What's the difference between a sissy bar and a backrest? I know some sissy bars are really just bars but some have pads on them that look just like a back rest. And also on some HDs there is a cap on the tank that looks like a gas cap but I heard it wasn't and Iv'e seen some that say do not twist on them, if you know what i'm talking about what is that cap for and why shouldn't you twist it?
