Many modern bows (both recurves and longbows) have reinforced limb tips to handle the added strain of a fast flight string because they have minimal stretching which puts more stress on the limb tips. Older bows which do not have these reinforced limb tips and can only shoot non-flast flight strings (Dacron). If you put a fast flight string on a bow not rated for it, you eventually will damage the tips and limbs. If you have any questions about what your bow will handle, call the manufacturer. Safety first!