Bicycle wheels contain a number of spokes. The spokes link the hub portion of the wheel to the outer rim. Spokes also keep the rim centred over the hub through the use of tension . A loose spoke often causes a deviation in the rim, as higher tension from an opposing spoke causes the rim to pull away from the loose spoke. Tightening the loose spoke removes the deviation from the rim and increases the overall strength of the wheel.
Place a drop of bicycle lubricant where the loose spoke meets the spoke nipple and where the nipple attaches to the rim. The "nipple" is a threaded nut used to join the spoke to the rim. Lube allows the spoke to thread in and out of the nipple without the danger of binding.
Apply a truing wrench to the spoke nipple. Turn the wrench and nipple counterclockwise to tighten the spoke. Work in small, half-turn increments to prevent over-tightening of the spoke.
Observe the effect of the adjustment upon the side of the rim. The increase in tension on the loose spoke should eliminate the deviation in the rim.
Continue to add tension to the loose spoke gradually, until the rim is even. As a visual reference, rotate the loose spoke portion of the wheel past the bicycle's brake pads. The adjusted portion of the wheel should pass the brake pads as evenly as other sections of the wheel.