On November 14, 2001, about 1420 mountain standard time, an Aviat (Pitts) S-2C, N4HV, departed the right side of runway 22R and struck a taxiway sign at Chandler, Arizona. Chandler Air Service was operating the rental airplane under the provisions of 14 CFR Part 91. The certified flight instructor pilot and commercial pilot undergoing instruction were not injured; the airplane sustained substantial damage. The local instructional flight departed Chandler about 1330. Visual meteorological conditions prevailed, and no flight plan had been filed. The instructor stated they completed airwork in the local practice area and returned to the landing pattern. The student was low on the first approach and the instructor had him complete a go-around. The second approach was satisfactory during the approach and touchdown. During the rollout, the nose of the airplane started to move to the left and the right, and the instructor advised the student that he was taking control of the airplane. The instructor straightened the nose and continued the rollout. As the airplane slowed, the airplane started to move to the right. The instructor pumped left rudder, but the airplane continued to the right. He applied and held full left rudder, but determined that the airplane would depart the runway. He released the left rudder pedal and intended to stop in the dirt. However, he did not see a runway sign, which collided with the lower right wing. The collision spun the airplane around and resulted in substantial damage to the lower wing.