Results 1 to 6 of 6
  1. #1
    mtbr member
    Reputation: Skid Solo's Avatar
    Join Date
    Feb 2004
    Posts
    56

    BB sizing question

    My Daughters mountain bike needs a new BB, I have taken the old one out and measured it but I am confused, these are the dimension :
    Shaft length 120mm
    Frame width 68mm (where the bottom bracket goes into the frame)
    The shaft of the BB is marked with 50.120 and 08.01A
    I cant find any BBs that have a shaft length of 120 (118 or 122 no problem)

    Is this a special size for a kids bike ?

  2. #2
    mtbr member
    Reputation:
    Join Date
    Jul 2003
    Posts
    6,693
    Quote Originally Posted by Skid Solo View Post
    My Daughters mountain bike needs a new BB, I have taken the old one out and measured it but I am confused, these are the dimension :
    Shaft length 120mm
    Frame width 68mm (where the bottom bracket goes into the frame)
    The shaft of the BB is marked with 50.120 and 08.01A
    I cant find any BBs that have a shaft length of 120 (118 or 122 no problem)

    Is this a special size for a kids bike ?
    Definitely an uncommon size. There is no "special" size standard for kids bikes. But the question is raised, how are you measuring the spindle? A dial or digital caliper is highly recommended. A flat rule is usually to inaccurate. If you are absolutely certain that your measurement is accurate, then you have two choices. Go with the 122mm. That will push the drive side chain line out 1mm and the non drive side crank arm outboard 1mm, which likely won't be a problem. Or you can go with the 118mm and install a 2.5mm spacer under the drive side flange. That will push the chainline out .5mm from stock, not a problem at all, but will move the non drive side crank arm inboard 2.5mm. Likely not a problem unless the non drive crank arm/chain stay clearance was extremely tight with the stock BB.

    In either case you'll likely have to adjust the front derailleur, if so equipped, for the slight change in chainline.

    Bottom line, make sure your measurement of the stock spindle is accurate. Then go with the BB that suits you. It's usually better to go with something that will leave the chainline as close to stock as possible. But either should work.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  3. #3
    mtbr member
    Reputation: Skid Solo's Avatar
    Join Date
    Feb 2004
    Posts
    56
    Measuring with a digital vernier so I know its accurate, and the 120 marking on the shaft to mind also confirm this

  4. #4
    local trails rider
    Reputation: perttime's Avatar
    Join Date
    Aug 2005
    Posts
    11,834
    Sheldon Brown's site does mention 120 mm spindles. In some standards, it is mainly the drive-side end of the spindle that varies. Spindle length mainly affects two things: chain line and clearance between cranks and chainstays. Is either of those critical on that bike?

    "it IS possible that you are faster or slower than anybody else who is having at least as much if not more or less fun"

  5. #5
    mtbr member
    Reputation: Skid Solo's Avatar
    Join Date
    Feb 2004
    Posts
    56
    Spindle leght is proably not critical not 1mm either side anyway so I will go with a 122. So bascially I can fit any Shimano 68mm shell with a 122 axle length or do the square tapers come in different sizes as well ?
    Sorry I havent used a non HT2 BB for 10 years on my bike and am a bit out of touch

  6. #6
    mtbr member
    Reputation:
    Join Date
    Jul 2003
    Posts
    6,693
    Quote Originally Posted by Skid Solo View Post
    Spindle leght is proably not critical not 1mm either side anyway so I will go with a 122. So bascially I can fit any Shimano 68mm shell with a 122 axle length or do the square tapers come in different sizes as well ?
    Sorry I havent used a non HT2 BB for 10 years on my bike and am a bit out of touch
    Nope spindles are spindles, you should be fine with the 122.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

Similar Threads

  1. KHS sizing question
    By bricke in forum KHS
    Replies: 1
    Last Post: 01-17-2011, 05:40 AM
  2. HD Sizing question
    By bedell99 in forum Ibis
    Replies: 3
    Last Post: 12-18-2010, 09:12 AM
  3. Replies: 37
    Last Post: 05-16-2010, 08:04 PM
  4. sizing question
    By bapesta in forum Downhill - Freeride
    Replies: 6
    Last Post: 05-21-2009, 09:09 PM
  5. A question of sizing.
    By bat on a bike in forum Titus
    Replies: 1
    Last Post: 02-23-2009, 01:25 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •