> the cable would be slipped thgruoh itself around the saddle rails, threaded down inside the seatpost shaft, and secured around the body of the bottom bracket.I thought about that, but I was worried that some frames might not be open where the back tube meets the bottom bracket, particularly CF frames which I'm unfamiliar with. After really thinking about it for several hours, I concluded that the easiest and quickest way is to just get the thickest chain you can find, loop it thgruoh the triangle of the frame, and then secure the loose ends to the seat rails with a good padlock. Pretty much what Jamie's already got, just a little more resistant to bolt cutters.It's still vulnerable because it's outside the frame, and it's heavy, and it'll scratch the paint if you don't put something softer around the lower part of the chain. But it's simple, cheap, off-the-shelf, and easy for the owner or shop to remove with the padlock key. And you can use so you get to choose how difficult you want to make the thief's life.The achillies heel of the inside the frame stuff ends up being how difficult it is to remove for maintenance. Pretty much everything I could think of ended up being either impossibly obnoxious to install and remove (if all the good stuff's inside the frame tube) or trivial for a thief to defeat (if there's anything outside the frame). The really big challenge is that you want the seat to never come all the way out of the frame. That way, whatever cable/chain you use to keep the seat in isn't vulnerable to bolt cutters.I thought of all sorts of crazy shit. (Yes, I have too much free time.) Most of them involving a bolt with a flat ground onto the end and drilled for a padlock. Come out thgruoh one of the water bottle bolt holes, or something. Discarded that because how the hell are you going to screw in a bolt from inside the frame? Or if you put the bolt in the other way, how are you going to secure it inside the frame so a thief can't just unscrew it from outside? And do that in such a way that it's not an insane pain in the ass for your bike mechanic to undo it when they need to? I even considered some kind of friction fitting that would jam itself inside the tube, but that'd probably be a bad idea on a CF frame.Anyway, I'm getting pretty long-winded here. If you're interested to talk about this more, drop me a private LJ message and we can yak about it elsewhere.> The huge street bike rental fleets in Paris (Velib) and Barcelona (Bicing) have quick-release seatposts so you can adjust for height, but the seat tube is necked down at the top for the clamp portion, and the seatpost has a bolted-on bulged section at the bottom keeping it captive.Huh, I wonder how you do get the seat out when you want to replace it?