Door Track
A bottom rolling system consists of two rollers (sometimes called a sheave) at the bottom of the door running on a track and two guides at the top running in a guide channel. As all the weight of the door is concentrated on the two bottom wheels, more force is needed to move the door than on a top hung system, where the door is hung at the top from two points, also requiring a track / stay roller at the bottom to prevent it from swinging sideways.