Air bending causes a permanent deformation of the workpiece.  An elastic area within this deformation causes the workpiece to spring back.

Spring-back depends on :

--the relation between r and s,

--the material of the workpiece,

--the quality of the surface

--the direction of rolling

These parameters cannot be calculated, neither practically, nor theoretically.Oehler determined the angle of spring-back b at 90° bendings in a V-die