Here's how I mounted my OnGuard U-lock. The lock's mounting bracket is mounted on the portion of the seat tube above the top tube. I could not get the bracket on the lower portion, the tube diameter seems too large (despite OnGuard's claim of universality), but by removing the seat clamp, I was able to slip it on that top portion without having to open the mounting bracket's clamp. I did not want it on my seat post, I have other stuff there (removed for the photo). On the seat stay, a length of velcro tape keeps it secure so it does not rattle and is more sturdy going over bumps, having two support points. In my case, the velcro also goes around the brake mounting point.