Results 1 to 8 of 8

Thread: Bottom brackets

  1. #1
    mtbr member
    Reputation:
    Join Date
    Aug 2010
    Posts
    11

    Bottom brackets

    Hi everyone

    Can anyone explain the difference to me between the widths of bottom bracket. I currently have 113mm on one setup, and 120mm on another. The current one is running nicely with a 9 speed x 3 setup. I want to build a new one with similar various bits I have, but not sure what difference the 120mm will make

    Many thanks

  2. #2
    mtbr member
    Reputation:
    Join Date
    Sep 2010
    Posts
    783
    Are you talking about the length of the spindle on a square taper BB? if so then the length will vary depending on the particular crank and frame. If you are building up a new frame then it's most likely you won't be using a square taper BB.

  3. #3
    mtbr member
    Reputation:
    Join Date
    Aug 2010
    Posts
    11
    Yes, that's what I meant. The length of the spindle. Well, I'm trying to build a spare bike out of spare parts I have, and the 120mm bb is already in the frame.
    I have to match the two up, and my crankset is a Shimano FC-M440 22/32/44 175mm, according to a website I found, it will accept SP LGTH 47.5 CN LINE: 110mm or SP LGTH 50 CN LINE: 113mm.
    So my 120mm BB is no good right?

    Plus, the BB is much cheaper to replace than a new crankset. So I just need the shell width of the frame, and then choose a square taper bb with a 110 or 113mm spindle.

    From what I've read, it seems unless your seatpost is oversized, you want a 47.5mm chainline, which for me means a 110mm spindle

    Does that sound about right?

  4. #4
    mtbr member
    Reputation:
    Join Date
    Sep 2010
    Posts
    783
    Based on the info you've given I'd say go with the 110-113 but I have to ask why the 120 is in the frame. If you already have the crank I'd bolt it on and measure the chainline. Is this possible?

  5. #5
    mtbr member
    Reputation:
    Join Date
    Sep 2010
    Posts
    783
    By the way , the reason I ask to test fit if you have the crank is because recently I had an older bike with a cracked chainring. I found a replacement crank that was cheaper then new chainrings. It was the same maker and model but different years but the chainline was 2mm different.

  6. #6
    mtbr member
    Reputation:
    Join Date
    Oct 2011
    Posts
    452
    Slap your cranks on the BB that is in there now and see how they fit. You can measure the chain line, front and rear and see how they line up:
    - All About Bicycle Chainline

    Also check for clearance with the chain stays with the cranks and the inner chain ring. Assuming symmetric spindles, the 120mm one will push the chain line 3.5mm farther out than the 113mm BB.

  7. #7
    mtbr member
    Reputation:
    Join Date
    Aug 2010
    Posts
    11
    I've read some guides on the web. You measure from the middle of the seatpost tube to the middle of the sprocket on the big chainring. If I have 3 chainrings, which one do I measure? And how do I measure the rear ones. And then, how much can they be different by? Many thanks from a novice

  8. #8
    mtbr member
    Reputation:
    Join Date
    Oct 2011
    Posts
    452
    One of the best:
    - All About Bicycle Chainline

    Front triple crank, measure to center of middle ring. In back, measure to middle of cassette, even number of cogs, say 10, measure halfway between cogs 5 and 6. Since you have no center point in back, measure in from the dropout then worm back to what it would be from the center. So on a 135mm axle, each dropout is 67.5mm out from the center. Then if the middle of the cassette is say 20mm in from the dropout, that mid-point is 47.5mm out from center. And up front, I just measure the seat tube dia., divide that in half and add that to a measurement from the outside of the tube to the chain ring.

    Basically you want the chain lines (front and rear) as close to equal as you can get, so that if you were in the middle ring up front and the middle cog in back, the chain would be dead straight. It can be off as well, say if your FD won't shift all the way high or low or if the cranks hit the chain stays, you may need to give up some chain line to let the bike shift and pedal. On a 10sp. cassette, cogs are spaced about 4mm apart so every 4mm the chain line is off is like being cross chained 1 extra gear. It is not like the chain is going to explode if it is off a bit, after all in a 3x10 type setup, there are probably only 3 front/rear combinations where the chain is perfectly straight and thus 27 combinations where it is not straight. So might only be riding with a straight chain 10% of the time if you used all gears equally.

Posting Permissions

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