On downwind the pilot noticed that the wind had picked up and that there was light turbulence. He planned to land beyond trees to avoid turbulence. The airplane went through a gust of wind on roll out. The pilot applied rudder and aileron corrections along with heavy braking. The airplane exited the runway and impacted a pipe rack where the airplane sustained substantial damage to its left wing. A postaccident examination of the airplane revealed that a tailwheel spring, part number 3432, was bent and its spring rate was weak, which allowed the tailwheel's fork to caster freely by not engaging into its machined locking detent.