I purchased the Kryptonite KC890 HD Chain Lock, for around $17 at Walmart. It has a 3' length which gives me more options like trees and posts to wrap it around. Just be sure when locking up a bike to keep the chain as high up as possible. Also, it is wrapped in a durable nylon webbing that helps to protect from scratching as I wrap it around the frame under the seat when riding . One drawback is the 5lbs weight, so be prepared for that. I need to shop the Kryptonite website to see if they make a stronger chain.