Join Date: Apr 2010
Location: Sunnyvale, CA USA
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How do I fix this? Do I need a new spoke? What might be causing it to just lose tension? Thanks.
The tension in that spoke is too low for friction to keep the nipple from turning when it unloads as it passes the bottom of the wheel.
The wheel could have been trued incorrectly resulting in uneven tension, the rim could have a bend in the direction of that spoke which makes it impossible to have the wheel true with sufficient tension in that spoke, the spoke could be failing, or the nipple could be pulling out of a failing socket.
You start by making sure the spokes have uniform tension on both sides of the wheel. If you can't do that without loosing true you'll need to loosen the spokes and bend the rim back or replace it. Then make sure they have enough tension. Light box section rims with 36 or fewer spokes can be set with Jobst Brandt's method of increasing tension then stress relieving until the rim deforms in waves, backing off half a turn, and re-truing. A tension meter is simplest.
My last front wheel stayed true for over ten years after I built it until I put a bend in it with the spokes around the bend needing to be twice as tight on one side after which it wouldn't stay true for a hundred miles.