Built-in spring design: Innovative built-in spring design, extends the life of the spring while providing a more beautiful appearance. The built-in spring clamp acts as a shock absorber when the wooden gate wheels roll over uneven or rough surfaces, reducing stress on gate hardware and extending the life of a gate's posts and hinges. Spring Gate Caster: Adjustable spring bracket with up to 1.77" spring-loaded vertical travel, automatically adjusts gate height with surrounding terrain throughout the opening and closing cycles, effectively prevents gates from sagging to keep the fence line level. Heavy Duty Gate Caster: Thick matte black plated steel plate and double ball bearings bring strong load-bearing, 220 lbs maximum load capacity for each gate wheel, making it easy to open or close large, heavy wooden gates, vinyl gates, metal pipe gates, or metal swing gates. 360° Swivel Caster: Aging-resistant rubber caster 4" with 360°rotating top plate, excellent mechanical strength and load