The words 'Good' and 'lightweight' do not go together with locks - all of the good ones weigh in at 1.5 kg + (or over 4 pounds for the Americans)

Cycle Plus magazine did a profile last November of the top 20 locks available, the New York Kryptonite was in the top 3, along with a couple others that could not be got through by professional theives in under 5 mins, some of the cheap lightweight ones only took 4 seconds !

At the end of the day, expect to spend about 10% of the cost of your bike on a lock or two