Solution to minimize overall height and maximize load capacity. Dual wheel casters also improve stability and swivel more easily than wider-face single wheels. However, this 40mm diameter caster is the most "ugly" one of the Series.
A real bargain for you: This one is strong and sturdy. Manganese raceway and Dust proof ring are not something you'll see everyday. This one will last very long at minimum maintenance effort. The wheel is soft and protected by 2 covers.