if Italian) and you still would not want to have a longer or shorter axle. Changing frame BB width is symmetric (equal in both directions) - it doesn't shift the frame BB left or right.
Wrong analysisNug
Mar 23, 2003 6:20 PM
My thought was half the spindle length minus half the bb shell = exposed spindle. If they reccommend a 68x113, that's 56.5-34= 22.5mm to mount the crankset on. If I go 73x113, that's only 56.5-36.5=20mm, or I've got 2.5mm more taken up by the shell, or 2.5 less to hold the triple. Going up to a 118 keeps me at 22.5mm exposed spindle (per side) to work with. Am I over thinking this, or does that make sense? Any wrenches, please respond! The easy answer would be to take out the current bottom bracket and order the same length, but I don't have the tools and thought I could get an answer here so I can order one and screw around with it only once.
Wrong analysisgtx
Mar 23, 2003 6:40 PM
If a Tiagra triple is supposed to go with a 113 and you have a 73 bb then you need a 73 x 113. It's that simple. Really.
Still wrong analysisKerry
Mar 23, 2003 6:41 PM
The length of the BB axle is used to place the rings in proper relationship to the rear cogs. The only reason to use a longer axle is if the small ring threatens to touch the chain stay. Nothing will touch the frame at the BB. Using a longer axle will move your rings too far to the right and screw up your chain line. It has nothing to do with "amount of exposed spindle" which seems to be a new criteria you just made up.