Spring-loaded design: Allows doors to open and close freely by automatically adjusting the height on uneven floors while eliminating door sag. Spring-loaded design: Allows doors to open and close freely by automatically adjusting the height on uneven floors while eliminating door sag. Heavy duty universal mounting plate with 4 holes for easy attachment to wooden doors or horizontal or vertical rails along tubular farmhouse doors. Made of high quality rubber and metal, strong and durable. Shock absorption: easy to move and roll, easy to open and close the gate. Heavy Duty Casters: The wheels have a maximum load capacity of 220 lbs and can also withstand exposure to chemicals and solvents. Castor diameter: 100 mm / 4 inches; Load capacity: 100 kg / 220 lb max.