The ratio between tube outside diameter and wall thickness. It is a rule of thumb for assessing the difficulty of a tube bend: The higher the wall factor, the more difficult the bend. The rationale behind this rule is that a wall that is thin relative to the tube outside diameter requires more support at the point of bend to prevent wrinkling or collapse. As a practical matter, the higher the wall factor, the more likely a mandrel and a wiper are needed to achieve good bend quality in rotary-draw bending. The wall factor needs to be considered in conjunction with other factors, such as the “D” of bend, to fully gauge the difficulty of a bend.