Bearings on a bicycle allow the parts to rotate relative to one another. The headset allows the fork to turn smoothly while riding. Bicycles, and all two wheeled vehicles, make small self-corrections in steering while traveling forward. If the headset is pitted or worn, these corrections are not made smoothly and handling suffers. Very worn headsets tend to "lock up" when the front wheel is pointing straight. Pick up the front of the bike, and gently swing the handlebars back and forth from center. Pitting in the cups will cause the headset to stick as it passes through center position
. A pitted headset should be replaced. New headsets are pressed into the frame and fork.