Hi all...I have that book. If anyone wants a reprint of this detailed article I would happily photocopy it and send you a copy. Email me your address at : kmaher80@hotmail.com. Also, AR2201, your elementary physics of the forces acting around the hinge pin are correct. The issue is complicated by other factors such as fitting tolerances, unequal bearing of different locking bits, etc but the jist of it is correct. Cheers Kevin.

"Any man who is willing to trade his liberty for a few pieces of gold, deserves neither and will probably lose both."