I've got a bunch of frames and its got me curious if some are worth adding lead to or not. Is there a rule of thumb? All the major manufacturers are finding ways to add stability with less weight through added stiffness. I know first generation of stiff racquets weren't very arm friendly, but the newer ones seem too be improving. Head racquets for a while were making their frames that vary in stiffness depending on the swing (youtek). And do racquets lose stiffness overtime?