Bifold Door Hinge Bottom Roller Folding Patio Doors ALU UK SYSTEM VARIANT A replacement bottom roller & hinge to repair existing ALUK bifold installations. Bifold door rollers are used at the bottom of folding door installations wherever two hinged panels are sitting directly onto the track. Each roller assembly works with a top guide to support the moving door and to ensure that the door stays in the track. (New version assembly works with a top guide to support the moving door and to ensure that the door stays in the track.)