Kryptonite Folding Key
U-locks are big, ugly and awkward to mount on a bike. The lock companies have tried for years to find a sufficiently strong, reliable alternative to them.
Heavy duty chain locks are tough enough but don't mount on a bike. They are almost always carried in a basket, pannier bag or backpack.
A few years ago the Abus brand invented the folding bike lock with its Bordo model.
This Kryptonite version of that lock is slightly larger and more affordable with the quality that has made Kryptonite the leading bike lock brand in the world.
Made of 3mm thick hardened steel plates and coated with thick rubber, the Keeper 585 unfolds like origami to a larger than U-lock size and is as tough for thieves to get thru as most U-locks.
If your bike has quick release wheels, consider a double ended cable for even greater security.