If you have to leave your bicycle somewhere, perhaps whilst at work on the cafe stop, a good quality lock is a wise investment.
If you have to leave your bicycle for a full day, especially in a city centre, a D-lock or chain is going to be your best option. These are difficult to get through and are going to need some serious equipment to cut through them. On the other hand, if you are going out on a road ride, and want to ensure that no one can walk away with your pride and joy whilst sat in a cafe, a much smaller and thinner cable lock should suffice.
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