Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  

Go Back   > >

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

User Tag List

Thread Tools Search this Thread
Old 09-05-09, 06:45 PM   #1
La Fin Du Monde!
Thread Starter
Join Date: Mar 2007
Bikes: A red one and a green one
Posts: 9
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
BB sizing

I have a crankset that was originally on a shimano square taper 68mm-113mm bottom bracket. I would like to use this crankset on a new frame whose bb shell is 73mm. My question is, should the new bottom bracket be a 73mm-113mm or a 73mm-118mm? I don't quite understand where the extra 5mm will go, just the shell, or the spindle length too. Thank you
proRow is offline   Reply With Quote
Old 09-05-09, 07:38 PM   #2
Senior Member
joejack951's Avatar
Join Date: May 2004
Location: Wilmington, DE
Bikes: 2016 Hong Fu FM-079-F, 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2016 Islabikes Beinn 20 (son's)
Posts: 11,504
Mentioned: 10 Post(s)
Tagged: 0 Thread(s)
Quoted: 897 Post(s)
The new spindle length will depend on what bike the crank came off and what it's going on to. You want the chainline to be correct for the new bike. If you are swapping it from one road bike to another (unlikely given the 73mm shell dimension), no change in spindle length should be needed. Same applies for MTB to MTB. If it's going from a road bike to a MTB, you'll want to account for the difference in chainline which will be between 2.5 and 5mm according to Sheldon Brown.

Assuming you want to move the crank outwards by 2.5mm, you'll need a spindle that's 2.5mm longer on the drive side. According to what I've read (I need to measure for myself some day), most square taper bottom brackets are asymmetrical. Assuming that's the case, you'll want a 115mm BB to get close to the ideal chainline. If the bike has a larger diameter seat tube (aluminum frame), you'll probably want to go with a 118mm BB to move the chainline out a full 5mm.
joejack951 is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

All times are GMT -6. The time now is 01:05 PM.

  • Ask a Question
    get answers from real people!
Click to start entering your question.