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If you don't have secure parking at your workplace, you should indeed have a serious lock, such as a Kryptonite. The thing is, you should not carry it home every night. The weight of a typical U-lock represents the difference between a $400 bike and a $700 bike.
Just leave the lock at work, locked to whatever you normally lock your bike to. Carry a light cable lock with you for quick errands or emergencies.
If you use both the U-lock and the cable lock at work, you are more than twice as safe as you would be with either of them alone. Either type of lock can be defeated, but each requires a different large, bulky tool which is useless against the other.
The cable lock will secure your front wheel to the frame and any convenient object, and the U-lock will secure your rear wheel and frame. If you have a quick-release seatpost bolt, replace it with an Allen head bolt, and stop worrying about having your saddle stolen. [If its a pre-softened Brooks titanium-rail Swallow, put a cheaper saddle on your commute bike, and then you won't worry -- John Allen]
The best cable locks are the ones that have the lock built-in, rather than relying on a padlock. The padlock is the weak link, easily cut with bolt cutters, the tool of choice for most bike thieves. A new, sharp bolt cutter will cut a cable too, but an old, worn-out one will only crush a cable.
The best U-locks, if you must carry one on the bike, are the smallest. My favorite is the Kryptonite Mini, which not all bike shops stock. The Mini is much smaller and lighter than the more popular models, but just as secure. It may be even more secure, because of the limited room to put a jack inside it. It also gives less purchase for leverage-based attacks.
People tend to buy the big clunky U-locks because they don't know how to use them properly. A U-lock should go around the rear rim and tire, somewhere inside the rear triangle of the frame. There is no need to loop it around the seat tube as well, because the wheel cannot be pulled through the rear triangle.
Some will object that felons might cut the rear rim and tire to remove the lock. Believe me, this just doesn't happen in the real world. It is indeed possible to cut the rim with a hacksaw, working from the outside to the inside, but first, the tire must be removed or cut through. It would be a lot of work to steal a frame without a usable rear wheel, the most expensive part of a bike, after the frame.
[On the other hand, a slightly longer lock would fit around the rim, and the chainstays or seatstays...or locking from the left side might make a little room because the chain doesn't get in the way...you might need a longer lock too, for when you can't get the bike as close to a pole.
Also note, a bicycle with a single main tube instead of a top tube and down tube poses a locking challenge. Some folding bicycles have separable seatstays too, and locking the rear wheel as shown will not secure the frame. You need to pay extra attention to be sure that the lock passes through a part that can't be separated. Or take advantage of the bike's folding and take it in with you. Advocate for secure, sheltered bicycle parking at your workplace: a bicycle is as easily lost by sitting out in the weather every day, as by theft. -- John Allen]
[Think about what kind of bicycle to ride to where you must lock it up. A bicycle that looks like a junker can ride like a dream. An older frame with scratched and chipped paint is good choice. Equip it with nice but undistinguished-looking components. Let it get dirty. Sheldon's Mead Ranger is a good example. That's the original paint, or what's left of it. This bicycle is really old. (1916! -- maybe too old, it's collectible, but most thieves wouldn't understand such subtleties. In any case, this isn't a daily ride-to-work bike.).
You may be well-advised to lock your bicycle at home, especially if you must leave it in a public area such as an apartment-building basement. Even if this is inside a locked perimeter, dozens, even hundreds of people you don't know -- not only your neighbors but also maintenance crews and who knows who else, will see your bicycle there day after day.In a high-crime location, it can make sense to disable a bicycle so it can't be ridden away. This can be inconvenient, but then taking the front wheel with you isn't much trouble if you are going to be at work all day. A folding bicycle offers special opportunities along these lines. The Raleigh Twenty shown below would be very low on the priority list for a thief. -- John Allen]
[My Raleigh Twenty, brush-painted with drippy marine enamel.
I left the bike here two days. I took the seatpost and saddle with me, along with
the touring bag. I took the hinge key too. The red arrow points to my
old Kryptonite lock, which secured the front wheel and the frame.
The rear wheel is hard to access, behind the front wheel, and
hard to remove, with its internal-gear hub's nutted axle -- John Allen]