Changing Bottom bracket

SteveB2005
SteveB2005 Posts: 6
edited July 2018 in Workshop
I currently have a cup and cone bottom bracket and I am planning on changing this.

The cup and cone is 125mm, from end to end, and I was wondering how important this is, if I went with one that was narrower.
Does this just mean my crankset would be closer to the bike?

Not sure how clear this is, if not will upload a sketch
Thanks

Comments

  • Nick Payne
    Nick Payne Posts: 288
    If you want to maintain the same chainline, you'll need pretty much the same BB spindle length. However, there are a couple of caveats:
    1. There are two different tapers in use, JIS and ISO. The taper angle is the same, but ISO is slimmer than JIS, so you can't fit a JIS crank on an ISO taper, as the crank will bottom out on the end of the taper before it is on tight. You can fit an ISO crank on a JIS taper, but you will need a spindle a few mm shorter to get the same chainline, as the crank won't pull so far onto the taper. You should be able to find which taper you have from your model of crank.
    2. Most modern square taper BBs are symmetrical (equal amounts of spindle left and right), but some older ones were longer on the right than the left, to reduce the tread (Q factor). With the cranks off, measure yours left and right to see.

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