section of the bow element 4 is a curve the particular shape shown in fig6 is a sector of a circle having radius r by contrast fig7 shows an alternative bow element 8 having a flat cross section which is less preferred a mathematical analysis to compare the stiffness of the sections can be performed for example using the parallel axis theorem consider an exemplary flat section having width 1 5 units and thickness 0 158 units this is similar to the embodiment of fig7 and has a second moment of area about the neutral axis ina given by equation 1 consider now a section having the same width and thickness but having a curvature of 3 56 units this is similar to the embodiment of fig6 and has a second moment of area about the neutral axis ina given by equation 2 it is thus follows that in the above examples curvature of the cross section shows some 20 increase in stiffness over the flat bar cross section of similar proportions the cold forming techniques made possible by the use of boron steel as the material of the centraliser provide an ability to adjust cross sectional curvature in turn this facilitates fine tuning of the flexive force resistance moreover transition regions from the selected cross sectional curve of the bows to the end collar portions can be shaped to maximise stiffness of the flexing construction it is also desirable in certain embodiments to form the bows to have a.