I offer no apologies for it, I carry an American Derringer in .40S&W everywhere and always. I sincerely hope that I never have to use it. Firing it is equal to your own private earthquake and hurts your palm and wrist like you wouldn't believe.

Beats pulling a prayer book outta your pocket.

I don't know about the size of the can and accessible mounting on a bike but, my wife carries a can of wasp and hornet spray between the driver's seat and the door. It beats any civilian "pepper spray" junk in sudden stopping power and has a range of somewhere around 20 feet. It's as legal a weapon as you could wish to carry.