Help with gears/chainline please.

lifeofbrian2007
lifeofbrian2007 Posts: 2
in MTB workshop & tech
I have an old mtb that I'd like to change the crank on, it is a 3x7 speed but there's not many(or any) 7 speed cranks with a 48.8mm chainline. Could I try maybe a 8/9 speed crankset if I can find one with the right chainline, or can I just get a crank with smaller chainline and use a spacer on the crank axle to make it 48? Or maybe get a wider BB? The BB is a square type.

Comments

  • firealno9
    firealno9 Posts: 7
    Get a slightly wider BB. You could also use an 8 speed chainset with no issues.
  • reaperactual
    reaperactual Posts: 1,185
    edited November 2020
    You can buy and fit a 7,8 or 9 speed triple crankset straight into your current square taper bb which will give you roughly the same chainline you already have.

    If you want to alter your chainline for some reason remove the bb, check the spindle length printed on it then buy a replacement square taper bb. Buy the same bb, match the numbers, eg shell width (68,73) etc but a longer or shorter spindle length as required. The spindle length is the bottom number:-




    Take into account that stated length of current spindle is the overall measurement so a new bb with a 20mm longer spindle will alter chainline out by 10mm and vice versa.

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