While I was volunteering at a bike co-op in town I would go through the trash pile looking for useful salvage. closest thing to a weapon i found is a bike seat post. it sounds stupid, but its heavy steel. think of it like a club. also i have a bike chain/lock that would make for an impromptu smiley, but as a bunch of folks are saying here, if you use your head you wont have such a need to carry designated weapons.. you can usually use your surroundings to defend yourself too. blunt objects like bricks and bottles whatever you have around you can save you in a pinch. I think you just have to be aware of your surroundings and you can pick up on stuff.