U-locks are the classic in the fight against bicycle thieves. For many years, these relatively heavy, also a little bulky, but secure and reliable locks have been the number one type of lock. Many models come with the corresponding holder for stowing on the frame. The materials used resist forcible opening well. In terms of anti-theft protection, how long it would take to break the lock plays an important role. The longer it takes and the more difficult it is to pick the U-lock, the less often it is attempted.