Changing the Shape of Hair 162 where r is the radius of the coil.13,14 These parameters can also be calculated on the curling rod, B(0) & T(0). Then the % set at time, t, for bending or torsion is given by: Of course, this procedure assumes that the hair coil maintains its helical shape as the set “falls out,” but at least it should give a good approximation for the first portion of the set release. Figure 4 shows the fallout of bending set of a helical coil of hair set from water and dried overnight at 20% RH.8 After removal from the setting rod the coil was hung at 40% RH and length and diameter were measured to calculate bending set from the equations above. At 30 min, %set was about 58% and it “fell out” to about 52% at 8 h. Figure 4 Bending set release in a helical coil at 40% RH. Ln (% Set) was linear over the first 4 hours.