A door roller comprising: a carriage member comprising a plurality of bores (242a, 242b); and a plurality of roller assemblies connected to the carriage member, each roller assembly of the plurality of roller assemblies comprising a pivot bearing rotatably disposed within a bore of the plurality of bores (242a, 242b) and a pair of rollers (226a, 226b) coupled to the pivot bearing, wherein the plurality of roller assemblies comprises an upper roller assembly (226a, 219, 240) configured to roll upon a topside of a roller track and a lower roller assembly (226b, 219, 240) configured to roll upon an underside of the roller track, characterized in that: the carriage member comprises a plurality of slots formed through walls of the bores (242a, 242b) and each bore (242a, 242b) of the carriage member is coupled to two slots of the plurality of slots; and each roller assembly of the plurality of roller assemblies comprises a pair of axles, and each axle of the pair of axles extends through one slot of the plurality of slots, wherein each of the plurality of slots has a length and a width dimensioned to allow a corresponding axle to move along the length of the slot so that a central axis of the corresponding roller assembly can pivot about its pivot bearing to allow the central axis to remain parallel with a face of a topside of a roller track when the carriage member pivots about the pivot bearings. a carriage member comprising a plurality of bores (242a, 242b); and a plurality of roller assemblies connected to the carriage member, each roller assembly of the plurality of roller assemblies comprising a pivot bearing rotatably disposed within a bore of the plurality of bores (242a, 242b) and a pair of rollers (226a, 226b) coupled to the pivot bearing, wherein the plurality of roller assemblies comprises an upper roller assembly (226a, 219, 240) configured to roll upon a topside of a roller track and a lower roller assembly (226b, 219, 240) configured to roll upon an underside of the roller track, characterized in that: the carriage member comprises a plurality of slots formed through walls of the bores (242a, 242b) and each bore (242a, 242b) of the carriage member is coupled to two slots of the plurality of slots; and each roller assembly of the plurality of roller assemblies comprises a pair of axles, and each axle of the pair of axles extends through one slot of the plurality of slots, wherein each of the plurality of slots has a length and a width dimensioned to allow a corresponding axle to move along the length of the slot so that a central axis of the corresponding roller assembly can pivot about its pivot bearing to allow the central axis to remain parallel with a face of a topside of a roller track when the carriage member pivots about the pivot bearings. the carriage member comprises a plurality of slots formed through walls of the bores (242a, 242b) and each bore (242a, 242b) of the carriage member is coupled to two slots of the plurality of slots; and each roller assembly of the plurality of roller assemblies comprises a pair of axles, and each axle of the pair of axles extends through one slot of the plurality of slots, wherein each of the plurality of slots has a length and a width dimensioned to allow a corresponding axle to move along the length of the slot so that a central axis of the corresponding roller assembly can pivot about its pivot bearing to allow the central axis to remain parallel with a face of a topside of a roller track when the carriage member pivots about the pivot bearings.