High-Security U-Lock with Double Bolt Mechanism and Hardened Steel Shackle Our best-selling U-locks feature a premium X2P Double Bolt lock mechanism for enhanced security and a robust 13mm (0.51'') hardened steel shackle that provides comprehensive protection against theft. The included cable with Bulldog U-Lock offers versatile locking options. The X2P Double Bolt Locking Mechanism secures the shackle on both sides for maximum strength. The innovative Z-Cylinder is bump-proof and resistant to picking, pulling, or drilling attempts. The sturdy 13mm steel shackle resists cutting, prying, and jacking attacks. Its rounded design and centered keyway eliminate pry points, while the double rubber-coated crossbar protects your bike's paint and finish. Additionally, the extra thick molded end caps safeguard against bumps and impacts.