A stationary outer frame member is attached to the bottom portion of a sliding closure, this outer frame having a pair of oppositely positioned vertical slots formed therein. An inner frame member having a pair of oppositely positioned canted slots formed therein is slidably mounted in the outer frame member. A roller wheel is mounted for rotation within the inner frame member on a pin which passes through the slots of both the inner and outer members, this pin holding these members and the wheel together. Vertical adjustment of the wheel without any lateral movement thereof is provided by means of an adjustment screw which causes the inner frame member to move relative to the outer frame member thereby causing vertical motion on the pin in the vertical slots formed in the outer frame member.