On March 7, 2014, about 1340 Pacific standard time, a Cessna 170B, N2252D, had the left main landing gear collapse during the landing roll at San Luis Obispo, California. The pilot was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The airline transport pilot and one passenger were not injured; the airplane sustained substantial damage. The local personal flight departed about 1300. Visual meteorological conditions prevailed, and no flight plan had been filed.The pilot reported that the touchdown was normal. Shortly thereafter, he heard and felt what he described as driving a car over the dummy dots on the road, and then the left main landing gear collapsed. The airframe sustained substantial damage in the area of the gearbox. A Federal Aviation Administration inspector retrieved and observed video from three surveillance cameras with different views of the landing. He felt that all three supported a normal landing. The inspector performed a postaccident examination of the airplane. Using an inspection mirror, he was able to see into the damaged belly of the fuselage, and observed corrosion of the structural bulkhead assembly in the damaged gearbox area.