In a forage harvester, a shear bar is adjusted relative to the knives of a rotating cutterhead by selectively energizing first and second motors to move one or the other end of the shear bar toward the cutterhead. A vibration sensor senses vibration of the shear bar and controls a circuit to deenergize an energized motor when vibration is induced in the shear bar. The vibration may be caused by impact of the cutterhead knives on the shear bar (a true hit) or by random noise (detected as a false hit). To discriminate between false hits and true hits, an energized motor is stopped when a first hit (true or false) is detected. A timer is set to time an interval at least as great as the time it takes the cutterhead to make one revolution, and during this interval the vibration sensor is repeatedly sampled to determine if a second hit has occurred. If a second hit is detected during the interval the first hit is assumed to be a true hit. If a second hit is not detected during the interval then the first hit is assumed to be a false hit and the deenergized motor is restarted.