ABUS Granit XPlus 540 U-Lock Bike Lock - High Security Level 15, Anti-Theft XPlus Cylinder, 23cm Black/Grey - Perfect for Urban Cycling, Commuting & Bike Parking Security

Expensive, but what a great lock. The key engages and turns like butter (important to me after replacing two cheaper u-locks, in two years, that the keys finally couldn't turn any more from binding-up). I've had this lock for a year, in Seattle weather, commuting at least five days a week without interruption (all seasons) and the lock appears to be weather proof. It's heavy, coming in at about 3-1/4 lbs. for the 9" and 3-3/4 lbs. for the longer 11" model (I own both). But when you weigh 250 lbs., and ride steel frames, it's less than 1% difference relative to the cheaper, dead, discarded u-locks I've had. If you're riding carbon, and are skinny, well, I can't imagine you would want this lock stealing back all the reduction in weight you're enjoying. This does not come with the frame mount, bought it separately (about $25). It, too, is excellent, no rattling, easy to remove and return to the mount. The lock's square shackle, hopefully, means that anyone using a power cutting wheel would have to make two cuts, because the shackle does not twist in the lock like round shackles can. The shackle feels stiff/strong enough that a thief would be hard-pressed to bend the shackle after a single cut.