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I have an old Univega MTB with a triple 28-38-48 crankset and 6-speed freewheel. The bottom bracket is square taper with cup and bearings. I want to remove the front derailleur and convert to 48T crank, keep the freewheel and rear derailler. The final setup will be 1x6. What adjustments do I have to make to the chain line (priority item) and the bottom bracket, if any. The fixed crankset will be 48T so I expect that chain line adjustment should be minimal, if any, on a bike with a rear derailleur.

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  • You may have to keep the front deraileur mech to act as a chain guide, Or you might be able to replace the bottom bracket axle with a shorter one, that moves the chainring inboard enough so the chainring is in-line with the middle of the cassette.
    – Criggie ♦
    Commented Mar 21, 2016 at 22:18
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    So are you going to change the front cranks? Or just remove chainrings? How will the removed chainrings affect the placement of the outer chainring? Can you change the placement of the outer chainring with different crankbolts/spacers?
    – Deleted User
    Commented Jun 22, 2016 at 22:25

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Assuming the chainline is spot on as it is now, you could remove the inner and middle chainrings, and place your 48t outer ring in the middle ring position. This will most likely require new chainring bolts intended for single ring use (they are shorter). This will only work if the middle and outer ring share the same bolt circle diameter - BCD (most triple cranks work like that, but I've come across a few where the inner and middle rings both had a smaller BCD than the outer ring.)

If above solution doesnt provide the chainline you want, you could replace the bottom bracket. Shimano makes a cheap and (in my experience) reliable bottom bracket for square taper interface: The BB-UN26. It comes in various spindle lengths and for both 68mm and 73mm shells.

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You'll probably have to buy a narrow wide chainring.. So that your chain does not fall off. And a longer shaft bb would also be a good idea.

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  • I doubt he'll need a narrow wide with a 1 x 6, the lateral movement will be minimal but i could be wrong.
    – Nate W
    Commented Mar 24, 2016 at 16:27
  • Yes but that is just a advice to be safe.
    – user25126
    Commented Mar 28, 2016 at 14:15
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    Who makes a 6 speed narrow wide?
    – paparazzo
    Commented May 31, 2017 at 13:35

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