Shackle diameter
The thickness of the shackle in a U-lock (see "Type").
The thicker the brace, the more difficult it is to saw through/bite, the more the lock makes it harder to steal the bike. The downside of this reliability is an increase in size and weight, and quite noticeable literally with every additional millimetre. Therefore, in most modern U-locks, the staple thickness is from 12 to 14 mm — this is considered the optimal compromise between strength and lightness.
Note that staples of the same thickness can be made from different materials, and the composition of the metal also affects reliability. Usually, the quality of the material also directly depends on the price category.
