Padlock Shackle Diameter: The Thickness at Which Hand Bolt Cutters Run Out of Jaw Force

A padlock is usually attacked through its shackle with bolt cutters, and whether the attack succeeds is a contest between the force the jaws can develop and the force the shackle's cross-section demands. This study models the demand as F = 330 d^2 newtons for shackle diameter d in millimetres, a stated relation combining hardened steel shear strength with jaw wedge action, and sets it against stated jaw capacities of 17,000 N for a compact 450 mm cutter and 38,000 N for a full-size 900 mm cutter. The compact cutter is defeated at a shackle diameter of 7.1 mm and the full-size cutter at 10.7 mm; an 8 mm shackle requires 21120 N and an 11 mm shackle 39930 N. Diameter is therefore a strong defence because demand grows with its square, but it is a defence with a ceiling, since a determined attacker can bring a larger tool. The purchasing rule is to specify a shackle of at least 10 mm in hardened steel, shrouded where possible so the jaws cannot seat, and to remember that the hasp and chain beside the lock must be graded to match. SIMULATED, NOT MEASURED.